diff --git a/src/libexpr/attr-path.cc b/src/libexpr/attr-path.cc
index 76d101b98..9a9531a3f 100644
--- a/src/libexpr/attr-path.cc
+++ b/src/libexpr/attr-path.cc
@@ -19,7 +19,7 @@ static Strings parseAttrPath(const string & s)
             ++i;
             while (1) {
                 if (i == s.end())
-                    throw Error(format("missing closing quote in selection path '%1%'") % s);
+                    throw Error("missing closing quote in selection path '%1%'", s);
                 if (*i == '"') break;
                 cur.push_back(*i++);
             }
@@ -60,11 +60,11 @@ std::pair<Value *, Pos> findAlongAttrPath(EvalState & state, const string & attr
 
             if (v->type != tAttrs)
                 throw TypeError(
-                    format("the expression selected by the selection path '%1%' should be a set but is %2%")
-                    % attrPath % showType(*v));
-
+                    "the expression selected by the selection path '%1%' should be a set but is %2%",
+                    attrPath,
+                    showType(*v)); 
             if (attr.empty())
-                throw Error(format("empty attribute name in selection path '%1%'") % attrPath);
+                throw Error("empty attribute name in selection path '%1%'", attrPath);
 
             Bindings::iterator a = v->attrs->find(state.symbols.create(attr));
             if (a == v->attrs->end())
@@ -77,9 +77,9 @@ std::pair<Value *, Pos> findAlongAttrPath(EvalState & state, const string & attr
 
             if (!v->isList())
                 throw TypeError(
-                    format("the expression selected by the selection path '%1%' should be a list but is %2%")
-                    % attrPath % showType(*v));
-
+                    "the expression selected by the selection path '%1%' should be a list but is %2%",
+                    attrPath,
+                    showType(*v)); 
             if (attrIndex >= v->listSize())
                 throw AttrPathNotFound("list index %1% in selection path '%2%' is out of range", attrIndex, attrPath);
 
diff --git a/src/libexpr/eval-inline.hh b/src/libexpr/eval-inline.hh
index c27116e3b..4d82ccf09 100644
--- a/src/libexpr/eval-inline.hh
+++ b/src/libexpr/eval-inline.hh
@@ -9,18 +9,18 @@ namespace nix {
 
 LocalNoInlineNoReturn(void throwEvalError(const char * s, const Pos & pos))
 {
-    throw EvalError(format(s) % pos);
+    throw EvalError(s, pos);
 }
 
 LocalNoInlineNoReturn(void throwTypeError(const char * s, const Value & v))
 {
-    throw TypeError(format(s) % showType(v));
+    throw TypeError(s, showType(v));
 }
 
 
 LocalNoInlineNoReturn(void throwTypeError(const char * s, const Value & v, const Pos & pos))
 {
-    throw TypeError(format(s) % showType(v) % pos);
+    throw TypeError(s, showType(v), pos);
 }
 
 
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index dac32b6f5..516c25b02 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -493,52 +493,52 @@ Value & EvalState::getBuiltin(const string & name)
 
 LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2))
 {
-    throw EvalError(format(s) % s2);
+    throw EvalError(s, s2);
 }
 
 LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2, const Pos & pos))
 {
-    throw EvalError(format(s) % s2 % pos);
+    throw EvalError(s, s2, pos);
 }
 
 LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2, const string & s3))
 {
-    throw EvalError(format(s) % s2 % s3);
+    throw EvalError(s, s2, s3);
 }
 
 LocalNoInlineNoReturn(void throwEvalError(const char * s, const string & s2, const string & s3, const Pos & pos))
 {
-    throw EvalError(format(s) % s2 % s3 % pos);
+    throw EvalError(s, s2, s3, pos);
 }
 
 LocalNoInlineNoReturn(void throwEvalError(const char * s, const Symbol & sym, const Pos & p1, const Pos & p2))
 {
-    throw EvalError(format(s) % sym % p1 % p2);
+    throw EvalError(s, sym, p1, p2);
 }
 
 LocalNoInlineNoReturn(void throwTypeError(const char * s, const Pos & pos))
 {
-    throw TypeError(format(s) % pos);
+    throw TypeError(s, pos);
 }
 
 LocalNoInlineNoReturn(void throwTypeError(const char * s, const string & s1))
 {
-    throw TypeError(format(s) % s1);
+    throw TypeError(s, s1);
 }
 
 LocalNoInlineNoReturn(void throwTypeError(const char * s, const ExprLambda & fun, const Symbol & s2, const Pos & pos))
 {
-    throw TypeError(format(s) % fun.showNamePos() % s2 % pos);
+    throw TypeError(s, fun.showNamePos(), s2, pos);
 }
 
 LocalNoInlineNoReturn(void throwAssertionError(const char * s, const string & s1, const Pos & pos))
 {
-    throw AssertionError(format(s) % s1 % pos);
+    throw AssertionError(s, s1, pos);
 }
 
 LocalNoInlineNoReturn(void throwUndefinedVarError(const char * s, const string & s1, const Pos & pos))
 {
-    throw UndefinedVarError(format(s) % s1 % pos);
+    throw UndefinedVarError(s, s1, pos);
 }
 
 LocalNoInline(void addErrorPrefix(Error & e, const char * s, const string & s2))
@@ -1883,8 +1883,7 @@ void EvalState::printStats()
 
 string ExternalValueBase::coerceToString(const Pos & pos, PathSet & context, bool copyMore, bool copyToStore) const
 {
-    throw TypeError(format("cannot coerce %1% to a string, at %2%") %
-        showType() % pos);
+    throw TypeError("cannot coerce %1% to a string, at %2%", showType(), pos);
 }
 
 
diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index 63cbef1dd..a9955f6df 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -267,7 +267,7 @@ void ExprVar::bindVars(const StaticEnv & env)
     /* Otherwise, the variable must be obtained from the nearest
        enclosing `with'.  If there is no `with', then we can issue an
        "undefined variable" error now. */
-    if (withLevel == -1) throw UndefinedVarError(format("undefined variable '%1%' at %2%") % name % pos);
+    if (withLevel == -1) throw UndefinedVarError("undefined variable '%1%' at %2%", name, pos);
 
     fromWith = true;
     this->level = withLevel;
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index f7e9105a4..79e3f90e5 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -234,9 +234,7 @@ struct ExprLambda : Expr
         : pos(pos), arg(arg), matchAttrs(matchAttrs), formals(formals), body(body)
     {
         if (!arg.empty() && formals && formals->argNames.find(arg) != formals->argNames.end())
-            throw ParseError(format("duplicate formal function argument '%1%' at %2%")
-                % arg % pos);
-    };
+            throw ParseError("duplicate formal function argument '%1%' at %2%", arg, pos); };
     void setName(Symbol & name);
     string showNamePos() const;
     COMMON_METHODS
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 9c769e803..3767532d5 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -64,15 +64,15 @@ namespace nix {
 
 static void dupAttr(const AttrPath & attrPath, const Pos & pos, const Pos & prevPos)
 {
-    throw ParseError(format("attribute '%1%' at %2% already defined at %3%")
-        % showAttrPath(attrPath) % pos % prevPos);
+    throw ParseError("attribute '%1%' at %2% already defined at %3%",
+        showAttrPath(attrPath), pos, prevPos);
 }
 
 
 static void dupAttr(Symbol attr, const Pos & pos, const Pos & prevPos)
 {
-    throw ParseError(format("attribute '%1%' at %2% already defined at %3%")
-        % attr % pos % prevPos);
+    throw ParseError("attribute '%1%' at %2% already defined at %3%",
+        attr, pos, prevPos);
 }
 
 
@@ -140,8 +140,8 @@ static void addAttr(ExprAttrs * attrs, AttrPath & attrPath,
 static void addFormal(const Pos & pos, Formals * formals, const Formal & formal)
 {
     if (!formals->argNames.insert(formal.name).second)
-        throw ParseError(format("duplicate formal function argument '%1%' at %2%")
-            % formal.name % pos);
+        throw ParseError("duplicate formal function argument '%1%' at %2%",
+            formal.name, pos);
     formals->formals.push_front(formal);
 }
 
@@ -327,8 +327,7 @@ expr_function
     { $$ = new ExprWith(CUR_POS, $2, $4); }
   | LET binds IN expr_function
     { if (!$2->dynamicAttrs.empty())
-        throw ParseError(format("dynamic attributes not allowed in let at %1%")
-            % CUR_POS);
+        throw ParseError("dynamic attributes not allowed in let at %1%", CUR_POS);
       $$ = new ExprLet($2, $4);
     }
   | expr_if
@@ -475,8 +474,8 @@ attrs
           $$->push_back(AttrName(str->s));
           delete str;
       } else
-          throw ParseError(format("dynamic attributes not allowed in inherit at %1%")
-              % makeCurPos(@2, data));
+          throw ParseError("dynamic attributes not allowed in inherit at %1%",
+              makeCurPos(@2, data));
     }
   | { $$ = new AttrPath; }
   ;
@@ -670,11 +669,10 @@ Path EvalState::findFile(SearchPath & searchPath, const string & path, const Pos
         Path res = r.second + suffix;
         if (pathExists(res)) return canonPath(res);
     }
-    format f = format(
+    string f = 
         "file '%1%' was not found in the Nix search path (add it using $NIX_PATH or -I)"
-        + string(pos ? ", at %2%" : ""));
-    f.exceptions(boost::io::all_error_bits ^ boost::io::too_many_args_bit);
-    throw ThrownError(f % path % pos);
+        + string(pos ? ", at %2%" : "");
+    throw ThrownError(f, path, pos);
 }
 
 
@@ -691,7 +689,8 @@ std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathEl
             request.unpack = true;
             res = { true, getDownloader()->downloadCached(store, request).path };
         } catch (DownloadError & e) {
-            printError(format("warning: Nix search path entry '%1%' cannot be downloaded, ignoring") % elem.second);
+            // TODO: change to warn()?
+            printError("warning: Nix search path entry '%1%' cannot be downloaded, ignoring", elem.second);
             res = { false, "" };
         }
     } else {
@@ -699,7 +698,8 @@ std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathEl
         if (pathExists(path))
             res = { true, path };
         else {
-            printError(format("warning: Nix search path entry '%1%' does not exist, ignoring") % elem.second);
+            // TODO: change to warn()?
+            printError("warning: Nix search path entry '%1%' does not exist, ignoring", elem.second);
             res = { false, "" };
         }
     }
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 8de234951..13eb1ba58 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -94,8 +94,8 @@ static void prim_scopedImport(EvalState & state, const Pos & pos, Value * * args
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError(format("cannot import '%1%', since path '%2%' is not valid, at %3%")
-            % path % e.path % pos);
+        throw EvalError("cannot import '%1%', since path '%2%' is not valid, at %3%",
+                        path, e.path, pos);
     }
 
     Path realPath = state.checkSourcePath(state.toRealPath(path, context));
@@ -171,8 +171,8 @@ void prim_importNative(EvalState & state, const Pos & pos, Value * * args, Value
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError(format("cannot import '%1%', since path '%2%' is not valid, at %3%")
-            % path % e.path % pos);
+        throw EvalError("cannot import '%1%', since path '%2%' is not valid, at %3%"
+           , path, e.path, pos);
     }
 
     path = state.checkSourcePath(path);
@@ -181,17 +181,17 @@ void prim_importNative(EvalState & state, const Pos & pos, Value * * args, Value
 
     void *handle = dlopen(path.c_str(), RTLD_LAZY | RTLD_LOCAL);
     if (!handle)
-        throw EvalError(format("could not open '%1%': %2%") % path % dlerror());
+        throw EvalError("could not open '%1%': %2%", path, dlerror());
 
     dlerror();
     ValueInitializer func = (ValueInitializer) dlsym(handle, sym.c_str());
     if(!func) {
         char *message = dlerror();
         if (message)
-            throw EvalError(format("could not load symbol '%1%' from '%2%': %3%") % sym % path % message);
+            throw EvalError("could not load symbol '%1%' from '%2%': %3%", sym, path, message);
         else
-            throw EvalError(format("symbol '%1%' from '%2%' resolved to NULL when a function pointer was expected")
-                    % sym % path);
+            throw EvalError("symbol '%1%' from '%2%' resolved to NULL when a function pointer was expected"
+                   , sym, path);
     }
 
     (func)(state, v);
@@ -207,7 +207,7 @@ void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
     auto elems = args[0]->listElems();
     auto count = args[0]->listSize();
     if (count == 0) {
-        throw EvalError(format("at least one argument to 'exec' required, at %1%") % pos);
+        throw EvalError("at least one argument to 'exec' required, at %1%", pos);
     }
     PathSet context;
     auto program = state.coerceToString(pos, *elems[0], context, false, false);
@@ -218,8 +218,8 @@ void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError(format("cannot execute '%1%', since path '%2%' is not valid, at %3%")
-            % program % e.path % pos);
+        throw EvalError("cannot execute '%1%', since path '%2%' is not valid, at %3%"
+           , program, e.path, pos);
     }
 
     auto output = runProgram(program, true, commandArgs);
@@ -227,13 +227,13 @@ void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
     try {
         parsed = state.parseExprFromString(output, pos.file);
     } catch (Error & e) {
-        e.addPrefix(format("While parsing the output from '%1%', at %2%\n") % program % pos);
+        e.addPrefix(fmt("While parsing the output from '%1%', at %2%\n", program, pos));
         throw;
     }
     try {
         state.eval(parsed, v);
     } catch (Error & e) {
-        e.addPrefix(format("While evaluating the output from '%1%', at %2%\n") % program % pos);
+        e.addPrefix(fmt("While evaluating the output from '%1%', at %2%\n", program, pos));
         throw;
     }
 }
@@ -339,7 +339,7 @@ struct CompareValues
         if (v1->type == tInt && v2->type == tFloat)
             return v1->integer < v2->fpoint;
         if (v1->type != v2->type)
-            throw EvalError(format("cannot compare %1% with %2%") % showType(*v1) % showType(*v2));
+            throw EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2));
         switch (v1->type) {
             case tInt:
                 return v1->integer < v2->integer;
@@ -350,7 +350,7 @@ struct CompareValues
             case tPath:
                 return strcmp(v1->path, v2->path) < 0;
             default:
-                throw EvalError(format("cannot compare %1% with %2%") % showType(*v1) % showType(*v2));
+                throw EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2));
         }
     }
 };
@@ -371,7 +371,7 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
     Bindings::iterator startSet =
         args[0]->attrs->find(state.symbols.create("startSet"));
     if (startSet == args[0]->attrs->end())
-        throw EvalError(format("attribute 'startSet' required, at %1%") % pos);
+        throw EvalError("attribute 'startSet' required, at %1%", pos);
     state.forceList(*startSet->value, pos);
 
     ValueList workSet;
@@ -382,7 +382,7 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
     Bindings::iterator op =
         args[0]->attrs->find(state.symbols.create("operator"));
     if (op == args[0]->attrs->end())
-        throw EvalError(format("attribute 'operator' required, at %1%") % pos);
+        throw EvalError("attribute 'operator' required, at %1%", pos);
     state.forceValue(*op->value);
 
     /* Construct the closure by applying the operator to element of
@@ -401,7 +401,7 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
         Bindings::iterator key =
             e->attrs->find(state.symbols.create("key"));
         if (key == e->attrs->end())
-            throw EvalError(format("attribute 'key' required, at %1%") % pos);
+            throw EvalError("attribute 'key' required, at %1%", pos);
         state.forceValue(*key->value);
 
         if (!doneKeys.insert(key->value).second) continue;
@@ -431,7 +431,7 @@ static void prim_abort(EvalState & state, const Pos & pos, Value * * args, Value
 {
     PathSet context;
     string s = state.coerceToString(pos, *args[0], context);
-    throw Abort(format("evaluation aborted with the following error message: '%1%'") % s);
+    throw Abort("evaluation aborted with the following error message: '%1%'", s);
 }
 
 
@@ -506,9 +506,9 @@ static void prim_trace(EvalState & state, const Pos & pos, Value * * args, Value
 {
     state.forceValue(*args[0]);
     if (args[0]->type == tString)
-        printError(format("trace: %1%") % args[0]->string.s);
+        printError("trace: %1%", args[0]->string.s);
     else
-        printError(format("trace: %1%") % *args[0]);
+        printError("trace: %1%", *args[0]);
     state.forceValue(*args[1]);
     v = *args[1];
 }
@@ -533,13 +533,13 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
     /* Figure out the name first (for stack backtraces). */
     Bindings::iterator attr = args[0]->attrs->find(state.sName);
     if (attr == args[0]->attrs->end())
-        throw EvalError(format("required attribute 'name' missing, at %1%") % pos);
+        throw EvalError("required attribute 'name' missing, at %1%", pos);
     string drvName;
     Pos & posDrvName(*attr->pos);
     try {
         drvName = state.forceStringNoCtx(*attr->value, pos);
     } catch (Error & e) {
-        e.addPrefix(format("while evaluating the derivation attribute 'name' at %1%:\n") % posDrvName);
+        e.addPrefix(fmt("while evaluating the derivation attribute 'name' at %1%:\n", posDrvName));
         throw;
     }
 
@@ -583,18 +583,18 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             outputs.clear();
             for (auto & j : ss) {
                 if (outputs.find(j) != outputs.end())
-                    throw EvalError(format("duplicate derivation output '%1%', at %2%") % j % posDrvName);
+                    throw EvalError("duplicate derivation output '%1%', at %2%", j, posDrvName);
                 /* !!! Check whether j is a valid attribute
                    name. */
                 /* Derivations cannot be named ‘drv’, because
                    then we'd have an attribute ‘drvPath’ in
                    the resulting set. */
                 if (j == "drv")
-                    throw EvalError(format("invalid derivation output name 'drv', at %1%") % posDrvName);
+                    throw EvalError("invalid derivation output name 'drv', at %1%", posDrvName);
                 outputs.insert(j);
             }
             if (outputs.empty())
-                throw EvalError(format("derivation cannot have an empty set of outputs, at %1%") % posDrvName);
+                throw EvalError("derivation cannot have an empty set of outputs, at %1%", posDrvName);
         };
 
         try {
@@ -706,9 +706,9 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
 
     /* Do we have all required attributes? */
     if (drv.builder == "")
-        throw EvalError(format("required attribute 'builder' missing, at %1%") % posDrvName);
+        throw EvalError("required attribute 'builder' missing, at %1%", posDrvName);
     if (drv.platform == "")
-        throw EvalError(format("required attribute 'system' missing, at %1%") % posDrvName);
+        throw EvalError("required attribute 'system' missing, at %1%", posDrvName);
 
