Add error context for most basic coercions

src/libcmd/installables.cc

@@ -524,7 +524,7 @@ ref<eval_cache::EvalCache> openEvalCache(
             auto vFlake = state.allocValue();
             flake::callFlake(state, *lockedFlake, *vFlake);
 
-            state.forceAttrs(*vFlake, noPos);
+            state.forceAttrs(*vFlake, noPos, "While evaluating a cached flake");
 
             auto aOutputs = vFlake->attrs->get(state.symbols.create("outputs"));
             assert(aOutputs);

src/libexpr/attr-path.cc

@@ -112,7 +112,7 @@ Pos findPackageFilename(EvalState & state, Value & v, std::string what)
 
     // FIXME: is it possible to extract the Pos object instead of doing this
     //        toString + parsing?
-    auto pos = state.forceString(*v2);
+    auto pos = state.forceString(*v2, noPos, "While evaluating the meta.position attribute of a derivation");
 
     auto colon = pos.rfind(':');
     if (colon == std::string::npos)

src/libexpr/eval-cache.cc

@@ -336,7 +336,7 @@ Value & AttrCursor::getValue()
     if (!_value) {
         if (parent) {
             auto & vParent = parent->first->getValue();
-            root->state.forceAttrs(vParent, noPos);
+            root->state.forceAttrs(vParent, noPos, "While evaluating the parent attr set");
             auto attr = vParent.attrs->get(parent->second);
             if (!attr)
                 throw Error("attribute '%s' is unexpectedly missing", getAttrPathStr());

src/libexpr/eval-inline.hh

@@ -15,10 +15,10 @@ LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s))
     });
 }
 
-LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const Value & v))
+LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const Value & v, const std::string & s2))
 {
     throw TypeError({
-        .msg = hintfmt(s, showType(v)),
+        .msg = hintfmt(s, showType(v), s2),
         .errPos = pos
     });
 }
@@ -52,26 +52,26 @@ void EvalState::forceValue(Value & v, Callable getPos)
 }
 
 
-inline void EvalState::forceAttrs(Value & v, const Pos & pos)
+inline void EvalState::forceAttrs(Value & v, const Pos & pos, const std::string & errorCtx)
 {
-    forceAttrs(v, [&]() { return pos; });
+    forceAttrs(v, [&]() { return pos; }, errorCtx);
 }
 
 
 template <typename Callable>
-inline void EvalState::forceAttrs(Value & v, Callable getPos)
+inline void EvalState::forceAttrs(Value & v, Callable getPos, const std::string & errorCtx)
 {
     forceValue(v, getPos);
     if (v.type() != nAttrs)
-        throwTypeError(getPos(), "value is %1% while a set was expected", v);
+        throwTypeError(getPos(), "%2%: value is %1% while a set was expected", v, errorCtx);
 }
 
 
-inline void EvalState::forceList(Value & v, const Pos & pos)
+inline void EvalState::forceList(Value & v, const Pos & pos, const std::string & errorCtx)
 {
     forceValue(v, pos);
     if (!v.isList())
-        throwTypeError(pos, "value is %1% while a list was expected", v);
+        throwTypeError(pos, "%2%: value is %1% while a list was expected", v, errorCtx);
 }
 
 /* Note: Various places expect the allocated memory to be zeroed. */

src/libexpr/eval.cc

@@ -297,7 +297,7 @@ static Symbol getName(const AttrName & name, EvalState & state, Env & env)
     } else {
         Value nameValue;
         name.expr->eval(state, env, nameValue);
-        state.forceStringNoCtx(nameValue);
+        state.forceStringNoCtx(nameValue, noPos, "While evaluating an attribute name");
         return state.symbols.create(nameValue.string.s);
     }
 }
@@ -763,9 +763,17 @@ LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const
     });
 }
 