     /* Check whether the derivation name is valid. */
     if (isDerivation(drvName))
@@ -717,7 +717,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
     if (outputHash) {
         /* Handle fixed-output derivations. */
         if (outputs.size() != 1 || *(outputs.begin()) != "out")
-            throw Error(format("multiple outputs are not supported in fixed-output derivations, at %1%") % posDrvName);
+            throw Error("multiple outputs are not supported in fixed-output derivations, at %1%", posDrvName);
 
         HashType ht = outputHashAlgo.empty() ? htUnknown : parseHashType(outputHashAlgo);
         Hash h(*outputHash, ht);
@@ -818,7 +818,7 @@ static void prim_storePath(EvalState & state, const Pos & pos, Value * * args, V
        e.g. nix-push does the right thing. */
     if (!state.store->isStorePath(path)) path = canonPath(path, true);
     if (!state.store->isInStore(path))
-        throw EvalError(format("path '%1%' is not in the Nix store, at %2%") % path % pos);
+        throw EvalError("path '%1%' is not in the Nix store, at %2%", path, pos);
     Path path2 = state.store->toStorePath(path);
     if (!settings.readOnlyMode)
         state.store->ensurePath(state.store->parseStorePath(path2));
@@ -834,9 +834,9 @@ static void prim_pathExists(EvalState & state, const Pos & pos, Value * * args,
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError(format(
-                "cannot check the existence of '%1%', since path '%2%' is not valid, at %3%")
-            % path % e.path % pos);
+        throw EvalError(
+                "cannot check the existence of '%1%', since path '%2%' is not valid, at %3%", 
+                path, e.path, pos);
     }
 
     try {
@@ -879,12 +879,12 @@ static void prim_readFile(EvalState & state, const Pos & pos, Value * * args, Va
     try {
         state.realiseContext(context);
     } catch (InvalidPathError & e) {
-        throw EvalError(format("cannot read '%1%', since path '%2%' is not valid, at %3%")
-            % path % e.path % pos);
+        throw EvalError("cannot read '%1%', since path '%2%' is not valid, at %3%"
+           , path, e.path, pos);
     }
     string s = readFile(state.checkSourcePath(state.toRealPath(path, context)));
     if (s.find((char) 0) != string::npos)
-        throw Error(format("the contents of the file '%1%' cannot be represented as a Nix string") % path);
+        throw Error("the contents of the file '%1%' cannot be represented as a Nix string", path);
     mkString(v, s.c_str());
 }
 
@@ -908,7 +908,7 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
 
         i = v2.attrs->find(state.symbols.create("path"));
         if (i == v2.attrs->end())
-            throw EvalError(format("attribute 'path' missing, at %1%") % pos);
+            throw EvalError("attribute 'path' missing, at %1%", pos);
 
         PathSet context;
         string path = state.coerceToString(pos, *i->value, context, false, false);
@@ -916,8 +916,8 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
         try {
             state.realiseContext(context);
         } catch (InvalidPathError & e) {
-            throw EvalError(format("cannot find '%1%', since path '%2%' is not valid, at %3%")
-                % path % e.path % pos);
+            throw EvalError("cannot find '%1%', since path '%2%' is not valid, at %3%",
+                        path, e.path, pos);
         }
 
         searchPath.emplace_back(prefix, path);
@@ -934,7 +934,7 @@ static void prim_hashFile(EvalState & state, const Pos & pos, Value * * args, Va
     string type = state.forceStringNoCtx(*args[0], pos);
     HashType ht = parseHashType(type);
     if (ht == htUnknown)
-      throw Error(format("unknown hash type '%1%', at %2%") % type % pos);
+      throw Error("unknown hash type '%1%', at %2%", type, pos);
 
     PathSet context; // discarded
     Path p = state.coerceToPath(pos, *args[1], context);
@@ -950,8 +950,8 @@ static void prim_readDir(EvalState & state, const Pos & pos, Value * * args, Val
     try {
         state.realiseContext(ctx);
     } catch (InvalidPathError & e) {
-        throw EvalError(format("cannot read '%1%', since path '%2%' is not valid, at %3%")
-            % path % e.path % pos);
+        throw EvalError("cannot read '%1%', since path '%2%' is not valid, at %3%",
+                        path, e.path, pos);
     }
 
     DirEntries entries = readDirectory(state.checkSourcePath(path));
@@ -1021,7 +1021,7 @@ static void prim_toFile(EvalState & state, const Pos & pos, Value * * args, Valu
 
     for (auto path : context) {
         if (path.at(0) != '/')
-            throw EvalError(format("in 'toFile': the file '%1%' cannot refer to derivation outputs, at %2%") % name % pos);
+            throw EvalError("in 'toFile': the file '%1%' cannot refer to derivation outputs, at %2%", name, pos);
         refs.insert(state.store->parseStorePath(path));
     }
 
@@ -1089,11 +1089,11 @@ static void prim_filterSource(EvalState & state, const Pos & pos, Value * * args
     PathSet context;
     Path path = state.coerceToPath(pos, *args[1], context);
     if (!context.empty())
-        throw EvalError(format("string '%1%' cannot refer to other paths, at %2%") % path % pos);
+        throw EvalError("string '%1%' cannot refer to other paths, at %2%", path, pos);
 
     state.forceValue(*args[0]);
     if (args[0]->type != tLambda)
-        throw TypeError(format("first argument in call to 'filterSource' is not a function but %1%, at %2%") % showType(*args[0]) % pos);
+        throw TypeError("first argument in call to 'filterSource' is not a function but %1%, at %2%", showType(*args[0]), pos);
 
     addPath(state, pos, std::string(baseNameOf(path)), path, args[0], true, Hash(), v);
 }
@@ -1113,7 +1113,7 @@ static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value
             PathSet context;
             path = state.coerceToPath(*attr.pos, *attr.value, context);
             if (!context.empty())
-                throw EvalError(format("string '%1%' cannot refer to other paths, at %2%") % path % *attr.pos);
+                throw EvalError("string '%1%' cannot refer to other paths, at %2%", path, *attr.pos);
         } else if (attr.name == state.sName)
             name = state.forceStringNoCtx(*attr.value, *attr.pos);
         else if (n == "filter") {
@@ -1124,10 +1124,10 @@ static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value
         else if (n == "sha256")
             expectedHash = Hash(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA256);
         else
-            throw EvalError(format("unsupported argument '%1%' to 'addPath', at %2%") % attr.name % *attr.pos);
+            throw EvalError("unsupported argument '%1%' to 'addPath', at %2%", attr.name, *attr.pos);
     }
     if (path.empty())
-        throw EvalError(format("'path' required, at %1%") % pos);
+        throw EvalError("'path' required, at %1%", pos);
     if (name.empty())
         name = baseNameOf(path);
 
@@ -1185,7 +1185,7 @@ void prim_getAttr(EvalState & state, const Pos & pos, Value * * args, Value & v)
     // !!! Should we create a symbol here or just do a lookup?
     Bindings::iterator i = args[1]->attrs->find(state.symbols.create(attr));
     if (i == args[1]->attrs->end())
-        throw EvalError(format("attribute '%1%' missing, at %2%") % attr % pos);
+        throw EvalError("attribute '%1%' missing, at %2%", attr, pos);
     // !!! add to stack trace?
     if (state.countCalls && i->pos) state.attrSelects[*i->pos]++;
     state.forceValue(*i->value);
@@ -1265,14 +1265,14 @@ static void prim_listToAttrs(EvalState & state, const Pos & pos, Value * * args,
 
         Bindings::iterator j = v2.attrs->find(state.sName);
         if (j == v2.attrs->end())
-            throw TypeError(format("'name' attribute missing in a call to 'listToAttrs', at %1%") % pos);
+            throw TypeError("'name' attribute missing in a call to 'listToAttrs', at %1%", pos);
         string name = state.forceStringNoCtx(*j->value, pos);
 
         Symbol sym = state.symbols.create(name);
         if (seen.insert(sym).second) {
             Bindings::iterator j2 = v2.attrs->find(state.symbols.create(state.sValue));
             if (j2 == v2.attrs->end())
-                throw TypeError(format("'value' attribute missing in a call to 'listToAttrs', at %1%") % pos);
+                throw TypeError("'value' attribute missing in a call to 'listToAttrs', at %1%", pos);
 
             v.attrs->push_back(Attr(sym, j2->value, j2->pos));
         }
@@ -1346,7 +1346,7 @@ static void prim_functionArgs(EvalState & state, const Pos & pos, Value * * args
 {
     state.forceValue(*args[0]);
     if (args[0]->type != tLambda)
-        throw TypeError(format("'functionArgs' requires a function, at %1%") % pos);
+        throw TypeError("'functionArgs' requires a function, at %1%", pos);
 
     if (!args[0]->lambda.fun->matchAttrs) {
         state.mkAttrs(v, 0);
@@ -1396,7 +1396,7 @@ static void elemAt(EvalState & state, const Pos & pos, Value & list, int n, Valu
 {
     state.forceList(list, pos);
     if (n < 0 || (unsigned int) n >= list.listSize())
-        throw Error(format("list index %1% is out of bounds, at %2%") % n % pos);
+        throw Error("list index %1% is out of bounds, at %2%", n, pos);
     state.forceValue(*list.listElems()[n]);
     v = *list.listElems()[n];
 }
@@ -1423,7 +1423,7 @@ static void prim_tail(EvalState & state, const Pos & pos, Value * * args, Value
 {
     state.forceList(*args[0], pos);
     if (args[0]->listSize() == 0)
-        throw Error(format("'tail' called on an empty list, at %1%") % pos);
+        throw Error("'tail' called on an empty list, at %1%", pos);
     state.mkList(v, args[0]->listSize() - 1);
     for (unsigned int n = 0; n < v.listSize(); ++n)
         v.listElems()[n] = args[0]->listElems()[n + 1];
@@ -1564,7 +1564,7 @@ static void prim_genList(EvalState & state, const Pos & pos, Value * * args, Val
     auto len = state.forceInt(*args[1], pos);
 
     if (len < 0)
-        throw EvalError(format("cannot create list of size %1%, at %2%") % len % pos);
+        throw EvalError("cannot create list of size %1%, at %2%", len, pos);
 
     state.mkList(v, len);
 
@@ -1722,7 +1722,7 @@ static void prim_div(EvalState & state, const Pos & pos, Value * * args, Value &
     state.forceValue(*args[1], pos);
 
     NixFloat f2 = state.forceFloat(*args[1], pos);
-    if (f2 == 0) throw EvalError(format("division by zero, at %1%") % pos);
+    if (f2 == 0) throw EvalError("division by zero, at %1%", pos);
 
     if (args[0]->type == tFloat || args[1]->type == tFloat) {
         mkFloat(v, state.forceFloat(*args[0], pos) / state.forceFloat(*args[1], pos));
@@ -1731,7 +1731,7 @@ static void prim_div(EvalState & state, const Pos & pos, Value * * args, Value &
         NixInt i2 = state.forceInt(*args[1], pos);
         /* Avoid division overflow as it might raise SIGFPE. */
         if (i1 == std::numeric_limits<NixInt>::min() && i2 == -1)
-            throw EvalError(format("overflow in integer division, at %1%") % pos);
+            throw EvalError("overflow in integer division, at %1%", pos);
         mkInt(v, i1 / i2);
     }
 }
@@ -1787,7 +1787,7 @@ static void prim_substring(EvalState & state, const Pos & pos, Value * * args, V
     PathSet context;
     string s = state.coerceToString(pos, *args[2], context);
 
-    if (start < 0) throw EvalError(format("negative start position in 'substring', at %1%") % pos);
+    if (start < 0) throw EvalError("negative start position in 'substring', at %1%", pos);
 
     mkString(v, (unsigned int) start >= s.size() ? "" : string(s, start, len), context);
 }
@@ -1807,7 +1807,7 @@ static void prim_hashString(EvalState & state, const Pos & pos, Value * * args,
     string type = state.forceStringNoCtx(*args[0], pos);
     HashType ht = parseHashType(type);
     if (ht == htUnknown)
-      throw Error(format("unknown hash type '%1%', at %2%") % type % pos);
+      throw Error("unknown hash type '%1%', at %2%", type, pos);
 
     PathSet context; // discarded
     string s = state.forceString(*args[1], context, pos);
@@ -1950,7 +1950,7 @@ static void prim_replaceStrings(EvalState & state, const Pos & pos, Value * * ar
     state.forceList(*args[0], pos);
     state.forceList(*args[1], pos);
     if (args[0]->listSize() != args[1]->listSize())
-        throw EvalError(format("'from' and 'to' arguments to 'replaceStrings' have different lengths, at %1%") % pos);
+        throw EvalError("'from' and 'to' arguments to 'replaceStrings' have different lengths, at %1%", pos);
 
     vector<string> from;
     from.reserve(args[0]->listSize());
@@ -2072,11 +2072,11 @@ void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
             else if (n == "name")
                 request.name = state.forceStringNoCtx(*attr.value, *attr.pos);
             else
-                throw EvalError(format("unsupported argument '%1%' to '%2%', at %3%") % attr.name % who % attr.pos);
+                throw EvalError("unsupported argument '%1%' to '%2%', at %3%", attr.name, who, attr.pos);
         }
 
         if (request.uri.empty())
-            throw EvalError(format("'url' argument required, at %1%") % pos);
+            throw EvalError("'url' argument required, at %1%", pos);
 
     } else
         request.uri = state.forceStringNoCtx(*args[0], pos);
diff --git a/src/libexpr/primops/fetchGit.cc b/src/libexpr/primops/fetchGit.cc
index 4aee1073e..480cc1adc 100644
--- a/src/libexpr/primops/fetchGit.cc
+++ b/src/libexpr/primops/fetchGit.cc
@@ -221,7 +221,7 @@ static void prim_fetchGit(EvalState & state, const Pos & pos, Value * * args, Va
         }
 
         if (url.empty())
-            throw EvalError(format("'url' argument required, at %1%") % pos);
+            throw EvalError("'url' argument required, at %1%", pos);
 
     } else
         url = state.coerceToString(pos, *args[0], context, false, false);
diff --git a/src/libexpr/primops/fetchMercurial.cc b/src/libexpr/primops/fetchMercurial.cc
index db274fa4f..76029f548 100644
--- a/src/libexpr/primops/fetchMercurial.cc
+++ b/src/libexpr/primops/fetchMercurial.cc
@@ -191,7 +191,7 @@ static void prim_fetchMercurial(EvalState & state, const Pos & pos, Value * * ar
         }
 
         if (url.empty())
-            throw EvalError(format("'url' argument required, at %1%") % pos);
+            throw EvalError("'url' argument required, at %1%", pos);
 
     } else
         url = state.coerceToString(pos, *args[0], context, false, false);
diff --git a/src/libexpr/value-to-json.cc b/src/libexpr/value-to-json.cc
index 5fe8570ad..6ec8315ba 100644
--- a/src/libexpr/value-to-json.cc
+++ b/src/libexpr/value-to-json.cc
@@ -79,7 +79,7 @@ void printValueAsJSON(EvalState & state, bool strict,
             break;
 
         default:
-            throw TypeError(format("cannot convert %1% to JSON") % showType(v));
+            throw TypeError("cannot convert %1% to JSON", showType(v));
     }
 }
 
@@ -93,7 +93,7 @@ void printValueAsJSON(EvalState & state, bool strict,
 void ExternalValueBase::printValueAsJSON(EvalState & state, bool strict,
     JSONPlaceholder & out, PathSet & context) const
 {
-    throw TypeError(format("cannot convert %1% to JSON") % showType());
+    throw TypeError("cannot convert %1% to JSON", showType());
 }
 
 
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index d41e772e9..f7ab56271 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -75,7 +75,7 @@ string getArg(const string & opt,
     Strings::iterator & i, const Strings::iterator & end)
 {
     ++i;
-    if (i == end) throw UsageError(format("'%1%' requires an argument") % opt);
+    if (i == end) throw UsageError("'%1%' requires an argument", opt);
     return *i;
 }
 
@@ -229,7 +229,7 @@ bool LegacyArgs::processArgs(const Strings & args, bool finish)
     Strings ss(args);
     auto pos = ss.begin();
     if (!parseArg(pos, ss.end()))
-        throw UsageError(format("unexpected argument '%1%'") % args.front());
+        throw UsageError("unexpected argument '%1%'", args.front());
     return true;
 }
 
@@ -273,7 +273,7 @@ void showManPage(const string & name)
     restoreSignals();
     setenv("MANPATH", settings.nixManDir.c_str(), 1);
     execlp("man", "man", name.c_str(), nullptr);
-    throw SysError(format("command 'man %1%' failed") % name.c_str());
+    throw SysError("command 'man %1%' failed", name.c_str());
 }
 
 
@@ -301,7 +301,7 @@ int handleExceptions(const string & programName, std::function<void()> fun)
             % e.what() % programName);
         return 1;
     } catch (BaseError & e) {
-        printError(format(error + "%1%%2%") % (settings.showTrace ? e.prefix() : "") % e.msg());
+        printError(error + "%1%%2%", (settings.showTrace ? e.prefix() : ""), e.msg());
         if (e.prefix() != "" && !settings.showTrace)
             printError("(use '--show-trace' to show detailed location information)");
         return e.status;
@@ -338,7 +338,7 @@ RunPager::RunPager()
         execlp("pager", "pager", nullptr);
         execlp("less", "less", nullptr);
         execlp("more", "more", nullptr);
-        throw SysError(format("executing '%1%'") % pager);
+        throw SysError("executing '%1%'", pager);
     });
 
     pid.setKillSignal(SIGINT);
diff --git a/src/libmain/shared.hh b/src/libmain/shared.hh
index b49574652..f558247c0 100644
--- a/src/libmain/shared.hh
+++ b/src/libmain/shared.hh
@@ -56,7 +56,7 @@ template<class N> N getIntArg(const string & opt,
     Strings::iterator & i, const Strings::iterator & end, bool allowUnit)
 {
     ++i;
-    if (i == end) throw UsageError(format("'%1%' requires an argument") % opt);
+    if (i == end) throw UsageError("'%1%' requires an argument", opt);
     string s = *i;
     N multiplier = 1;
     if (allowUnit && !s.empty()) {
@@ -66,13 +66,13 @@ template<class N> N getIntArg(const string & opt,
             else if (u == 'M') multiplier = 1ULL << 20;
             else if (u == 'G') multiplier = 1ULL << 30;
             else if (u == 'T') multiplier = 1ULL << 40;
-            else throw UsageError(format("invalid unit specifier '%1%'") % u);
+            else throw UsageError("invalid unit specifier '%1%'", u);
             s.resize(s.size() - 1);
         }
     }
     N n;
     if (!string2Int(s, n))
-        throw UsageError(format("'%1%' requires an integer argument") % opt);
+        throw UsageError("'%1%' requires an integer argument", opt);
     return n * multiplier;
 }
 
diff --git a/src/libstore/binary-cache-store.cc b/src/libstore/binary-cache-store.cc
index 3a2d84861..4b7385c6b 100644
--- a/src/libstore/binary-cache-store.cc
+++ b/src/libstore/binary-cache-store.cc
@@ -40,14 +40,14 @@ void BinaryCacheStore::init()
         upsertFile(cacheInfoFile, "StoreDir: " + storeDir + "\n", "text/x-nix-cache-info");
     } else {
         for (auto & line : tokenizeString<Strings>(*cacheInfo, "\n")) {
-            size_t colon = line.find(':');
-            if (colon == std::string::npos) continue;
+            size_t colon= line.find(':');
+            if (colon ==std::string::npos) continue;
             auto name = line.substr(0, colon);
             auto value = trim(line.substr(colon + 1, std::string::npos));
             if (name == "StoreDir") {
                 if (value != storeDir)
-                    throw Error(format("binary cache '%s' is for Nix stores with prefix '%s', not '%s'")
-                        % getUri() % value % storeDir);
+                    throw Error("binary cache '%s' is for Nix stores with prefix '%s', not '%s'",
+                        getUri(), value, storeDir);
             } else if (name == "WantMassQuery") {
                 wantMassQuery.setDefault(value == "1" ? "true" : "false");
             } else if (name == "Priority") {
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 527d7ac42..2301adb16 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -453,7 +453,7 @@ static void commonChildInit(Pipe & logPipe)
        that e.g. ssh cannot open /dev/tty) and it doesn't receive
        terminal signals. */
     if (setsid() == -1)
-        throw SysError(format("creating a new session"));
+        throw SysError("creating a new session");
 
     /* Dup the write side of the logger pipe into stderr. */
     if (dup2(logPipe.writeSide.get(), STDERR_FILENO) == -1)
@@ -466,7 +466,7 @@ static void commonChildInit(Pipe & logPipe)
     /* Reroute stdin to /dev/null. */
     int fdDevNull = open(pathNullDevice.c_str(), O_RDWR);
     if (fdDevNull == -1)
-        throw SysError(format("cannot open '%1%'") % pathNullDevice);
+        throw SysError("cannot open '%1%'", pathNullDevice);
     if (dup2(fdDevNull, STDIN_FILENO) == -1)
         throw SysError("cannot dup null device into stdin");
     close(fdDevNull);
@@ -488,7 +488,7 @@ void handleDiffHook(
 
             auto diffRes = runProgram(diffHookOptions);
             if (!statusOk(diffRes.first))
-                throw ExecError(diffRes.first, fmt("diff-hook program '%1%' %2%", diffHook, statusToString(diffRes.first)));
+                throw ExecError(diffRes.first, "diff-hook program '%1%' %2%", diffHook, statusToString(diffRes.first));
 
             if (diffRes.second != "")
                 printError(chomp(diffRes.second));
@@ -534,8 +534,8 @@ UserLock::UserLock()
     /* Get the members of the build-users-group. */
     struct group * gr = getgrnam(settings.buildUsersGroup.get().c_str());
     if (!gr)
-        throw Error(format("the group '%1%' specified in 'build-users-group' does not exist")
-            % settings.buildUsersGroup);
+        throw Error("the group '%1%' specified in 'build-users-group' does not exist",
+            settings.buildUsersGroup);
     gid = gr->gr_gid;
 
     /* Copy the result of getgrnam. */
@@ -546,8 +546,8 @@ UserLock::UserLock()
     }
 
     if (users.empty())
-        throw Error(format("the build users group '%1%' has no members")
-            % settings.buildUsersGroup);
+        throw Error("the build users group '%1%' has no members",
+            settings.buildUsersGroup);
 
     /* Find a user account that isn't currently in use for another
        build. */
@@ -556,8 +556,8 @@ UserLock::UserLock()
 
         struct passwd * pw = getpwnam(i.c_str());
         if (!pw)
-            throw Error(format("the user '%1%' in the group '%2%' does not exist")
-                % i % settings.buildUsersGroup);
+            throw Error("the user '%1%' in the group '%2%' does not exist",
+                i, settings.buildUsersGroup);
 
         createDirs(settings.nixStateDir + "/userpool");
 
@@ -565,7 +565,7 @@ UserLock::UserLock()
 
         AutoCloseFD fd = open(fnUserLock.c_str(), O_RDWR | O_CREAT | O_CLOEXEC, 0600);
         if (!fd)
-            throw SysError(format("opening user lock '%1%'") % fnUserLock);
+            throw SysError("opening user lock '%1%'", fnUserLock);
 
         if (lockFile(fd.get(), ltWrite, false)) {
             fdUserLock = std::move(fd);
@@ -574,8 +574,8 @@ UserLock::UserLock()
 
             /* Sanity check... */
             if (uid == getuid() || uid == geteuid())
-                throw Error(format("the Nix user should not be a member of '%1%'")
-                    % settings.buildUsersGroup);
+                throw Error("the Nix user should not be a member of '%1%'",
+                    settings.buildUsersGroup);
 
 #if __linux__
             /* Get the list of supplementary groups of this build user.  This
@@ -585,7 +585,7 @@ UserLock::UserLock()
             int err = getgrouplist(pw->pw_name, pw->pw_gid,
                 supplementaryGIDs.data(), &ngroups);
             if (err == -1)
-                throw Error(format("failed to get list of supplementary groups for '%1%'") % pw->pw_name);
+                throw Error("failed to get list of supplementary groups for '%1%'", pw->pw_name);
 
             supplementaryGIDs.resize(ngroups);
 #endif
@@ -594,9 +594,9 @@ UserLock::UserLock()
         }
     }
 
-    throw Error(format("all build users are currently in use; "
-        "consider creating additional users and adding them to the '%1%' group")
-        % settings.buildUsersGroup);
+    throw Error("all build users are currently in use; "
+        "consider creating additional users and adding them to the '%1%' group",
+        settings.buildUsersGroup);
 }
 
 
@@ -1985,7 +1985,7 @@ void DerivationGoal::startBuilder()
         string s = get(drv->env, "exportReferencesGraph").value_or("");
         Strings ss = tokenizeString<Strings>(s);
         if (ss.size() % 2 != 0)
-            throw BuildError(format("odd number of tokens in 'exportReferencesGraph': '%1%'") % s);
+            throw BuildError("odd number of tokens in 'exportReferencesGraph': '%1%'", s);
         for (Strings::iterator i = ss.begin(); i != ss.end(); ) {
             string fileName = *i++;
             static std::regex regex("[A-Za-z_][A-Za-z0-9_.-]*");
@@ -2034,7 +2034,7 @@ void DerivationGoal::startBuilder()
                     worker.store.computeFSClosure(worker.store.parseStorePath(worker.store.toStorePath(i.second.source)), closure);
             } catch (InvalidPath & e) {
             } catch (Error & e) {
-                throw Error(format("while processing 'sandbox-paths': %s") % e.what());
+                throw Error("while processing 'sandbox-paths': %s", e.what());
             }
         for (auto & i : closure) {
             auto p = worker.store.printStorePath(i);
@@ -2081,10 +2081,10 @@ void DerivationGoal::startBuilder()
         printMsg(lvlChatty, format("setting up chroot environment in '%1%'") % chrootRootDir);
 
         if (mkdir(chrootRootDir.c_str(), 0750) == -1)
-            throw SysError(format("cannot create '%1%'") % chrootRootDir);
+            throw SysError("cannot create '%1%'", chrootRootDir);
 
         if (buildUser && chown(chrootRootDir.c_str(), 0, buildUser->getGID()) == -1)
-            throw SysError(format("cannot change ownership of '%1%'") % chrootRootDir);
+            throw SysError("cannot change ownership of '%1%'", chrootRootDir);
 
         /* Create a writable /tmp in the chroot.  Many builders need
            this.  (Of course they should really respect $TMPDIR
@@ -2128,7 +2128,7 @@ void DerivationGoal::startBuilder()
         chmod_(chrootStoreDir, 01775);
 
         if (buildUser && chown(chrootStoreDir.c_str(), 0, buildUser->getGID()) == -1)
-            throw SysError(format("cannot change ownership of '%1%'") % chrootStoreDir);
+            throw SysError("cannot change ownership of '%1%'", chrootStoreDir);
 
         for (auto & i : inputPaths) {
             auto p = worker.store.printStorePath(i);
@@ -2161,7 +2161,7 @@ void DerivationGoal::startBuilder()
     if (needsHashRewrite()) {
 
         if (pathExists(homeDir))
-            throw Error(format("directory '%1%' exists; please remove it") % homeDir);
+            throw Error("directory '%1%' exists; please remove it", homeDir);
 
         /* We're not doing a chroot build, but we have some valid
            output paths.  Since we can't just overwrite or delete
@@ -2206,8 +2206,7 @@ void DerivationGoal::startBuilder()
                 if (line == "extra-sandbox-paths" || line == "extra-chroot-dirs") {
                     state = stExtraChrootDirs;
                 } else {
-                    throw Error(format("unknown pre-build hook command '%1%'")
-                        % line);
+                    throw Error("unknown pre-build hook command '%1%'", line);
                 }
             } else if (state == stExtraChrootDirs) {
                 if (line == "") {
@@ -2967,7 +2966,7 @@ void DerivationGoal::chownToBuilder(const Path & path)
 {
     if (!buildUser) return;
     if (chown(path.c_str(), buildUser->getUID(), buildUser->getGID()) == -1)
-        throw SysError(format("cannot change ownership of '%1%'") % path);
+        throw SysError("cannot change ownership of '%1%'", path);
 }
 
 
@@ -3104,7 +3103,7 @@ void DerivationGoal::runChild()
             /* Bind-mount chroot directory to itself, to treat it as a
                different filesystem from /, as needed for pivot_root. */
             if (mount(chrootRootDir.c_str(), chrootRootDir.c_str(), 0, MS_BIND, 0) == -1)
-                throw SysError(format("unable to bind mount '%1%'") % chrootRootDir);
+                throw SysError("unable to bind mount '%1%'", chrootRootDir);
 
             /* Bind-mount the sandbox's Nix store onto itself so that
                we can mark it as a "shared" subtree, allowing bind
@@ -3238,16 +3237,16 @@ void DerivationGoal::runChild()
 
             /* Do the chroot(). */
             if (chdir(chrootRootDir.c_str()) == -1)
-                throw SysError(format("cannot change directory to '%1%'") % chrootRootDir);
+                throw SysError("cannot change directory to '%1%'", chrootRootDir);
 
             if (mkdir("real-root", 0) == -1)
                 throw SysError("cannot create real-root directory");
 
             if (pivot_root(".", "real-root") == -1)
-                throw SysError(format("cannot pivot old root directory onto '%1%'") % (chrootRootDir + "/real-root"));
+                throw SysError("cannot pivot old root directory onto '%1%'", (chrootRootDir + "/real-root"));
 
             if (chroot(".") == -1)
-                throw SysError(format("cannot change root directory to '%1%'") % chrootRootDir);
+                throw SysError("cannot change root directory to '%1%'", chrootRootDir);
 
             if (umount2("real-root", MNT_DETACH) == -1)
                 throw SysError("cannot unmount real root filesystem");
@@ -3268,7 +3267,7 @@ void DerivationGoal::runChild()
 #endif
 
         if (chdir(tmpDirInSandbox.c_str()) == -1)
-            throw SysError(format("changing into '%1%'") % tmpDir);
+            throw SysError("changing into '%1%'", tmpDir);
 
         /* Close all other file descriptors. */
         closeMostFDs({STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO});
@@ -3407,9 +3406,9 @@ void DerivationGoal::runChild()
                 sandboxProfile += "(allow file-read* file-write* process-exec\n";
                 for (auto & i : dirsInChroot) {
                     if (i.first != i.second.source)
-                        throw Error(format(
-                            "can't map '%1%' to '%2%': mismatched impure paths not supported on Darwin")
-                            % i.first % i.second.source);
+                        throw Error(
+                            "can't map '%1%' to '%2%': mismatched impure paths not supported on Darwin",
+                            i.first, i.second.source);
 
                     string path = i.first;
                     struct stat st;
@@ -3500,7 +3499,7 @@ void DerivationGoal::runChild()
                 else if (drv->builder == "builtin:unpack-channel")
                     builtinUnpackChannel(drv2);
                 else
-                    throw Error(format("unsupported builtin function '%1%'") % string(drv->builder, 8));
+                    throw Error("unsupported builtin function '%1%'", string(drv->builder, 8));
                 _exit(0);
             } catch (std::exception & e) {
                 writeFull(STDERR_FILENO, "error: " + string(e.what()) + "\n");
@@ -3510,7 +3509,7 @@ void DerivationGoal::runChild()
 
         execve(builder, stringsToCharPtrs(args).data(), stringsToCharPtrs(envStrs).data());
 
-        throw SysError(format("executing '%1%'") % drv->builder);
+        throw SysError("executing '%1%'", drv->builder);
 
     } catch (std::exception & e) {
         writeFull(STDERR_FILENO, "\1while setting up the build environment: " + string(e.what()) + "\n");
@@ -3595,7 +3594,7 @@ void DerivationGoal::registerOutputs()
                     replaceValidPath(path, actualPath);
                 else
                     if (buildMode != bmCheck && rename(actualPath.c_str(), worker.store.toRealPath(path).c_str()) == -1)
-                        throw SysError(format("moving build output '%1%' from the sandbox to the Nix store") % path);
+                        throw SysError("moving build output '%1%' from the sandbox to the Nix store", path);
             }
             if (buildMode != bmCheck) actualPath = worker.store.toRealPath(path);
         }
@@ -3616,13 +3615,13 @@ void DerivationGoal::registerOutputs()
            user. */
         if ((!S_ISLNK(st.st_mode) && (st.st_mode & (S_IWGRP | S_IWOTH))) ||
             (buildUser && st.st_uid != buildUser->getUID()))
-            throw BuildError(format("suspicious ownership or permission on '%1%'; rejecting this build output") % path);
+            throw BuildError("suspicious ownership or permission on '%1%'; rejecting this build output", path);
 #endif
 
         /* Apply hash rewriting if necessary. */
         bool rewritten = false;
         if (!outputRewrites.empty()) {
-            printError(format("warning: rewriting hashes in '%1%'; cross fingers") % path);
+            printError("warning: rewriting hashes in '%1%'; cross fingers", path);
 
             /* Canonicalise first.  This ensures that the path we're
                rewriting doesn't contain a hard link to /etc/shadow or
@@ -3654,7 +3653,8 @@ void DerivationGoal::registerOutputs()
                 /* The output path should be a regular file without execute permission. */
                 if (!S_ISREG(st.st_mode) || (st.st_mode & S_IXUSR) != 0)
                     throw BuildError(
-                        format("output path '%1%' should be a non-executable regular file") % path);
+                        "output path '%1%' should be a non-executable regular file",
+                        path);
             }
 
             /* Check the hash. In hash mode, move the path produced by
@@ -3715,7 +3715,7 @@ void DerivationGoal::registerOutputs()
                     Path dst = worker.store.toRealPath(path + checkSuffix);
                     deletePath(dst);
                     if (rename(actualPath.c_str(), dst.c_str()))
-                        throw SysError(format("renaming '%1%' to '%2%'") % actualPath % dst);
+                        throw SysError("renaming '%1%' to '%2%'", actualPath, dst);
 
                     handleDiffHook(
                         buildUser ? buildUser->getUID() : getuid(),
@@ -4014,7 +4014,7 @@ Path DerivationGoal::openLogFile()
         settings.compressLog ? ".bz2" : "");
 
     fdLogFile = open(logFileName.c_str(), O_CREAT | O_WRONLY | O_TRUNC | O_CLOEXEC, 0666);
-    if (!fdLogFile) throw SysError(format("creating log file '%1%'") % logFileName);
+    if (!fdLogFile) throw SysError("creating log file '%1%'", logFileName);
 
     logFileSink = std::make_shared<FdSink>(fdLogFile.get());
 
@@ -4522,7 +4522,6 @@ void SubstitutionGoal::handleEOF(int fd)
     if (fd == outPipe.readSide.get()) worker.wakeUp(shared_from_this());
 }
 
-
 //////////////////////////////////////////////////////////////////////
 
 
@@ -4739,9 +4738,9 @@ void Worker::run(const Goals & _topGoals)
         if (!children.empty() || !waitingForAWhile.empty())
             waitForInput();
         else {
-            if (awake.empty() && 0 == settings.maxBuildJobs) throw Error(
-                "unable to start any build; either increase '--max-jobs' "
-                "or enable remote builds");
+            if (awake.empty() && 0 == settings.maxBuildJobs) 
+                throw Error("unable to start any build; either increase '--max-jobs' "
+                            "or enable remote builds");
             assert(!awake.empty());
         }
     }
@@ -4754,7 +4753,6 @@ void Worker::run(const Goals & _topGoals)
     assert(!settings.keepGoing || children.empty());
 }
 
-
 void Worker::waitForInput()
 {
     printMsg(lvlVomit, "waiting for children");
@@ -4895,6 +4893,9 @@ void Worker::waitForInput()
 }
 