-LocalNoInlineNoReturn(void throwTypeError(const char * s, const Value & v))
+//LocalNoInlineNoReturn(void throwTypeError(const char * s, const Value & v))
+//{
+//    throw TypeError(s, showType(v));
+//}
+
+LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const Value & v))
 {
-    throw TypeError(s, showType(v));
+    throw AssertionError({
+        .msg = hintfmt(s, showType(v)),
+        .errPos = pos
+    });
 }
 
 LocalNoInlineNoReturn(void throwAssertionError(const Pos & pos, const char * s, const std::string & s1))
@@ -1060,7 +1068,7 @@ inline bool EvalState::evalBool(Env & env, Expr * e, const Pos & pos, const std:
     Value v;
     e->eval(*this, env, v);
     if (v.type() != nBool)
-        throwTypeError(pos, (location + ": value is %1% while a Boolean was expected").c_str(), v);
+        throwTypeError(pos, "%2%: value is %1% while a Boolean was expected", v, location);
     return v.boolean;
 }
 
@@ -1069,7 +1077,7 @@ inline void EvalState::evalAttrs(Env & env, Expr * e, Value & v, const Pos & pos
 {
     e->eval(*this, env, v);
     if (v.type() != nAttrs)
-        throwTypeError(pos, (location + ": value is %1% while a set was expected").c_str(), v);
+        throwTypeError(pos, "%2%: value is %1% while a set was expected", v, location);
 }
 
 
@@ -1142,7 +1150,7 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
            Hence we need __overrides.) */
         if (hasOverrides) {
             Value * vOverrides = (*v.attrs)[overrides->second.displ].value;
-            state.forceAttrs(*vOverrides, [&]() { return vOverrides->determinePos(noPos); });
+            state.forceAttrs(*vOverrides, [&]() { return vOverrides->determinePos(noPos); }, "While evaluating the `__overrides` attribute");
             Bindings * newBnds = state.allocBindings(v.attrs->capacity() + vOverrides->attrs->size());
             for (auto & i : *v.attrs)
                 newBnds->push_back(i);
@@ -1170,7 +1178,7 @@ void ExprAttrs::eval(EvalState & state, Env & env, Value & v)
         state.forceValue(nameVal, i.pos);
         if (nameVal.type() == nNull)
             continue;
-        state.forceStringNoCtx(nameVal);
+        state.forceStringNoCtx(nameVal, i.pos, "While evaluating the name of a dynamic attribute");
         Symbol nameSym = state.symbols.create(nameVal.string.s);
         Bindings::iterator j = v.attrs->find(nameSym);
         if (j != v.attrs->end())
@@ -1260,7 +1268,7 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
                     return;
                 }
             } else {
-                state.forceAttrs(*vAttrs, pos);
+                state.forceAttrs(*vAttrs, pos, "While selecting an attribute");
                 if ((j = vAttrs->attrs->find(name)) == vAttrs->attrs->end())
                     throwEvalError(pos, "attribute '%1%' missing", name);
             }
@@ -1351,7 +1359,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 env2.values[displ++] = args[0];
 
             else {
-                forceAttrs(*args[0], pos);
+                forceAttrs(*args[0], pos, "While evaluating the value passed as argument to a function expecting an attribute set");
 
                 if (!lambda.arg.empty())
                     env2.values[displ++] = args[0];
@@ -1567,7 +1575,8 @@ void ExprWith::eval(EvalState & state, Env & env, Value & v)
 
 void ExprIf::eval(EvalState & state, Env & env, Value & v)
 {
-    (state.evalBool(env, cond, pos, "In the condition of the if operator") ? then : else_)->eval(state, env, v);
+    // We cheat in the parser, an pass the position of the condition as the position of the if itself.
+    (state.evalBool(env, cond, pos, "") ? then : else_)->eval(state, env, v);
 }
 
 
@@ -1665,18 +1674,18 @@ void ExprOpConcatLists::eval(EvalState & state, Env & env, Value & v)
     Value v1; e1->eval(state, env, v1);
     Value v2; e2->eval(state, env, v2);
     Value * lists[2] = { &v1, &v2 };
-    state.concatLists(v, 2, lists, pos);
+    state.concatLists(v, 2, lists, pos, "While evaluating one of the elements to concatenate");
 }
 