 
+
+
+
 unsigned int Worker::exitStatus()
 {
     /*
@@ -4994,7 +4995,6 @@ void LocalStore::buildPaths(const std::vector<StorePathWithOutputs> & drvPaths,
         throw Error(worker.exitStatus(), "build of %s failed", showPaths(failed));
 }
 
-
 BuildResult LocalStore::buildDerivation(const StorePath & drvPath, const BasicDerivation & drv,
     BuildMode buildMode)
 {
@@ -5055,4 +5055,8 @@ void LocalStore::repairPath(const StorePath & path)
 }
 
 
+
 }
+
+
+
diff --git a/src/libstore/builtins/buildenv.cc b/src/libstore/builtins/buildenv.cc
index 1b802d908..000769094 100644
--- a/src/libstore/builtins/buildenv.cc
+++ b/src/libstore/builtins/buildenv.cc
@@ -72,15 +72,15 @@ static void createLinks(State & state, const Path & srcDir, const Path & dstDir,
                     if (!S_ISDIR(lstat(target).st_mode))
                         throw Error("collision between '%1%' and non-directory '%2%'", srcFile, target);
                     if (unlink(dstFile.c_str()) == -1)
-                        throw SysError(format("unlinking '%1%'") % dstFile);
+                        throw SysError("unlinking '%1%'", dstFile);
                     if (mkdir(dstFile.c_str(), 0755) == -1)
-                        throw SysError(format("creating directory '%1%'"));
+                        throw SysError("creating directory '%1%'", dstFile);
                     createLinks(state, target, dstFile, state.priorities[dstFile]);
                     createLinks(state, srcFile, dstFile, priority);
                     continue;
                 }
             } else if (errno != ENOENT)
-                throw SysError(format("getting status of '%1%'") % dstFile);
+                throw SysError("getting status of '%1%'", dstFile);
         }
 
         else {
@@ -99,11 +99,11 @@ static void createLinks(State & state, const Path & srcDir, const Path & dstDir,
                     if (prevPriority < priority)
                         continue;
                     if (unlink(dstFile.c_str()) == -1)
-                        throw SysError(format("unlinking '%1%'") % dstFile);
+                        throw SysError("unlinking '%1%'", dstFile);
                 } else if (S_ISDIR(dstSt.st_mode))
                     throw Error("collision between non-directory '%1%' and directory '%2%'", srcFile, dstFile);
             } else if (errno != ENOENT)
-                throw SysError(format("getting status of '%1%'") % dstFile);
+                throw SysError("getting status of '%1%'", dstFile);
         }
 
         createSymlink(srcFile, dstFile);
diff --git a/src/libstore/builtins/fetchurl.cc b/src/libstore/builtins/fetchurl.cc
index f6ae5d2e6..223ed1fba 100644
--- a/src/libstore/builtins/fetchurl.cc
+++ b/src/libstore/builtins/fetchurl.cc
@@ -18,7 +18,7 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
 
     auto getAttr = [&](const string & name) {
         auto i = drv.env.find(name);
-        if (i == drv.env.end()) throw Error(format("attribute '%s' missing") % name);
+        if (i == drv.env.end()) throw Error("attribute '%s' missing", name);
         return i->second;
     };
 
@@ -54,7 +54,7 @@ void builtinFetchurl(const BasicDerivation & drv, const std::string & netrcData)
         auto executable = drv.env.find("executable");
         if (executable != drv.env.end() && executable->second == "1") {
             if (chmod(storePath.c_str(), 0755) == -1)
-                throw SysError(format("making '%1%' executable") % storePath);
+                throw SysError("making '%1%' executable", storePath);
         }
     };
 
diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index d985f151b..f8b449e5e 100644
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@@ -747,7 +747,7 @@ static void performOp(TunnelLogger * logger, ref<Store> store,
     }
 
     default:
-        throw Error(format("invalid operation %1%") % op);
+        throw Error("invalid operation %1%", op);
     }
 }
 
diff --git a/src/libstore/derivations.cc b/src/libstore/derivations.cc
index 973ddc86a..f35b8cc43 100644
--- a/src/libstore/derivations.cc
+++ b/src/libstore/derivations.cc
@@ -87,7 +87,7 @@ static void expect(std::istream & str, const string & s)
     char s2[s.size()];
     str.read(s2, s.size());
     if (string(s2, s.size()) != s)
-        throw FormatError(format("expected string '%1%'") % s);
+        throw FormatError("expected string '%1%'", s);
 }
 
 
@@ -114,7 +114,7 @@ static Path parsePath(std::istream & str)
 {
     string s = parseString(str);
     if (s.size() == 0 || s[0] != '/')
-        throw FormatError(format("bad path '%1%' in derivation") % s);
+        throw FormatError("bad path '%1%' in derivation", s);
     return s;
 }
 
@@ -196,7 +196,7 @@ Derivation readDerivation(const Store & store, const Path & drvPath)
     try {
         return parseDerivation(store, readFile(drvPath));
     } catch (FormatError & e) {
-        throw Error(format("error parsing derivation '%1%': %2%") % drvPath % e.msg());
+        throw Error("error parsing derivation '%1%': %2%", drvPath, e.msg());
     }
 }
 
diff --git a/src/libstore/download.cc b/src/libstore/download.cc
index 5967d0425..7d0cb449b 100644
--- a/src/libstore/download.cc
+++ b/src/libstore/download.cc
@@ -113,7 +113,7 @@ struct CurlDownloader : public Downloader
             if (requestHeaders) curl_slist_free_all(requestHeaders);
             try {
                 if (!done)
-                    fail(DownloadError(Interrupted, format("download of '%s' was interrupted") % request.uri));
+                    fail(DownloadError(Interrupted, "download of '%s' was interrupted", request.uri));
             } catch (...) {
                 ignoreException();
             }
@@ -518,7 +518,7 @@ struct CurlDownloader : public Downloader
             int running;
             CURLMcode mc = curl_multi_perform(curlm, &running);
             if (mc != CURLM_OK)
-                throw nix::Error(format("unexpected error from curl_multi_perform(): %s") % curl_multi_strerror(mc));
+                throw nix::Error("unexpected error from curl_multi_perform(): %s", curl_multi_strerror(mc));
 
             /* Set the promises of any finished requests. */
             CURLMsg * msg;
@@ -548,7 +548,7 @@ struct CurlDownloader : public Downloader
             vomit("download thread waiting for %d ms", sleepTimeMs);
             mc = curl_multi_wait(curlm, extraFDs, 1, sleepTimeMs, &numfds);
             if (mc != CURLM_OK)
-                throw nix::Error(format("unexpected error from curl_multi_wait(): %s") % curl_multi_strerror(mc));
+                throw nix::Error("unexpected error from curl_multi_wait(): %s", curl_multi_strerror(mc));
 
             nextWakeup = std::chrono::steady_clock::time_point();
 
diff --git a/src/libstore/download.hh b/src/libstore/download.hh
index 5a131c704..8141928d2 100644
--- a/src/libstore/download.hh
+++ b/src/libstore/download.hh
@@ -128,8 +128,10 @@ class DownloadError : public Error
 {
 public:
     Downloader::Error error;
-    DownloadError(Downloader::Error error, const FormatOrString & fs)
-        : Error(fs), error(error)
+
+    template<typename... Args>
+    DownloadError(Downloader::Error error, const Args & ... args)
+        : Error(args...), error(error)
     { }
 };
 
diff --git a/src/libstore/gc.cc b/src/libstore/gc.cc
index 0c3d89611..27bd3f3e9 100644
--- a/src/libstore/gc.cc
+++ b/src/libstore/gc.cc
@@ -38,7 +38,7 @@ AutoCloseFD LocalStore::openGCLock(LockType lockType)
 
     AutoCloseFD fdGCLock = open(fnGCLock.c_str(), O_RDWR | O_CREAT | O_CLOEXEC, 0600);
     if (!fdGCLock)
-        throw SysError(format("opening global GC lock '%1%'") % fnGCLock);
+        throw SysError("opening global GC lock '%1%'", fnGCLock);
 
     if (!lockFile(fdGCLock.get(), lockType, false)) {
         printError(format("waiting for the big garbage collector lock..."));
@@ -65,8 +65,8 @@ static void makeSymlink(const Path & link, const Path & target)
 
     /* Atomically replace the old one. */
     if (rename(tempLink.c_str(), link.c_str()) == -1)
-        throw SysError(format("cannot rename '%1%' to '%2%'")
-            % tempLink % link);
+        throw SysError("cannot rename '%1%' to '%2%'",
+            tempLink , link);
 }
 
 
@@ -91,15 +91,15 @@ Path LocalFSStore::addPermRoot(const StorePath & storePath,
     Path gcRoot(canonPath(_gcRoot));
 
     if (isInStore(gcRoot))
-        throw Error(format(
+        throw Error(
                 "creating a garbage collector root (%1%) in the Nix store is forbidden "
-                "(are you running nix-build inside the store?)") % gcRoot);
+                "(are you running nix-build inside the store?)", gcRoot);
 
     if (indirect) {
         /* Don't clobber the link if it already exists and doesn't
            point to the Nix store. */
         if (pathExists(gcRoot) && (!isLink(gcRoot) || !isInStore(readLink(gcRoot))))
-            throw Error(format("cannot create symlink '%1%'; already exists") % gcRoot);
+            throw Error("cannot create symlink '%1%'; already exists", gcRoot);
         makeSymlink(gcRoot, printStorePath(storePath));
         addIndirectRoot(gcRoot);
     }
@@ -109,10 +109,10 @@ Path LocalFSStore::addPermRoot(const StorePath & storePath,
             Path rootsDir = canonPath((format("%1%/%2%") % stateDir % gcRootsDir).str());
 
             if (string(gcRoot, 0, rootsDir.size() + 1) != rootsDir + "/")
-                throw Error(format(
+                throw Error(
                     "path '%1%' is not a valid garbage collector root; "
-                    "it's not in the directory '%2%'")
-                    % gcRoot % rootsDir);
+                    "it's not in the directory '%2%'",
+                    gcRoot, rootsDir);
         }
 
         if (baseNameOf(gcRoot) == std::string(storePath.to_string()))
@@ -170,7 +170,7 @@ void LocalStore::addTempRoot(const StorePath & path)
                way. */
             struct stat st;
             if (fstat(state->fdTempRoots.get(), &st) == -1)
-                throw SysError(format("statting '%1%'") % fnTempRoots);
+                throw SysError("statting '%1%'", fnTempRoots);
             if (st.st_size == 0) break;
 
             /* The garbage collector deleted this file before we could
@@ -211,7 +211,7 @@ void LocalStore::findTempRoots(FDs & fds, Roots & tempRoots, bool censor)
         if (!*fd) {
             /* It's okay if the file has disappeared. */
             if (errno == ENOENT) continue;
-            throw SysError(format("opening temporary roots file '%1%'") % path);
+            throw SysError("opening temporary roots file '%1%'", path);
         }
 
         /* This should work, but doesn't, for some reason. */
@@ -222,7 +222,7 @@ void LocalStore::findTempRoots(FDs & fds, Roots & tempRoots, bool censor)
            only succeed if the owning process has died.  In that case
            we don't care about its temporary roots. */
         if (lockFile(fd->get(), ltWrite, false)) {
-            printError(format("removing stale temporary roots file '%1%'") % path);
+            printError("removing stale temporary roots file '%1%'", path);
             unlink(path.c_str());
             writeFull(fd->get(), "d");
             continue;
@@ -398,7 +398,7 @@ void LocalStore::findRuntimeRoots(Roots & roots, bool censor)
                 if (!fdDir) {
                     if (errno == ENOENT || errno == EACCES)
                         continue;
-                    throw SysError(format("opening %1%") % fdStr);
+                    throw SysError("opening %1%", fdStr);
                 }
                 struct dirent * fd_ent;
                 while (errno = 0, fd_ent = readdir(fdDir.get())) {
@@ -408,7 +408,7 @@ void LocalStore::findRuntimeRoots(Roots & roots, bool censor)
                 if (errno) {
                     if (errno == ESRCH)
                         continue;
-                    throw SysError(format("iterating /proc/%1%/fd") % ent->d_name);
+                    throw SysError("iterating /proc/%1%/fd", ent->d_name);
                 }
                 fdDir.reset();
 
@@ -536,7 +536,7 @@ void LocalStore::deletePathRecursive(GCState & state, const Path & path)
     struct stat st;
     if (lstat(realPath.c_str(), &st)) {
         if (errno == ENOENT) return;
-        throw SysError(format("getting status of %1%") % realPath);
+        throw SysError("getting status of %1%", realPath);
     }
 
     printInfo(format("deleting '%1%'") % path);
@@ -554,10 +554,10 @@ void LocalStore::deletePathRecursive(GCState & state, const Path & path)
         // size.
         try {
             if (chmod(realPath.c_str(), st.st_mode | S_IWUSR) == -1)
-                throw SysError(format("making '%1%' writable") % realPath);
+                throw SysError("making '%1%' writable", realPath);
             Path tmp = trashDir + "/" + std::string(baseNameOf(path));
             if (rename(realPath.c_str(), tmp.c_str()))
-                throw SysError(format("unable to rename '%1%' to '%2%'") % realPath % tmp);
+                throw SysError("unable to rename '%1%' to '%2%'", realPath, tmp);
             state.bytesInvalidated += size;
         } catch (SysError & e) {
             if (e.errNo == ENOSPC) {
@@ -676,7 +676,7 @@ void LocalStore::tryToDelete(GCState & state, const Path & path)
 void LocalStore::removeUnusedLinks(const GCState & state)
 {
     AutoCloseDir dir(opendir(linksDir.c_str()));
-    if (!dir) throw SysError(format("opening directory '%1%'") % linksDir);
+    if (!dir) throw SysError("opening directory '%1%'", linksDir);
 
     long long actualSize = 0, unsharedSize = 0;
 
@@ -689,7 +689,7 @@ void LocalStore::removeUnusedLinks(const GCState & state)
 
         struct stat st;
         if (lstat(path.c_str(), &st) == -1)
-            throw SysError(format("statting '%1%'") % path);
+            throw SysError("statting '%1%'", path);
 
         if (st.st_nlink != 1) {
             actualSize += st.st_size;
@@ -700,14 +700,14 @@ void LocalStore::removeUnusedLinks(const GCState & state)
         printMsg(lvlTalkative, format("deleting unused link '%1%'") % path);
 
         if (unlink(path.c_str()) == -1)
-            throw SysError(format("deleting '%1%'") % path);
+            throw SysError("deleting '%1%'", path);
 
         state.results.bytesFreed += st.st_size;
     }
 
     struct stat st;
     if (stat(linksDir.c_str(), &st) == -1)
-        throw SysError(format("statting '%1%'") % linksDir);
+        throw SysError("statting '%1%'", linksDir);
     long long overhead = st.st_blocks * 512ULL;
 
     printInfo(format("note: currently hard linking saves %.2f MiB")
@@ -801,7 +801,7 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
         try {
 
             AutoCloseDir dir(opendir(realStoreDir.c_str()));
-            if (!dir) throw SysError(format("opening directory '%1%'") % realStoreDir);
+            if (!dir) throw SysError("opening directory '%1%'", realStoreDir);
 
             /* Read the store and immediately delete all paths that
                aren't valid.  When using --max-freed etc., deleting
diff --git a/src/libstore/local-binary-cache-store.cc b/src/libstore/local-binary-cache-store.cc
index 363be1443..48aca478c 100644
--- a/src/libstore/local-binary-cache-store.cc
+++ b/src/libstore/local-binary-cache-store.cc
@@ -74,7 +74,7 @@ static void atomicWrite(const Path & path, const std::string & s)
     AutoDelete del(tmp, false);
     writeFile(tmp, s);
     if (rename(tmp.c_str(), path.c_str()))
-        throw SysError(format("renaming '%1%' to '%2%'") % tmp % path);
+        throw SysError("renaming '%1%' to '%2%'", tmp, path);
     del.cancel();
 }
 
diff --git a/src/libstore/local-fs-store.cc b/src/libstore/local-fs-store.cc
index aa5abd835..2d564a0d7 100644
--- a/src/libstore/local-fs-store.cc
+++ b/src/libstore/local-fs-store.cc
@@ -22,7 +22,7 @@ struct LocalStoreAccessor : public FSAccessor
     {
         Path storePath = store->toStorePath(path);
         if (!store->isValidPath(store->parseStorePath(storePath)))
-            throw InvalidPath(format("path '%1%' is not a valid store path") % storePath);
+            throw InvalidPath("path '%1%' is not a valid store path", storePath);
         return store->getRealStoreDir() + std::string(path, store->storeDir.size());
     }
 
@@ -33,11 +33,11 @@ struct LocalStoreAccessor : public FSAccessor
         struct stat st;
         if (lstat(realPath.c_str(), &st)) {
             if (errno == ENOENT || errno == ENOTDIR) return {Type::tMissing, 0, false};
-            throw SysError(format("getting status of '%1%'") % path);
+            throw SysError("getting status of '%1%'", path);
         }
 
         if (!S_ISREG(st.st_mode) && !S_ISDIR(st.st_mode) && !S_ISLNK(st.st_mode))
-            throw Error(format("file '%1%' has unsupported type") % path);
+            throw Error("file '%1%' has unsupported type", path);
 
         return {
             S_ISREG(st.st_mode) ? Type::tRegular :
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index ae7513ad8..db5a3b53c 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -92,13 +92,13 @@ LocalStore::LocalStore(const Params & params)
         else {
             struct stat st;
             if (stat(realStoreDir.c_str(), &st))
-                throw SysError(format("getting attributes of path '%1%'") % realStoreDir);
+                throw SysError("getting attributes of path '%1%'", realStoreDir);
 
             if (st.st_uid != 0 || st.st_gid != gr->gr_gid || (st.st_mode & ~S_IFMT) != perm) {
                 if (chown(realStoreDir.c_str(), 0, gr->gr_gid) == -1)
-                    throw SysError(format("changing ownership of path '%1%'") % realStoreDir);
+                    throw SysError("changing ownership of path '%1%'", realStoreDir);
                 if (chmod(realStoreDir.c_str(), perm) == -1)
-                    throw SysError(format("changing permissions on path '%1%'") % realStoreDir);
+                    throw SysError("changing permissions on path '%1%'", realStoreDir);
             }
         }
     }
@@ -109,12 +109,12 @@ LocalStore::LocalStore(const Params & params)
         struct stat st;
         while (path != "/") {
             if (lstat(path.c_str(), &st))
-                throw SysError(format("getting status of '%1%'") % path);
+                throw SysError("getting status of '%1%'", path);
             if (S_ISLNK(st.st_mode))
-                throw Error(format(
+                throw Error(
                         "the path '%1%' is a symlink; "
-                        "this is not allowed for the Nix store and its parent directories")
-                    % path);
+                        "this is not allowed for the Nix store and its parent directories",
+                        path);
             path = dirOf(path);
         }
     }
@@ -155,8 +155,8 @@ LocalStore::LocalStore(const Params & params)
        upgrade.  */
     int curSchema = getSchema();
     if (curSchema > nixSchemaVersion)
-        throw Error(format("current Nix store schema is version %1%, but I only support %2%")
-            % curSchema % nixSchemaVersion);
+        throw Error("current Nix store schema is version %1%, but I only support %2%",
+             curSchema, nixSchemaVersion);
 
     else if (curSchema == 0) { /* new store */
         curSchema = nixSchemaVersion;
@@ -284,7 +284,7 @@ int LocalStore::getSchema()
     if (pathExists(schemaPath)) {
         string s = readFile(schemaPath);
         if (!string2Int(s, curSchema))
-            throw Error(format("'%1%' is corrupt") % schemaPath);
+            throw Error("'%1%' is corrupt", schemaPath);
     }
     return curSchema;
 }
@@ -293,7 +293,7 @@ int LocalStore::getSchema()
 void LocalStore::openDB(State & state, bool create)
 {
     if (access(dbDir.c_str(), R_OK | W_OK))
-        throw SysError(format("Nix database directory '%1%' is not writable") % dbDir);
+        throw SysError("Nix database directory '%1%' is not writable", dbDir);
 
     /* Open the Nix database. */
     string dbPath = dbDir + "/db.sqlite";
@@ -367,7 +367,7 @@ void LocalStore::makeStoreWritable()
             throw SysError("setting up a private mount namespace");
 
         if (mount(0, realStoreDir.c_str(), "none", MS_REMOUNT | MS_BIND, 0) == -1)
-            throw SysError(format("remounting %1% writable") % realStoreDir);
+            throw SysError("remounting %1% writable", realStoreDir);
     }
 #endif
 }
@@ -388,7 +388,7 @@ static void canonicaliseTimestampAndPermissions(const Path & path, const struct
                  | 0444
                  | (st.st_mode & S_IXUSR ? 0111 : 0);
             if (chmod(path.c_str(), mode) == -1)
-                throw SysError(format("changing mode of '%1%' to %2$o") % path % mode);
+                throw SysError("changing mode of '%1%' to %2$o", path, mode);
         }
 
     }
@@ -406,7 +406,7 @@ static void canonicaliseTimestampAndPermissions(const Path & path, const struct
 #else
         if (!S_ISLNK(st.st_mode) && utimes(path.c_str(), times) == -1)
 #endif
-            throw SysError(format("changing modification time of '%1%'") % path);
+            throw SysError("changing modification time of '%1%'", path);
     }
 }
 
@@ -415,7 +415,7 @@ void canonicaliseTimestampAndPermissions(const Path & path)
 {
     struct stat st;
     if (lstat(path.c_str(), &st))
-        throw SysError(format("getting attributes of path '%1%'") % path);
+        throw SysError("getting attributes of path '%1%'", path);
     canonicaliseTimestampAndPermissions(path, st);
 }
 
@@ -430,17 +430,17 @@ static void canonicalisePathMetaData_(const Path & path, uid_t fromUid, InodesSe
        setattrlist() to remove other attributes as well. */
     if (lchflags(path.c_str(), 0)) {
         if (errno != ENOTSUP)
-            throw SysError(format("clearing flags of path '%1%'") % path);
+            throw SysError("clearing flags of path '%1%'", path);
     }
 #endif
 
     struct stat st;
     if (lstat(path.c_str(), &st))
-        throw SysError(format("getting attributes of path '%1%'") % path);
+        throw SysError("getting attributes of path '%1%'", path);
 
     /* Really make sure that the path is of a supported type. */
     if (!(S_ISREG(st.st_mode) || S_ISDIR(st.st_mode) || S_ISLNK(st.st_mode)))
-        throw Error(format("file '%1%' has an unsupported type") % path);
+        throw Error("file '%1%' has an unsupported type", path);
 
 #if __linux__
     /* Remove extended attributes / ACLs. */
@@ -474,7 +474,7 @@ static void canonicalisePathMetaData_(const Path & path, uid_t fromUid, InodesSe
     if (fromUid != (uid_t) -1 && st.st_uid != fromUid) {
         assert(!S_ISDIR(st.st_mode));
         if (inodesSeen.find(Inode(st.st_dev, st.st_ino)) == inodesSeen.end())
-            throw BuildError(format("invalid ownership on file '%1%'") % path);
+            throw BuildError("invalid ownership on file '%1%'", path);
         mode_t mode = st.st_mode & ~S_IFMT;
         assert(S_ISLNK(st.st_mode) || (st.st_uid == geteuid() && (mode == 0444 || mode == 0555) && st.st_mtime == mtimeStore));
         return;
@@ -498,8 +498,8 @@ static void canonicalisePathMetaData_(const Path & path, uid_t fromUid, InodesSe
         if (!S_ISLNK(st.st_mode) &&
             chown(path.c_str(), geteuid(), getegid()) == -1)
 #endif
-            throw SysError(format("changing owner of '%1%' to %2%")
-                % path % geteuid());
+            throw SysError("changing owner of '%1%' to %2%",
+                  path, geteuid());
     }
 
     if (S_ISDIR(st.st_mode)) {
@@ -518,11 +518,11 @@ void canonicalisePathMetaData(const Path & path, uid_t fromUid, InodesSeen & ino
        be a symlink, since we can't change its ownership. */
     struct stat st;
     if (lstat(path.c_str(), &st))
-        throw SysError(format("getting attributes of path '%1%'") % path);
+        throw SysError("getting attributes of path '%1%'", path);
 
     if (st.st_uid != geteuid()) {
         assert(S_ISLNK(st.st_mode));
-        throw Error(format("wrong ownership of top-level store path '%1%'") % path);
+        throw Error("wrong ownership of top-level store path '%1%'", path);
     }
 }
 
@@ -1392,7 +1392,7 @@ static void makeMutable(const Path & path)
     AutoCloseFD fd = open(path.c_str(), O_RDONLY | O_NOFOLLOW | O_CLOEXEC);
     if (fd == -1) {
         if (errno == ELOOP) return; // it's a symlink
-        throw SysError(format("opening file '%1%'") % path);
+        throw SysError("opening file '%1%'", path);
     }
 
     unsigned int flags = 0, old;
diff --git a/src/libstore/nar-accessor.cc b/src/libstore/nar-accessor.cc
index b74480684..ca663d837 100644
--- a/src/libstore/nar-accessor.cc
+++ b/src/libstore/nar-accessor.cc
@@ -184,7 +184,7 @@ struct NarAccessor : public FSAccessor
         auto i = get(path);
 
         if (i.type != FSAccessor::Type::tDirectory)
-            throw Error(format("path '%1%' inside NAR file is not a directory") % path);
+            throw Error("path '%1%' inside NAR file is not a directory", path);
 
         StringSet res;
         for (auto & child : i.children)
@@ -197,7 +197,7 @@ struct NarAccessor : public FSAccessor
     {
         auto i = get(path);
         if (i.type != FSAccessor::Type::tRegular)
-            throw Error(format("path '%1%' inside NAR file is not a regular file") % path);
+            throw Error("path '%1%' inside NAR file is not a regular file", path);
 
         if (getNarBytes) return getNarBytes(i.start, i.size);
 
@@ -209,7 +209,7 @@ struct NarAccessor : public FSAccessor
     {
         auto i = get(path);
         if (i.type != FSAccessor::Type::tSymlink)
-            throw Error(format("path '%1%' inside NAR file is not a symlink") % path);
+            throw Error("path '%1%' inside NAR file is not a symlink", path);
         return i.target;
     }
 };
diff --git a/src/libstore/nar-info.cc b/src/libstore/nar-info.cc
index 1375094b5..fbdcd7786 100644
--- a/src/libstore/nar-info.cc
+++ b/src/libstore/nar-info.cc
@@ -7,7 +7,7 @@ NarInfo::NarInfo(const Store & store, const std::string & s, const std::string &
     : ValidPathInfo(StorePath::dummy.clone()) // FIXME: hack
 {
     auto corrupt = [&]() {
-        throw Error(format("NAR info file '%1%' is corrupt") % whence);
+        throw Error("NAR info file '%1%' is corrupt", whence);
     };
 
     auto parseHashField = [&](const string & s) {
diff --git a/src/libstore/optimise-store.cc b/src/libstore/optimise-store.cc
index 8ac382e9d..f9cf2bb6b 100644
--- a/src/libstore/optimise-store.cc
+++ b/src/libstore/optimise-store.cc
@@ -19,9 +19,9 @@ static void makeWritable(const Path & path)
 {
     struct stat st;
     if (lstat(path.c_str(), &st))
-        throw SysError(format("getting attributes of path '%1%'") % path);
+        throw SysError("getting attributes of path '%1%'", path);
     if (chmod(path.c_str(), st.st_mode | S_IWUSR) == -1)
-        throw SysError(format("changing writability of '%1%'") % path);
+        throw SysError("changing writability of '%1%'", path);
 }
 
 
@@ -47,7 +47,7 @@ LocalStore::InodeHash LocalStore::loadInodeHash()
     InodeHash inodeHash;
 
     AutoCloseDir dir(opendir(linksDir.c_str()));
-    if (!dir) throw SysError(format("opening directory '%1%'") % linksDir);
+    if (!dir) throw SysError("opening directory '%1%'", linksDir);
 
     struct dirent * dirent;
     while (errno = 0, dirent = readdir(dir.get())) { /* sic */
@@ -55,7 +55,7 @@ LocalStore::InodeHash LocalStore::loadInodeHash()
         // We don't care if we hit non-hash files, anything goes
         inodeHash.insert(dirent->d_ino);
     }
-    if (errno) throw SysError(format("reading directory '%1%'") % linksDir);
+    if (errno) throw SysError("reading directory '%1%'", linksDir);
 
     printMsg(lvlTalkative, format("loaded %1% hash inodes") % inodeHash.size());
 
@@ -68,7 +68,7 @@ Strings LocalStore::readDirectoryIgnoringInodes(const Path & path, const InodeHa
     Strings names;
 
     AutoCloseDir dir(opendir(path.c_str()));
-    if (!dir) throw SysError(format("opening directory '%1%'") % path);
+    if (!dir) throw SysError("opening directory '%1%'", path);
 
     struct dirent * dirent;
     while (errno = 0, dirent = readdir(dir.get())) { /* sic */
@@ -83,7 +83,7 @@ Strings LocalStore::readDirectoryIgnoringInodes(const Path & path, const InodeHa
         if (name == "." || name == "..") continue;
         names.push_back(name);
     }
-    if (errno) throw SysError(format("reading directory '%1%'") % path);
+    if (errno) throw SysError("reading directory '%1%'", path);
 
     return names;
 }
@@ -96,7 +96,7 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
 
     struct stat st;
     if (lstat(path.c_str(), &st))
-        throw SysError(format("getting attributes of path '%1%'") % path);
+        throw SysError("getting attributes of path '%1%'", path);
 
 #if __APPLE__
     /* HFS/macOS has some undocumented security feature disabling hardlinking for
@@ -130,7 +130,7 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
        NixOS (example: $fontconfig/var/cache being modified).  Skip
        those files.  FIXME: check the modification time. */
     if (S_ISREG(st.st_mode) && (st.st_mode & S_IWUSR)) {
-        printError(format("skipping suspicious writable file '%1%'") % path);
+        printError("skipping suspicious writable file '%1%'", path);
         return;
     }
 
@@ -186,7 +186,7 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
        current file with a hard link to that file. */
     struct stat stLink;
     if (lstat(linkPath.c_str(), &stLink))
-        throw SysError(format("getting attributes of path '%1%'") % linkPath);
+        throw SysError("getting attributes of path '%1%'", linkPath);
 
     if (st.st_ino == stLink.st_ino) {
         debug(format("'%1%' is already linked to '%2%'") % path % linkPath);
@@ -194,7 +194,7 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
     }
 
     if (st.st_size != stLink.st_size) {
-        printError(format("removing corrupted link '%1%'") % linkPath);
+        printError("removing corrupted link '%1%'", linkPath);
         unlink(linkPath.c_str());
         goto retry;
     }
@@ -229,7 +229,7 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
     /* Atomically replace the old file with the new hard link. */
     if (rename(tempLink.c_str(), path.c_str()) == -1) {
         if (unlink(tempLink.c_str()) == -1)
-            printError(format("unable to unlink '%1%'") % tempLink);
+            printError("unable to unlink '%1%'", tempLink);
         if (errno == EMLINK) {
             /* Some filesystems generate too many links on the rename,
                rather than on the original link.  (Probably it
@@ -238,7 +238,7 @@ void LocalStore::optimisePath_(Activity * act, OptimiseStats & stats,
             debug("'%s' has reached maximum number of links", linkPath);
             return;
         }
-        throw SysError(format("cannot rename '%1%' to '%2%'") % tempLink % path);
+        throw SysError("cannot rename '%1%' to '%2%'", tempLink, path);
     }
 
     stats.filesLinked++;
diff --git a/src/libstore/pathlocks.cc b/src/libstore/pathlocks.cc
index 2635e3940..52d430ffd 100644
--- a/src/libstore/pathlocks.cc
+++ b/src/libstore/pathlocks.cc
@@ -20,7 +20,7 @@ AutoCloseFD openLockFile(const Path & path, bool create)
 
     fd = open(path.c_str(), O_CLOEXEC | O_RDWR | (create ? O_CREAT : 0), 0600);
     if (!fd && (create || errno != ENOENT))
-        throw SysError(format("opening lock file '%1%'") % path);
+        throw SysError("opening lock file '%1%'", path);
 
     return fd;
 }
@@ -51,7 +51,7 @@ bool lockFile(int fd, LockType lockType, bool wait)
         while (flock(fd, type) != 0) {
             checkInterrupt();
             if (errno != EINTR)
-                throw SysError(format("acquiring/releasing lock"));
+                throw SysError("acquiring/releasing lock");
             else
                 return false;
         }
@@ -60,7 +60,7 @@ bool lockFile(int fd, LockType lockType, bool wait)
             checkInterrupt();
             if (errno == EWOULDBLOCK) return false;
             if (errno != EINTR)
-                throw SysError(format("acquiring/releasing lock"));
+                throw SysError("acquiring/releasing lock");
         }
     }
 
@@ -124,7 +124,7 @@ bool PathLocks::lockPaths(const PathSet & paths,
                hasn't been unlinked). */
             struct stat st;
             if (fstat(fd.get(), &st) == -1)
-                throw SysError(format("statting lock file '%1%'") % lockPath);
+                throw SysError("statting lock file '%1%'", lockPath);
             if (st.st_size != 0)
                 /* This lock file has been unlinked, so we're holding
                    a lock on a deleted file.  This means that other
diff --git a/src/libstore/profiles.cc b/src/libstore/profiles.cc
index 2bef51878..6cfe393a4 100644
--- a/src/libstore/profiles.cc
+++ b/src/libstore/profiles.cc
@@ -50,7 +50,7 @@ Generations findGenerations(Path profile, int & curGen)
             gen.number = n;
             struct stat st;
             if (lstat(gen.path.c_str(), &st) != 0)
-                throw SysError(format("statting '%1%'") % gen.path);
+                throw SysError("statting '%1%'", gen.path);
             gen.creationTime = st.st_mtime;
             gens.push_back(gen);
         }
@@ -117,7 +117,7 @@ Path createGeneration(ref<LocalFSStore> store, Path profile, Path outPath)
 static void removeFile(const Path & path)
 {
     if (remove(path.c_str()) == -1)
-        throw SysError(format("cannot unlink '%1%'") % path);
+        throw SysError("cannot unlink '%1%'", path);
 }
 
 
@@ -149,7 +149,7 @@ void deleteGenerations(const Path & profile, const std::set<unsigned int> & gens
     Generations gens = findGenerations(profile, curGen);
 
     if (gensToDelete.find(curGen) != gensToDelete.end())
-        throw Error(format("cannot delete current generation of profile %1%'") % profile);
+        throw Error("cannot delete current generation of profile %1%'", profile);
 
     for (auto & i : gens) {
         if (gensToDelete.find(i.number) == gensToDelete.end()) continue;
@@ -226,7 +226,7 @@ void deleteGenerationsOlderThan(const Path & profile, const string & timeSpec, b
     int days;
 
     if (!string2Int(strDays, days) || days < 1)
-        throw Error(format("invalid number of days specifier '%1%'") % timeSpec);
+        throw Error("invalid number of days specifier '%1%'", timeSpec);
 
     time_t oldTime = curTime - days * 24 * 3600;
 
diff --git a/src/libstore/references.cc b/src/libstore/references.cc
index 102e15921..a10d536a3 100644
--- a/src/libstore/references.cc
+++ b/src/libstore/references.cc
@@ -92,7 +92,7 @@ PathSet scanForReferences(const string & path,
         auto baseName = std::string(baseNameOf(i));
         string::size_type pos = baseName.find('-');
         if (pos == string::npos)
-            throw Error(format("bad reference '%1%'") % i);
+            throw Error("bad reference '%1%'", i);
         string s = string(baseName, 0, pos);
         assert(s.size() == refLength);
         assert(backMap.find(s) == backMap.end());
diff --git a/src/libstore/remote-fs-accessor.cc b/src/libstore/remote-fs-accessor.cc
index 5a2d103b9..9277a8e6b 100644
--- a/src/libstore/remote-fs-accessor.cc
+++ b/src/libstore/remote-fs-accessor.cc
@@ -51,7 +51,7 @@ std::pair<ref<FSAccessor>, Path> RemoteFSAccessor::fetch(const Path & path_)
     std::string restPath = std::string(path, storePath.size());
 
     if (!store->isValidPath(store->parseStorePath(storePath)))
-        throw InvalidPath(format("path '%1%' is not a valid store path") % storePath);
+        throw InvalidPath("path '%1%' is not a valid store path", storePath);
 
     auto i = nars.find(storePath);
     if (i != nars.end()) return {i->second, restPath};
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 8c55da268..0f8126aee 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -116,11 +116,11 @@ ref<RemoteStore::Connection> UDSRemoteStore::openConnection()
     struct sockaddr_un addr;
     addr.sun_family = AF_UNIX;
     if (socketPath.size() + 1 >= sizeof(addr.sun_path))
-        throw Error(format("socket path '%1%' is too long") % socketPath);
+        throw Error("socket path '%1%' is too long", socketPath);
     strcpy(addr.sun_path, socketPath.c_str());
 
     if (::connect(conn->fd.get(), (struct sockaddr *) &addr, sizeof(addr)) == -1)
-        throw SysError(format("cannot connect to daemon at '%1%'") % socketPath);
+        throw SysError("cannot connect to daemon at '%1%'", socketPath);
 
     conn->from.fd = conn->fd.get();
     conn->to.fd = conn->fd.get();
diff --git a/src/libstore/s3-binary-cache-store.cc b/src/libstore/s3-binary-cache-store.cc
index f2e4b63e0..0326821f6 100644
--- a/src/libstore/s3-binary-cache-store.cc
+++ b/src/libstore/s3-binary-cache-store.cc
@@ -32,8 +32,10 @@ namespace nix {
 struct S3Error : public Error
 {
     Aws::S3::S3Errors err;
-    S3Error(Aws::S3::S3Errors err, const FormatOrString & fs)
-        : Error(fs), err(err) { };
+
+    template<typename... Args>
+    S3Error(Aws::S3::S3Errors err, const Args & ... args)
+        : Error(args...), err(err) { };
 };
 
 /* Helper: given an Outcome<R, E>, return R in case of success, or
@@ -249,7 +251,7 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
                 // If bucket listing is disabled, 404s turn into 403s
                 || error.GetErrorType() == Aws::S3::S3Errors::ACCESS_DENIED)
                 return false;
-            throw Error(format("AWS error fetching '%s': %s") % path % error.GetMessage());
+            throw Error("AWS error fetching '%s': %s", path, error.GetMessage());
         }
 
         return true;
diff --git a/src/libstore/sqlite.cc b/src/libstore/sqlite.cc
index eb1daafc5..3407e5826 100644
--- a/src/libstore/sqlite.cc
+++ b/src/libstore/sqlite.cc
@@ -29,7 +29,7 @@ SQLite::SQLite(const Path & path, bool create)
 {
     if (sqlite3_open_v2(path.c_str(), &db,
             SQLITE_OPEN_READWRITE | (create ? SQLITE_OPEN_CREATE : 0), 0) != SQLITE_OK)
-        throw Error(format("cannot open SQLite database '%s'") % path);
+        throw Error("cannot open SQLite database '%s'", path);
 
     if (sqlite3_busy_timeout(db, 60 * 60 * 1000) != SQLITE_OK)
         throwSQLiteError(db, "setting timeout");
diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index b9e894a9a..a04d5013e 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -22,7 +22,7 @@ bool Store::isInStore(const Path & path) const
 Path Store::toStorePath(const Path & path) const
 {
     if (!isInStore(path))
-        throw Error(format("path '%1%' is not in the Nix store") % path);
+        throw Error("path '%1%' is not in the Nix store", path);
     Path::size_type slash = path.find('/', storeDir.size() + 1);
     if (slash == Path::npos)
         return path;
@@ -40,7 +40,7 @@ Path Store::followLinksToStore(std::string_view _path) const
         path = absPath(target, dirOf(path));
     }
     if (!isInStore(path))
-        throw Error(format("path '%1%' is not in the Nix store") % path);
+        throw Error("path '%1%' is not in the Nix store", path);
     return path;
 }
 
diff --git a/src/libutil/archive.cc b/src/libutil/archive.cc
index db544a212..6a8484705 100644
--- a/src/libutil/archive.cc
+++ b/src/libutil/archive.cc
@@ -46,7 +46,7 @@ static void dumpContents(const Path & path, size_t size,
     sink << "contents" << size;
 
     AutoCloseFD fd = open(path.c_str(), O_RDONLY | O_CLOEXEC);
-    if (!fd) throw SysError(format("opening file '%1%'") % path);
+    if (!fd) throw SysError("opening file '%1%'", path);
 
     std::vector<unsigned char> buf(65536);
     size_t left = size;
@@ -68,7 +68,7 @@ static void dump(const Path & path, Sink & sink, PathFilter & filter)
 
     struct stat st;
     if (lstat(path.c_str(), &st))
-        throw SysError(format("getting attributes of path '%1%'") % path);
+        throw SysError("getting attributes of path '%1%'", path);
 
     sink << "(";
 
@@ -94,8 +94,9 @@ static void dump(const Path & path, Sink & sink, PathFilter & filter)
                     name.erase(pos);
                 }
                 if (unhacked.find(name) != unhacked.end())
-                    throw Error(format("file name collision in between '%1%' and '%2%'")
-                        % (path + "/" + unhacked[name]) % (path + "/" + i.name));
+                    throw Error("file name collision in between '%1%' and '%2%'",
+                       (path + "/" + unhacked[name]),
+                       (path + "/" + i.name));
                 unhacked[name] = i.name;
             } else
                 unhacked[i.name] = i.name;
@@ -111,7 +112,7 @@ static void dump(const Path & path, Sink & sink, PathFilter & filter)
     else if (S_ISLNK(st.st_mode))
         sink << "type" << "symlink" << "target" << readLink(path);
 
-    else throw Error(format("file '%1%' has an unsupported type") % path);
+    else throw Error("file '%1%' has an unsupported type", path);
 
     sink << ")";
 }
@@ -247,7 +248,7 @@ static void parse(ParseSink & sink, Source & source, const Path & path)
                 } else if (s == "name") {
                     name = readString(source);
                     if (name.empty() || name == "." || name == ".." || name.find('/') != string::npos || name.find((char) 0) != string::npos)
-                        throw Error(format("NAR contains invalid file name '%1%'") % name);
+                        throw Error("NAR contains invalid file name '%1%'", name);
                     if (name <= prevName)
                         throw Error("NAR directory is not sorted");
                     prevName = name;
@@ -303,14 +304,14 @@ struct RestoreSink : ParseSink
     {
         Path p = dstPath + path;
         if (mkdir(p.c_str(), 0777) == -1)
-            throw SysError(format("creating directory '%1%'") % p);
+            throw SysError("creating directory '%1%'", p);
     };
 
     void createRegularFile(const Path & path)
     {
         Path p = dstPath + path;
         fd = open(p.c_str(), O_CREAT | O_EXCL | O_WRONLY | O_CLOEXEC, 0666);
-        if (!fd) throw SysError(format("creating file '%1%'") % p);
+        if (!fd) throw SysError("creating file '%1%'", p);
     }
 
     void isExecutable()
@@ -332,7 +333,7 @@ struct RestoreSink : ParseSink
                OpenSolaris).  Since preallocation is just an
                optimisation, ignore it. */
             if (errno && errno != EINVAL && errno != EOPNOTSUPP && errno != ENOSYS)
-                throw SysError(format("preallocating file of %1% bytes") % len);
+                throw SysError("preallocating file of %1% bytes", len);
         }
 #endif
     }
diff --git a/src/libutil/args.cc b/src/libutil/args.cc
index ba15ea571..4e705dd64 100644
--- a/src/libutil/args.cc
+++ b/src/libutil/args.cc
@@ -47,7 +47,7 @@ void Args::parseCmdline(const Strings & _cmdline)
         }
         else if (!dashDash && std::string(arg, 0, 1) == "-") {
             if (!processFlag(pos, cmdline.end()))
-                throw UsageError(format("unrecognised flag '%1%'") % arg);
+                throw UsageError("unrecognised flag '%1%'", arg);
         }
         else {
             pendingArgs.push_back(*pos++);
@@ -104,8 +104,9 @@ bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
         for (size_t n = 0 ; n < flag.arity; ++n) {
             if (pos == end) {
                 if (flag.arity == ArityAny) break;
-                throw UsageError(format("flag '%1%' requires %2% argument(s)")
-                    % name % flag.arity);
+                throw UsageError("flag '%1%' requires %2% argument(s)",
+                    name,
+                    flag.arity);
             }
             args.push_back(*pos++);
         }
@@ -133,7 +134,7 @@ bool Args::processArgs(const Strings & args, bool finish)
 {
     if (expectedArgs.empty()) {
         if (!args.empty())
-            throw UsageError(format("unexpected argument '%1%'") % args.front());
+            throw UsageError("unexpected argument '%1%'", args.front());
         return true;
     }
 
diff --git a/src/libutil/compression.cc b/src/libutil/compression.cc
index 860b04adb..a117ddc72 100644
--- a/src/libutil/compression.cc
+++ b/src/libutil/compression.cc
@@ -481,7 +481,7 @@ ref<CompressionSink> makeCompressionSink(const std::string & method, Sink & next
     else if (method == "br")
         return make_ref<BrotliCompressionSink>(nextSink);
     else
-        throw UnknownCompressionMethod(format("unknown compression method '%s'") % method);
+        throw UnknownCompressionMethod("unknown compression method '%s'", method);
 }
 
 ref<std::string> compress(const std::string & method, const std::string & in, const bool parallel)
diff --git a/src/libutil/fmt.hh b/src/libutil/fmt.hh
index 08edffee0..d65215063 100644
--- a/src/libutil/fmt.hh
+++ b/src/libutil/fmt.hh
@@ -92,15 +92,15 @@ public:
     {
         fmt.exceptions(boost::io::all_error_bits ^ boost::io::too_many_args_bit);
     }
+    hintformat(const hintformat &hf)
+    : fmt(hf.fmt)
+    {}
     template<class T>
     hintformat& operator%(const T &value)
     {
         fmt % yellowify(value);
         return *this;
     }
-    hintformat(const hintformat &hf)
-    : fmt(hf.fmt)
-    {}
 
 
     std::string str() const
diff --git a/src/libutil/types.hh b/src/libutil/types.hh
index 633c6bdf7..2261adb82 100644
--- a/src/libutil/types.hh
+++ b/src/libutil/types.hh
@@ -93,7 +93,9 @@ public:
 
     template<typename... Args>
     BaseError(unsigned int status, const Args & ... args)
-        : err(hintfmt(args...))
+        : err { .level = lvlError,
+                .hint = hintfmt(args...)
+              }
         , status(status)
     {
     }
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 332c1c43a..bffcbcccf 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -129,7 +129,7 @@ Path canonPath(const Path & path, bool resolveSymlinks)
     string s;
 
     if (path[0] != '/')
-        throw Error(format("not an absolute path: '%1%'") % path);
+        throw Error("not an absolute path: '%1%'", path);
 
     string::const_iterator i = path.begin(), end = path.end();
     string temp;
@@ -165,7 +165,7 @@ Path canonPath(const Path & path, bool resolveSymlinks)
                the symlink target might contain new symlinks). */
             if (resolveSymlinks && isLink(s)) {
                 if (++followCount >= maxFollow)
-                    throw Error(format("infinite symlink recursion in path '%1%'") % path);
+                    throw Error("infinite symlink recursion in path '%1%'", path);
                 temp = absPath(readLink(s), dirOf(s))
                     + string(i, end);
                 i = temp.begin(); /* restart */
@@ -226,7 +226,7 @@ struct stat lstat(const Path & path)
 {
     struct stat st;
     if (lstat(path.c_str(), &st))
-        throw SysError(format("getting status of '%1%'") % path);
+        throw SysError("getting status of '%1%'", path);
     return st;
 }
 
@@ -238,7 +238,7 @@ bool pathExists(const Path & path)
     res = lstat(path.c_str(), &st);
     if (!res) return true;
     if (errno != ENOENT && errno != ENOTDIR)
-        throw SysError(format("getting status of %1%") % path);
+        throw SysError("getting status of %1%", path);
     return false;
 }
 
@@ -274,7 +274,7 @@ DirEntries readDirectory(const Path & path)
     entries.reserve(64);
 
     AutoCloseDir dir(opendir(path.c_str()));
-    if (!dir) throw SysError(format("opening directory '%1%'") % path);
+    if (!dir) throw SysError("opening directory '%1%'", path);
 
     struct dirent * dirent;
     while (errno = 0, dirent = readdir(dir.get())) { /* sic */
@@ -289,7 +289,7 @@ DirEntries readDirectory(const Path & path)
 #endif
         );
     }
-    if (errno) throw SysError(format("reading directory '%1%'") % path);
+    if (errno) throw SysError("reading directory '%1%'", path);
 
     return entries;
 }
@@ -322,7 +322,7 @@ string readFile(const Path & path, bool drain)
 {
     AutoCloseFD fd = open(path.c_str(), O_RDONLY | O_CLOEXEC);
     if (!fd)
-        throw SysError(format("opening file '%1%'") % path);
+        throw SysError("opening file '%1%'", path);
     return drain ? drainFD(fd.get()) : readFile(fd.get());
 }
 
@@ -339,7 +339,7 @@ void writeFile(const Path & path, const string & s, mode_t mode)
 {
     AutoCloseFD fd = open(path.c_str(), O_WRONLY | O_TRUNC | O_CREAT | O_CLOEXEC, mode);
     if (!fd)
-        throw SysError(format("opening file '%1%'") % path);
+        throw SysError("opening file '%1%'", path);
     writeFull(fd.get(), s);
 }
 
@@ -348,7 +348,7 @@ void writeFile(const Path & path, Source & source, mode_t mode)
 {
     AutoCloseFD fd = open(path.c_str(), O_WRONLY | O_TRUNC | O_CREAT | O_CLOEXEC, mode);
     if (!fd)
-        throw SysError(format("opening file '%1%'") % path);
+        throw SysError("opening file '%1%'", path);
 
     std::vector<unsigned char> buf(64 * 1024);
 
@@ -396,7 +396,7 @@ static void _deletePath(const Path & path, unsigned long long & bytesFreed)
     struct stat st;
     if (lstat(path.c_str(), &st) == -1) {
         if (errno == ENOENT) return;
-        throw SysError(format("getting status of '%1%'") % path);
+        throw SysError("getting status of '%1%'", path);
     }
 
     if (!S_ISDIR(st.st_mode) && st.st_nlink == 1)
@@ -407,7 +407,7 @@ static void _deletePath(const Path & path, unsigned long long & bytesFreed)
         const auto PERM_MASK = S_IRUSR | S_IWUSR | S_IXUSR;
         if ((st.st_mode & PERM_MASK) != PERM_MASK) {
             if (chmod(path.c_str(), st.st_mode | PERM_MASK) == -1)
-                throw SysError(format("chmod '%1%'") % path);
+                throw SysError("chmod '%1%'", path);
         }
 
         for (auto & i : readDirectory(path))
@@ -416,7 +416,7 @@ static void _deletePath(const Path & path, unsigned long long & bytesFreed)
 
     if (remove(path.c_str()) == -1) {
         if (errno == ENOENT) return;
-        throw SysError(format("cannot unlink '%1%'") % path);
+        throw SysError("cannot unlink '%1%'", path);
     }
 }
 
@@ -468,12 +468,12 @@ Path createTempDir(const Path & tmpRoot, const Path & prefix,
                "wheel", then "tar" will fail to unpack archives that
                have the setgid bit set on directories. */
             if (chown(tmpDir.c_str(), (uid_t) -1, getegid()) != 0)
-                throw SysError(format("setting group of directory '%1%'") % tmpDir);
+                throw SysError("setting group of directory '%1%'", tmpDir);
 #endif
             return tmpDir;
         }
         if (errno != EEXIST)
-            throw SysError(format("creating directory '%1%'") % tmpDir);
+            throw SysError("creating directory '%1%'", tmpDir);
     }
 }
 
@@ -555,15 +555,15 @@ Paths createDirs(const Path & path)
     if (lstat(path.c_str(), &st) == -1) {
         created = createDirs(dirOf(path));
         if (mkdir(path.c_str(), 0777) == -1 && errno != EEXIST)
-            throw SysError(format("creating directory '%1%'") % path);
+            throw SysError("creating directory '%1%'", path);
         st = lstat(path);
         created.push_back(path);
     }
 
     if (S_ISLNK(st.st_mode) && stat(path.c_str(), &st) == -1)
-        throw SysError(format("statting symlink '%1%'") % path);
+        throw SysError("statting symlink '%1%'", path);
 
-    if (!S_ISDIR(st.st_mode)) throw Error(format("'%1%' is not a directory") % path);
+    if (!S_ISDIR(st.st_mode)) throw Error("'%1%' is not a directory", path);
 
     return created;
 }
@@ -572,7 +572,7 @@ Paths createDirs(const Path & path)
 void createSymlink(const Path & target, const Path & link)
 {
     if (symlink(target.c_str(), link.c_str()))
-        throw SysError(format("creating symlink from '%1%' to '%2%'") % link % target);
+        throw SysError("creating symlink from '%1%' to '%2%'", link, target);
 }
 
 
@@ -589,7 +589,7 @@ void replaceSymlink(const Path & target, const Path & link)
         }
 
         if (rename(tmp.c_str(), link.c_str()) != 0)
-            throw SysError(format("renaming '%1%' to '%2%'") % tmp % link);
+            throw SysError("renaming '%1%' to '%2%'", tmp, link);
 
         break;
     }
@@ -694,7 +694,7 @@ AutoDelete::~AutoDelete()
                 deletePath(path);
             else {
                 if (remove(path.c_str()) == -1)
-                    throw SysError(format("cannot unlink '%1%'") % path);
+                    throw SysError("cannot unlink '%1%'", path);
             }
         }
     } catch (...) {
@@ -760,7 +760,7 @@ void AutoCloseFD::close()
     if (fd != -1) {
         if (::close(fd) == -1)
             /* This should never happen. */
-            throw SysError(format("closing file descriptor %1%") % fd);
+            throw SysError("closing file descriptor %1%", fd);
     }
 }
 
@@ -920,7 +920,7 @@ void killUser(uid_t uid)
 #endif
             if (errno == ESRCH) break; /* no more processes */
             if (errno != EINTR)
-                throw SysError(format("cannot kill processes for uid '%1%'") % uid);
+                throw SysError("cannot kill processes for uid '%1%'", uid);
         }
 
         _exit(0);
@@ -928,7 +928,7 @@ void killUser(uid_t uid)
 
     int status = pid.wait();
     if (status != 0)
-        throw Error(format("cannot kill processes for uid '%1%': %2%") % uid % statusToString(status));
+        throw Error("cannot kill processes for uid '%1%': %2%", uid, statusToString(status));
 
     /* !!! We should really do some check to make sure that there are
        no processes left running under `uid', but there is no portable
@@ -1322,7 +1322,7 @@ void ignoreException()
     try {
         throw;
     } catch (std::exception & e) {
-        printError(format("error (ignored): %1%") % e.what());
+        printError("error (ignored): %1%", e.what());
     }
 }
 
@@ -1440,7 +1440,7 @@ void callFailure(const std::function<void(std::exception_ptr exc)> & failure, st
     try {
         failure(exc);
     } catch (std::exception & e) {
-        printError(format("uncaught exception: %s") % e.what());
+        printError("uncaught exception: %s", e.what());
         abort();
     }
 }
diff --git a/src/nix-channel/nix-channel.cc b/src/nix-channel/nix-channel.cc
index a2639579d..c403e3780 100755
--- a/src/nix-channel/nix-channel.cc
+++ b/src/nix-channel/nix-channel.cc
@@ -37,7 +37,8 @@ static void writeChannels()
 {
     auto channelsFD = AutoCloseFD{open(channelsList.c_str(), O_WRONLY | O_CLOEXEC | O_CREAT | O_TRUNC, 0644)};
     if (!channelsFD)
-        throw SysError(format("opening '%1%' for writing") % channelsList);
+        throw Error("");
+        // throw SysError("opening '%1%' for writing", channelsList);
     for (const auto & channel : channels)
         writeFull(channelsFD.get(), channel.second + " " + channel.first + "\n");
 }
@@ -46,9 +47,11 @@ static void writeChannels()
 static void addChannel(const string & url, const string & name)
 {
     if (!regex_search(url, std::regex("^(file|http|https)://")))
-        throw Error(format("invalid channel URL '%1%'") % url);
+        throw Error("");
+        // throw Error("invalid channel URL '%1%'", url);
     if (!regex_search(name, std::regex("^[a-zA-Z0-9_][a-zA-Z0-9_\\.-]*$")))
-        throw Error(format("invalid channel identifier '%1%'") % name);
+        throw Error("");
+        // throw Error("invalid channel identifier '%1%'", name);
     readChannels();
     channels[name] = url;
     writeChannels();
@@ -140,9 +143,11 @@ static void update(const StringSet & channelNames)
         if (S_ISLNK(st.st_mode))
             // old-skool ~/.nix-defexpr
             if (unlink(nixDefExpr.c_str()) == -1)
-                throw SysError(format("unlinking %1%") % nixDefExpr);
+                throw Error("");
+        //         throw SysError("unlinking %1%", nixDefExpr);
     } else if (errno != ENOENT) {
-        throw SysError(format("getting status of %1%") % nixDefExpr);
+        throw Error("");
+        // throw SysError("getting status of %1%", nixDefExpr);
     }
     createDirs(nixDefExpr);
     auto channelLink = nixDefExpr + "/channels";
@@ -194,10 +199,12 @@ static int _main(int argc, char ** argv)
 
         switch (cmd) {
             case cNone:
-                throw UsageError("no command specified");
+                throw Error("");
+        //         throw UsageError("no command specified");
             case cAdd:
                 if (args.size() < 1 || args.size() > 2)
-                    throw UsageError("'--add' requires one or two arguments");
+                    throw Error("");
+        //             throw UsageError("'--add' requires one or two arguments");
                 {
                 auto url = args[0];
                 std::string name;
@@ -213,12 +220,14 @@ static int _main(int argc, char ** argv)
                 break;
             case cRemove:
                 if (args.size() != 1)
-                    throw UsageError("'--remove' requires one argument");
+                    throw Error("");
+        //             throw UsageError("'--remove' requires one argument");
                 removeChannel(args[0]);
                 break;
             case cList:
                 if (!args.empty())
-                    throw UsageError("'--list' expects no arguments");
+                    throw Error("");
+        //             throw UsageError("'--list' expects no arguments");
                 readChannels();
                 for (const auto & channel : channels)
                     std::cout << channel.first << ' ' << channel.second << '\n';
@@ -228,7 +237,8 @@ static int _main(int argc, char ** argv)
                 break;
             case cRollback:
                 if (args.size() > 1)
-                    throw UsageError("'--rollback' has at most one argument");
+                    throw Error("");
+        //             throw UsageError("'--rollback' has at most one argument");
                 Strings envArgs{"--profile", profile};
                 if (args.size() == 1) {
                     envArgs.push_back("--switch-generation");
diff --git a/src/nix-daemon/nix-daemon.cc b/src/nix-daemon/nix-daemon.cc
index e68d1b1be..5f78ab464 100644
--- a/src/nix-daemon/nix-daemon.cc
+++ b/src/nix-daemon/nix-daemon.cc
@@ -36,7 +36,7 @@ using namespace nix::daemon;
 #define SPLICE_F_MOVE 0
 static ssize_t splice(int fd_in, void *off_in, int fd_out, void *off_out, size_t len, unsigned int flags)
 {
-    /* We ignore most parameters, we just have them for conformance with the linux syscall */
+    // We ignore most parameters, we just have them for conformance with the linux syscall 
     std::vector<char> buf(8192);
     auto read_count = read(fd_in, buf.data(), buf.size());
     if (read_count == -1)
@@ -57,7 +57,7 @@ static void sigChldHandler(int sigNo)
 {
     // Ensure we don't modify errno of whatever we've interrupted
     auto saved_errno = errno;
-    /* Reap all dead children. */
+    //  Reap all dead children. 
     while (waitpid(-1, 0, WNOHANG) > 0) ;
     errno = saved_errno;
 }
@@ -106,7 +106,7 @@ struct PeerInfo
 };
 
 
-/* Get the identity of the caller, if possible. */
+//  Get the identity of the caller, if possible. 
 static PeerInfo getPeerInfo(int remote)
 {
     PeerInfo peer = { false, 0, false, 0, false, 0 };
@@ -154,13 +154,12 @@ static void daemonLoop(char * * argv)
     if (chdir("/") == -1)
         throw SysError("cannot change current directory");
 
-    /* Get rid of children automatically; don't let them become
-       zombies. */
+    //  Get rid of children automatically; don't let them become zombies. 
     setSigChldAction(true);
 
     AutoCloseFD fdSocket;
 
-    /* Handle socket-based activation by systemd. */
+    //  Handle socket-based activation by systemd. 
     auto listenFds = getEnv("LISTEN_FDS");
     if (listenFds) {
         if (getEnv("LISTEN_PID") != std::to_string(getpid()) || listenFds != "1")
@@ -169,17 +168,17 @@ static void daemonLoop(char * * argv)
         closeOnExec(fdSocket.get());
     }
 
-    /* Otherwise, create and bind to a Unix domain socket. */
+    //  Otherwise, create and bind to a Unix domain socket. 
     else {
         createDirs(dirOf(settings.nixDaemonSocketFile));
         fdSocket = createUnixDomainSocket(settings.nixDaemonSocketFile, 0666);
     }
 
-    /* Loop accepting connections. */
+    //  Loop accepting connections. 
     while (1) {
 
         try {
-            /* Accept a connection. */
+            //  Accept a connection. 
             struct sockaddr_un remoteAddr;
             socklen_t remoteAddrLen = sizeof(remoteAddr);
 
@@ -209,13 +208,13 @@ static void daemonLoop(char * * argv)
                 trusted = Trusted;
 
             if ((!trusted && !matchUser(user, group, allowedUsers)) || group == settings.buildUsersGroup)
-                throw Error(format("user '%1%' is not allowed to connect to the Nix daemon") % user);
+                throw Error("user '%1%' is not allowed to connect to the Nix daemon", user);
 
             printInfo(format((string) "accepted connection from pid %1%, user %2%" + (trusted ? " (trusted)" : ""))
                 % (peer.pidKnown ? std::to_string(peer.pid) : "<unknown>")
                 % (peer.uidKnown ? user : "<unknown>"));
 
-            /* Fork a child to handle the connection. */
+            //  Fork a child to handle the connection. 
             ProcessOptions options;
             options.errorPrefix = "unexpected Nix daemon error: ";
             options.dieWithParent = false;
@@ -224,20 +223,20 @@ static void daemonLoop(char * * argv)
             startProcess([&]() {
                 fdSocket = -1;
 
-                /* Background the daemon. */
+                //  Background the daemon. 
                 if (setsid() == -1)
-                    throw SysError(format("creating a new session"));
+                    throw SysError("creating a new session");
 
-                /* Restore normal handling of SIGCHLD. */
+                //  Restore normal handling of SIGCHLD. 
                 setSigChldAction(false);
 
-                /* For debugging, stuff the pid into argv[1]. */
+                //  For debugging, stuff the pid into argv[1]. 
                 if (peer.pidKnown && argv[1]) {
                     string processName = std::to_string(peer.pid);
                     strncpy(argv[1], processName.c_str(), strlen(argv[1]));
                 }
 
-                /* Handle the connection. */
+                //  Handle the connection. 
                 FdSource from(remote.get());
                 FdSink to(remote.get());
                 processConnection(openUncachedStore(), from, to, trusted, NotRecursive, user, peer.uid);
@@ -248,7 +247,7 @@ static void daemonLoop(char * * argv)
         } catch (Interrupted & e) {
             return;
         } catch (Error & e) {
-            printError(format("error processing connection: %1%") % e.msg());
+            printError("error processing connection: %1%", e.msg());
         }
     }
 }
@@ -261,7 +260,7 @@ static int _main(int argc, char * * argv)
 
         parseCmdLine(argc, argv, [&](Strings::iterator & arg, const Strings::iterator & end) {
             if (*arg == "--daemon")
-                ; /* ignored for backwards compatibility */
+                ; //  ignored for backwards compatibility 
             else if (*arg == "--help")
                 showManPage("nix-daemon");
             else if (*arg == "--version")
@@ -276,7 +275,7 @@ static int _main(int argc, char * * argv)
 
         if (stdio) {
             if (getStoreType() == tDaemon) {
-                /* Forward on this connection to the real daemon */
+                //  Forward on this connection to the real daemon 
                 auto socketPath = settings.nixDaemonSocketFile;
                 auto s = socket(PF_UNIX, SOCK_STREAM, 0);
                 if (s == -1)
@@ -284,17 +283,17 @@ static int _main(int argc, char * * argv)
 
                 auto socketDir = dirOf(socketPath);
                 if (chdir(socketDir.c_str()) == -1)
-                    throw SysError(format("changing to socket directory '%1%'") % socketDir);
+                    throw SysError("changing to socket directory '%1%'", socketDir);
 
                 auto socketName = std::string(baseNameOf(socketPath));
                 auto addr = sockaddr_un{};
                 addr.sun_family = AF_UNIX;
                 if (socketName.size() + 1 >= sizeof(addr.sun_path))
-                    throw Error(format("socket name %1% is too long") % socketName);
+                    throw Error("socket name %1% is too long", socketName);
                 strcpy(addr.sun_path, socketName.c_str());
 
                 if (connect(s, (struct sockaddr *) &addr, sizeof(addr)) == -1)
-                    throw SysError(format("cannot connect to daemon at %1%") % socketPath);
+                    throw SysError("cannot connect to daemon at %1%", socketPath);
 
                 auto nfds = (s > STDIN_FILENO ? s : STDIN_FILENO) + 1;
                 while (true) {
diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index 1a2bb42a3..dde8875f1 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -25,7 +25,6 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
-
 using namespace nix;
 using std::cout;
 
@@ -70,8 +69,7 @@ typedef void (* Operation) (Globals & globals,
 static string needArg(Strings::iterator & i,
     Strings & args, const string & arg)
 {
-    if (i == args.end()) throw UsageError(
-        format("'%1%' requires an argument") % arg);
+    if (i == args.end()) throw UsageError( "'%1%' requires an argument", arg);
     return *i++;
 }
 
@@ -133,7 +131,7 @@ static void getAllExprs(EvalState & state,
             Value & vArg(*state.allocValue());
             mkString(vArg, path2);
             if (v.attrs->size() == v.attrs->capacity())
-                throw Error(format("too many Nix expressions in directory '%1%'") % path);
+                throw Error("too many Nix expressions in directory '%1%'", path);
             mkApp(*state.allocAttr(v, state.symbols.create(attrName)), vFun, vArg);
         }
         else if (S_ISDIR(st.st_mode))
@@ -144,11 +142,12 @@ static void getAllExprs(EvalState & state,
 }
 
 
+
 static void loadSourceExpr(EvalState & state, const Path & path, Value & v)
 {
     struct stat st;
     if (stat(path.c_str(), &st) == -1)
-        throw SysError(format("getting information about '%1%'") % path);
+        throw SysError("getting inon about '%1%'", path);
 
     if (isNixExpr(path, st))
         state.evalFile(path, v);
@@ -221,7 +220,7 @@ static void checkSelectorUse(DrvNames & selectors)
     /* Check that all selectors have been used. */
     for (auto & i : selectors)
         if (i.hits == 0 && i.fullName != "*")
-            throw Error(format("selector '%1%' matches no derivations") % i.fullName);
+            throw Error("selector '%1%' matches no derivations", i.fullName);
 }
 
 
@@ -507,7 +506,7 @@ static void opInstall(Globals & globals, Strings opFlags, Strings opArgs)
             globals.preserveInstalled = true;
         else if (arg == "--remove-all" || arg == "-r")
             globals.removeAll = true;
-        else throw UsageError(format("unknown flag '%1%'") % arg);
+        else throw UsageError("unknown flag '%1%'", arg);
     }
 
     installDerivations(globals, opArgs, globals.profile);
@@ -618,7 +617,7 @@ static void opUpgrade(Globals & globals, Strings opFlags, Strings opArgs)
         else if (arg == "--leq") upgradeType = utLeq;
         else if (arg == "--eq") upgradeType = utEq;
         else if (arg == "--always") upgradeType = utAlways;
-        else throw UsageError(format("unknown flag '%1%'") % arg);
+        else throw UsageError("unknown flag '%1%'", arg);
     }
 
     upgradeDerivations(globals, opArgs, upgradeType);
@@ -637,7 +636,7 @@ static void setMetaFlag(EvalState & state, DrvInfo & drv,
 static void opSetFlag(Globals & globals, Strings opFlags, Strings opArgs)
 {
     if (opFlags.size() > 0)
-        throw UsageError(format("unknown flag '%1%'") % opFlags.front());
+        throw UsageError("unknown flag '%1%'", opFlags.front());
     if (opArgs.size() < 2)
         throw UsageError("not enough arguments to '--set-flag'");
 
@@ -680,7 +679,7 @@ static void opSet(Globals & globals, Strings opFlags, Strings opArgs)
     for (Strings::iterator i = opFlags.begin(); i != opFlags.end(); ) {
         string arg = *i++;
         if (parseInstallSourceOptions(globals, i, opFlags, arg)) ;
-        else throw UsageError(format("unknown flag '%1%'") % arg);
+        else throw UsageError("unknown flag '%1%'", arg);
     }
 
     DrvInfos elems;
@@ -748,7 +747,7 @@ static void uninstallDerivations(Globals & globals, Strings & selectors,
 static void opUninstall(Globals & globals, Strings opFlags, Strings opArgs)
 {
     if (opFlags.size() > 0)
-        throw UsageError(format("unknown flag '%1%'") % opFlags.front());
+        throw UsageError("unknown flag '%1%'", opFlags.front());
     uninstallDerivations(globals, opArgs, globals.profile);
 }
 
@@ -911,7 +910,7 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
         else if (arg == "--attr" || arg == "-A")
             attrPath = needArg(i, opFlags, arg);
         else
-            throw UsageError(format("unknown flag '%1%'") % arg);
+            throw UsageError("unknown flag '%1%'", arg);
     }
 
     if (printAttrPath && source != sAvailable)
@@ -1177,9 +1176,9 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
 static void opSwitchProfile(Globals & globals, Strings opFlags, Strings opArgs)
 {
     if (opFlags.size() > 0)
-        throw UsageError(format("unknown flag '%1%'") % opFlags.front());
+        throw UsageError("unknown flag '%1%'", opFlags.front());
     if (opArgs.size() != 1)
-        throw UsageError(format("exactly one argument expected"));
+        throw UsageError("exactly one argument expected");
 
     Path profile = absPath(opArgs.front());
     Path profileLink = getHome() + "/.nix-profile";
@@ -1207,10 +1206,10 @@ static void switchGeneration(Globals & globals, int dstGen)
 
     if (!dst) {
         if (dstGen == prevGen)
-            throw Error(format("no generation older than the current (%1%) exists")
-                % curGen);
+            throw Error("no generation older than the current (%1%) exists",
+                        curGen);
         else
-            throw Error(format("generation %1% does not exist") % dstGen);
+            throw Error("generation %1% does not exist", dstGen);
     }
 
     printInfo(format("switching from generation %1% to %2%")
@@ -1225,13 +1224,13 @@ static void switchGeneration(Globals & globals, int dstGen)
 static void opSwitchGeneration(Globals & globals, Strings opFlags, Strings opArgs)
 {
     if (opFlags.size() > 0)
-        throw UsageError(format("unknown flag '%1%'") % opFlags.front());
+        throw UsageError("unknown flag '%1%'", opFlags.front());
     if (opArgs.size() != 1)
-        throw UsageError(format("exactly one argument expected"));
+        throw UsageError("exactly one argument expected");
 
     int dstGen;
     if (!string2Int(opArgs.front(), dstGen))
-        throw UsageError(format("expected a generation number"));
+        throw UsageError("expected a generation number");
 
     switchGeneration(globals, dstGen);
 }
@@ -1240,9 +1239,9 @@ static void opSwitchGeneration(Globals & globals, Strings opFlags, Strings opArg
 static void opRollback(Globals & globals, Strings opFlags, Strings opArgs)
 {
     if (opFlags.size() > 0)
-        throw UsageError(format("unknown flag '%1%'") % opFlags.front());
+        throw UsageError("unknown flag '%1%'", opFlags.front());
     if (opArgs.size() != 0)
-        throw UsageError(format("no arguments expected"));
+        throw UsageError("no arguments expected");
 
     switchGeneration(globals, prevGen);
 }
@@ -1251,9 +1250,9 @@ static void opRollback(Globals & globals, Strings opFlags, Strings opArgs)
 static void opListGenerations(Globals & globals, Strings opFlags, Strings opArgs)
 {
     if (opFlags.size() > 0)
-        throw UsageError(format("unknown flag '%1%'") % opFlags.front());
+        throw UsageError("unknown flag '%1%'", opFlags.front());
     if (opArgs.size() != 0)
-        throw UsageError(format("no arguments expected"));
+        throw UsageError("no arguments expected");
 
     PathLocks lock;
     lockProfile(lock, globals.profile);
@@ -1278,7 +1277,7 @@ static void opListGenerations(Globals & globals, Strings opFlags, Strings opArgs
 static void opDeleteGenerations(Globals & globals, Strings opFlags, Strings opArgs)
 {
     if (opFlags.size() > 0)
-        throw UsageError(format("unknown flag '%1%'") % opFlags.front());
+        throw UsageError("unknown flag '%1%'", opFlags.front());
 
     if (opArgs.size() == 1 && opArgs.front() == "old") {
         deleteOldGenerations(globals.profile, globals.dryRun);
@@ -1286,18 +1285,18 @@ static void opDeleteGenerations(Globals & globals, Strings opFlags, Strings opAr
         deleteGenerationsOlderThan(globals.profile, opArgs.front(), globals.dryRun);
     } else if (opArgs.size() == 1 && opArgs.front().find('+') != string::npos) {
         if(opArgs.front().size() < 2)
-            throw Error(format("invalid number of generations ‘%1%’") % opArgs.front());
+            throw Error("invalid number of generations ‘%1%’", opArgs.front());
         string str_max = string(opArgs.front(), 1, opArgs.front().size());
         int max;
         if (!string2Int(str_max, max) || max == 0)
-            throw Error(format("invalid number of generations to keep ‘%1%’") % opArgs.front());
+            throw Error("invalid number of generations to keep ‘%1%’", opArgs.front());
         deleteGenerationsGreaterThan(globals.profile, max, globals.dryRun);
     } else {
         std::set<unsigned int> gens;
         for (auto & i : opArgs) {
             unsigned int n;
             if (!string2Int(i, n))
-                throw UsageError(format("invalid generation number '%1%'") % i);
+                throw UsageError("invalid generation number '%1%'", i);
             gens.insert(n);
         }
         deleteGenerations(globals.profile, gens, globals.dryRun);
diff --git a/src/nix-instantiate/nix-instantiate.cc b/src/nix-instantiate/nix-instantiate.cc
index 6c99d1181..bf353677a 100644
--- a/src/nix-instantiate/nix-instantiate.cc
+++ b/src/nix-instantiate/nix-instantiate.cc
@@ -66,7 +66,7 @@ void processExpr(EvalState & state, const Strings & attrPaths,
                 /* What output do we want? */
                 string outputName = i.queryOutputName();
                 if (outputName == "")
-                    throw Error(format("derivation '%1%' lacks an 'outputName' attribute ") % drvPath);
+                    throw Error("derivation '%1%' lacks an 'outputName' attribute ", drvPath);
 
                 if (gcRoot == "")
                     printGCWarning();
@@ -166,7 +166,7 @@ static int _main(int argc, char * * argv)
         if (findFile) {
             for (auto & i : files) {
                 Path p = state->findFile(i);
-                if (p == "") throw Error(format("unable to find '%1%'") % i);
+                if (p == "") throw Error("unable to find '%1%'", i);
                 std::cout << p << std::endl;
             }
             return 0;
diff --git a/src/nix-prefetch-url/nix-prefetch-url.cc b/src/nix-prefetch-url/nix-prefetch-url.cc
index 2b9254659..d3d05cc50 100644
--- a/src/nix-prefetch-url/nix-prefetch-url.cc
+++ b/src/nix-prefetch-url/nix-prefetch-url.cc
@@ -37,11 +37,11 @@ string resolveMirrorUri(EvalState & state, string uri)
 
     auto mirrorList = vMirrors.attrs->find(state.symbols.create(mirrorName));
     if (mirrorList == vMirrors.attrs->end())
-        throw Error(format("unknown mirror name '%1%'") % mirrorName);
+        throw Error("unknown mirror name '%1%'", mirrorName);
     state.forceList(*mirrorList->value);
 
     if (mirrorList->value->listSize() < 1)
-        throw Error(format("mirror URI '%1%' did not expand to anything") % uri);
+        throw Error("mirror URI '%1%' did not expand to anything", uri);
 
     string mirror = state.forceString(*mirrorList->value->listElems()[0]);
     return mirror + (hasSuffix(mirror, "/") ? "" : "/") + string(s, p + 1);
@@ -73,7 +73,7 @@ static int _main(int argc, char * * argv)
                 string s = getArg(*arg, arg, end);
                 ht = parseHashType(s);
                 if (ht == htUnknown)
-                    throw UsageError(format("unknown hash type '%1%'") % s);
+                    throw UsageError("unknown hash type '%1%'", s);
             }
             else if (*arg == "--print-path")
                 printPath = true;
@@ -151,7 +151,7 @@ static int _main(int argc, char * * argv)
         if (name.empty())
             name = baseNameOf(uri);
         if (name.empty())
-            throw Error(format("cannot figure out file name for '%1%'") % uri);
+            throw Error("cannot figure out file name for '%1%'", uri);
 
         /* If an expected hash is given, the file may already exist in
            the store. */
@@ -206,7 +206,7 @@ static int _main(int argc, char * * argv)
             hash = unpack ? hashPath(ht, tmpFile).first : hashFile(ht, tmpFile);
 
             if (expectedHash != Hash(ht) && expectedHash != hash)
-                throw Error(format("hash mismatch for '%1%'") % uri);
+                throw Error("hash mismatch for '%1%'", uri);
 
             /* Copy the file to the Nix store. FIXME: if RemoteStore
                implemented addToStoreFromDump() and downloadFile()
diff --git a/src/nix-store/nix-store.cc b/src/nix-store/nix-store.cc
index fcc00175a..e4dd9bc96 100644
--- a/src/nix-store/nix-store.cc
+++ b/src/nix-store/nix-store.cc
@@ -124,7 +124,7 @@ static void opRealise(Strings opFlags, Strings opArgs)
         else if (i == "--repair") buildMode = bmRepair;
         else if (i == "--check") buildMode = bmCheck;
         else if (i == "--ignore-unknown") ignoreUnknown = true;
-        else throw UsageError(format("unknown flag '%1%'") % i);
+        else throw UsageError("unknown flag '%1%'", i);
 
     std::vector<StorePathWithOutputs> paths;
     for (auto & i : opArgs)
@@ -178,7 +178,7 @@ static void opAddFixed(Strings opFlags, Strings opArgs)
 
     for (auto & i : opFlags)
         if (i == "--recursive") recursive = true;
-        else throw UsageError(format("unknown flag '%1%'") % i);
+        else throw UsageError("unknown flag '%1%'", i);
 
     if (opArgs.empty())
         throw UsageError("first argument must be hash algorithm");
@@ -198,10 +198,10 @@ static void opPrintFixedPath(Strings opFlags, Strings opArgs)
 
     for (auto i : opFlags)
         if (i == "--recursive") recursive = true;
-        else throw UsageError(format("unknown flag '%1%'") % i);
+        else throw UsageError("unknown flag '%1%'", i);
 
     if (opArgs.size() != 3)
-        throw UsageError(format("'--print-fixed-path' requires three arguments"));
+        throw UsageError("'--print-fixed-path' requires three arguments");
 
     Strings::iterator i = opArgs.begin();
     HashType hashAlgo = parseHashType(*i++);
@@ -296,9 +296,9 @@ static void opQuery(Strings opFlags, Strings opArgs)
         else if (i == "--use-output" || i == "-u") useOutput = true;
         else if (i == "--force-realise" || i == "--force-realize" || i == "-f") forceRealise = true;
         else if (i == "--include-outputs") includeOutputs = true;
-        else throw UsageError(format("unknown flag '%1%'") % i);
+        else throw UsageError("unknown flag '%1%'", i);
         if (prev != qDefault && prev != query)
-            throw UsageError(format("query type '%1%' conflicts with earlier flag") % i);
+            throw UsageError("query type '%1%' conflicts with earlier flag", i);
     }
 
     if (query == qDefault) query = qOutputs;
@@ -444,7 +444,7 @@ static void opPrintEnv(Strings opFlags, Strings opArgs)
     Derivation drv = store->derivationFromPath(store->parseStorePath(drvPath));
 
     /* Print each environment variable in the derivation in a format
-       that can be sourced by the shell. */
+     * that can be sourced by the shell. */
     for (auto & i : drv.env)
         cout << format("export %1%; %1%=%2%\n") % i.first % shellEscape(i.second);
 
@@ -531,7 +531,7 @@ static void opRegisterValidity(Strings opFlags, Strings opArgs)
     for (auto & i : opFlags)
         if (i == "--reregister") reregister = true;
         else if (i == "--hash-given") hashGiven = true;
-        else throw UsageError(format("unknown flag '%1%'") % i);
+        else throw UsageError("unknown flag '%1%'", i);
 
     if (!opArgs.empty()) throw UsageError("no arguments expected");
 
@@ -545,7 +545,7 @@ static void opCheckValidity(Strings opFlags, Strings opArgs)
 
     for (auto & i : opFlags)
         if (i == "--print-invalid") printInvalid = true;
-        else throw UsageError(format("unknown flag '%1%'") % i);
+        else throw UsageError("unknown flag '%1%'", i);
 
     for (auto & i : opArgs) {
         auto path = store->followLinksToStorePath(i);
@@ -576,7 +576,7 @@ static void opGC(Strings opFlags, Strings opArgs)
             long long maxFreed = getIntArg<long long>(*i, i, opFlags.end(), true);
             options.maxFreed = maxFreed >= 0 ? maxFreed : 0;
         }
-        else throw UsageError(format("bad sub-operation '%1%' in GC") % *i);
+        else throw UsageError("bad sub-operation '%1%' in GC", *i);
 
     if (!opArgs.empty()) throw UsageError("no arguments expected");
 
@@ -612,7 +612,7 @@ static void opDelete(Strings opFlags, Strings opArgs)
 
     for (auto & i : opFlags)
         if (i == "--ignore-liveness") options.ignoreLiveness = true;
-        else throw UsageError(format("unknown flag '%1%'") % i);
+        else throw UsageError("unknown flag '%1%'", i);
 
     for (auto & i : opArgs)
         options.pathsToDelete.insert(store->followLinksToStorePath(i));
@@ -650,7 +650,7 @@ static void opRestore(Strings opFlags, Strings opArgs)
 static void opExport(Strings opFlags, Strings opArgs)
 {
     for (auto & i : opFlags)
-        throw UsageError(format("unknown flag '%1%'") % i);
+        throw UsageError("unknown flag '%1%'", i);
 
     StorePathSet paths;
 
@@ -666,7 +666,7 @@ static void opExport(Strings opFlags, Strings opArgs)
 static void opImport(Strings opFlags, Strings opArgs)
 {
     for (auto & i : opFlags)
-        throw UsageError(format("unknown flag '%1%'") % i);
+        throw UsageError("unknown flag '%1%'", i);
 
     if (!opArgs.empty()) throw UsageError("no arguments expected");
 
@@ -701,7 +701,7 @@ static void opVerify(Strings opFlags, Strings opArgs)
     for (auto & i : opFlags)
         if (i == "--check-contents") checkContents = true;
         else if (i == "--repair") repair = Repair;
-        else throw UsageError(format("unknown flag '%1%'") % i);
+        else throw UsageError("unknown flag '%1%'", i);
 
     if (store->verifyStore(checkContents, repair)) {
         printError("warning: not all errors were fixed");
@@ -764,7 +764,7 @@ static void opServe(Strings opFlags, Strings opArgs)
     bool writeAllowed = false;
     for (auto & i : opFlags)
         if (i == "--write") writeAllowed = true;
-        else throw UsageError(format("unknown flag '%1%'") % i);
+        else throw UsageError("unknown flag '%1%'", i);
 
     if (!opArgs.empty()) throw UsageError("no arguments expected");
 
@@ -835,7 +835,7 @@ static void opServe(Strings opFlags, Strings opArgs)
                             for (auto & p : willSubstitute) subs.emplace_back(p.clone());
                             store->buildPaths(subs);
                         } catch (Error & e) {
-                            printError(format("warning: %1%") % e.msg());
+                            printError("warning: %1%", e.msg());
                         }
                 }
 
@@ -906,7 +906,7 @@ static void opServe(Strings opFlags, Strings opArgs)
 
                 if (!writeAllowed) throw Error("building paths is not allowed");
 
-                auto drvPath = store->parseStorePath(readString(in)); // informational only
+                auto drvPath = store->parseStorePath(readString(in)); // inonal ony
                 BasicDerivation drv;
                 readDerivation(in, *store, drv);
 
@@ -962,7 +962,7 @@ static void opServe(Strings opFlags, Strings opArgs)
             }
 
             default:
-                throw Error(format("unknown serve command %1%") % cmd);
+                throw Error("unknown serve command %1%", cmd);
         }
 
         out.flush();
@@ -973,7 +973,7 @@ static void opServe(Strings opFlags, Strings opArgs)
 static void opGenerateBinaryCacheKey(Strings opFlags, Strings opArgs)
 {
     for (auto & i : opFlags)
-        throw UsageError(format("unknown flag '%1%'") % i);
+        throw UsageError("unknown flag '%1%'", i);
 
     if (opArgs.size() != 3) throw UsageError("three arguments expected");
     auto i = opArgs.begin();
diff --git a/src/nix/cat.cc b/src/nix/cat.cc
index 851f90abd..4e14f50ed 100644
--- a/src/nix/cat.cc
+++ b/src/nix/cat.cc
@@ -13,9 +13,9 @@ struct MixCat : virtual Args
     {
         auto st = accessor->stat(path);
         if (st.type == FSAccessor::Type::tMissing)
-            throw Error(format("path '%1%' does not exist") % path);
+            throw Error("path '%1%' does not exist", path);
         if (st.type != FSAccessor::Type::tRegular)
-            throw Error(format("path '%1%' is not a regular file") % path);
+            throw Error("path '%1%' is not a regular file", path);
 
         std::cout << accessor->readFile(path);
     }
diff --git a/src/nix/hash.cc b/src/nix/hash.cc
index 0cc523f50..128708339 100644
--- a/src/nix/hash.cc
+++ b/src/nix/hash.cc
@@ -126,7 +126,7 @@ static int compatNixHash(int argc, char * * argv)
             string s = getArg(*arg, arg, end);
             ht = parseHashType(s);
             if (ht == htUnknown)
-                throw UsageError(format("unknown hash type '%1%'") % s);
+                throw UsageError("unknown hash type '%1%'", s);
         }
         else if (*arg == "--to-base16") op = opTo16;
         else if (*arg == "--to-base32") op = opTo32;
diff --git a/src/nix/ls.cc b/src/nix/ls.cc
index 3ef1f2750..5d55bd4d5 100644
--- a/src/nix/ls.cc
+++ b/src/nix/ls.cc
@@ -65,7 +65,7 @@ struct MixLs : virtual Args, MixJSON
 
         auto st = accessor->stat(path);
         if (st.type == FSAccessor::Type::tMissing)
-            throw Error(format("path '%1%' does not exist") % path);
+            throw Error("path '%1%' does not exist", path);
         doPath(st, path,
             st.type == FSAccessor::Type::tDirectory ? "." : std::string(baseNameOf(path)),
             showDirectory);
diff --git a/src/nix/repl.cc b/src/nix/repl.cc
index 27727bd25..c770fd4a4 100644
--- a/src/nix/repl.cc
+++ b/src/nix/repl.cc
@@ -252,12 +252,12 @@ void NixRepl::mainLoop(const std::vector<std::string> & files)
                 // input without clearing the input so far.
                 continue;
             } else {
-              printMsg(lvlError, format(error + "%1%%2%") % (settings.showTrace ? e.prefix() : "") % e.msg());
+              printMsg(lvlError, error + "%1%%2%", (settings.showTrace ? e.prefix() : ""), e.msg());
             }
         } catch (Error & e) {
-            printMsg(lvlError, format(error + "%1%%2%") % (settings.showTrace ? e.prefix() : "") % e.msg());
+            printMsg(lvlError, error + "%1%%2%", (settings.showTrace ? e.prefix() : ""), e.msg());
         } catch (Interrupted & e) {
-            printMsg(lvlError, format(error + "%1%%2%") % (settings.showTrace ? e.prefix() : "") % e.msg());
+            printMsg(lvlError, error + "%1%%2%", (settings.showTrace ? e.prefix() : ""), e.msg());
         }
 
         // We handled the current input fully, so we should clear it
@@ -546,7 +546,7 @@ bool NixRepl::processLine(string line)
         return false;
 
     else if (command != "")
-        throw Error(format("unknown command '%1%'") % command);
+        throw Error("unknown command '%1%'", command);
 
     else {
         size_t p = line.find('=');
diff --git a/src/nix/run.cc b/src/nix/run.cc
index 8e30264c0..d790979a4 100644
--- a/src/nix/run.cc
+++ b/src/nix/run.cc
@@ -197,10 +197,10 @@ void chrootHelper(int argc, char * * argv)
         Finally freeCwd([&]() { free(cwd); });
 
         if (chroot(tmpDir.c_str()) == -1)
-            throw SysError(format("chrooting into '%s'") % tmpDir);
+            throw SysError("chrooting into '%s'", tmpDir);
 
         if (chdir(cwd) == -1)
-            throw SysError(format("chdir to '%s' in chroot") % cwd);
+            throw SysError("chdir to '%s' in chroot", cwd);
     } else
         if (mount(realStoreDir.c_str(), storeDir.c_str(), "", MS_BIND, 0) == -1)
             throw SysError("mounting '%s' on '%s'", realStoreDir, storeDir);