 
-void EvalState::concatLists(Value & v, size_t nrLists, Value * * lists, const Pos & pos)
+void EvalState::concatLists(Value & v, size_t nrLists, Value * * lists, const Pos & pos, const std::string & errorCtx)
 {
     nrListConcats++;
 
     Value * nonEmpty = 0;
     size_t len = 0;
     for (size_t n = 0; n < nrLists; ++n) {
-        forceList(*lists[n], pos);
+        forceList(*lists[n], pos, errorCtx);
         auto l = lists[n]->listSize();
         len += l;
         if (l) nonEmpty = lists[n];
@@ -1824,31 +1833,31 @@ void EvalState::forceValueDeep(Value & v)
 }
 
 
-NixInt EvalState::forceInt(Value & v, const Pos & pos)
+NixInt EvalState::forceInt(Value & v, const Pos & pos, const std::string & errorCtx)
 {
     forceValue(v, pos);
     if (v.type() != nInt)
-        throwTypeError(pos, "value is %1% while an integer was expected", v);
+        throwTypeError(pos, "%2%: value is %1% while an integer was expected", v, errorCtx);
     return v.integer;
 }
 
 
-NixFloat EvalState::forceFloat(Value & v, const Pos & pos)
+NixFloat EvalState::forceFloat(Value & v, const Pos & pos, const std::string & errorCtx)
 {
     forceValue(v, pos);
     if (v.type() == nInt)
         return v.integer;
     else if (v.type() != nFloat)
-        throwTypeError(pos, "value is %1% while a float was expected", v);
+        throwTypeError(pos, "%2%: value is %1% while a float was expected", v, errorCtx);
     return v.fpoint;
 }
 
 
-bool EvalState::forceBool(Value & v, const Pos & pos)
+bool EvalState::forceBool(Value & v, const Pos & pos, const std::string & errorCtx)
 {
     forceValue(v, pos);
     if (v.type() != nBool)
-        throwTypeError(pos, "value is %1% while a Boolean was expected", v);
+        throwTypeError(pos, "%2%: value is %1% while a Boolean was expected", v, errorCtx);
     return v.boolean;
 }
 
@@ -1859,22 +1868,19 @@ bool EvalState::isFunctor(Value & fun)
 }
 
 
-void EvalState::forceFunction(Value & v, const Pos & pos)
+void EvalState::forceFunction(Value & v, const Pos & pos, const std::string & errorCtx)
 {
     forceValue(v, pos);
     if (v.type() != nFunction && !isFunctor(v))
-        throwTypeError(pos, "value is %1% while a function was expected", v);
+        throwTypeError(pos, "%2%: value is %1% while a function was expected", v, errorCtx);
 }
 
 
-std::string_view EvalState::forceString(Value & v, const Pos & pos)
+std::string_view EvalState::forceString(Value & v, const Pos & pos, const std::string & errorCtx)
 {
     forceValue(v, pos);
     if (v.type() != nString) {
-        if (pos)
-            throwTypeError(pos, "value is %1% while a string was expected", v);
-        else
-            throwTypeError("value is %1% while a string was expected", v);
+        throwTypeError(pos, "%2%: value is %1% while a string was expected", v, errorCtx);
     }
     return v.string.s;
 }
@@ -1911,23 +1917,23 @@ std::vector<std::pair<Path, std::string>> Value::getContext()
 }
 
 
-std::string_view EvalState::forceString(Value & v, PathSet & context, const Pos & pos)
+std::string_view EvalState::forceString(Value & v, PathSet & context, const Pos & pos, const std::string & errorCtx)
 {
-    auto s = forceString(v, pos);
+    auto s = forceString(v, pos, errorCtx);
     copyContext(v, context);
     return s;
 }
 
 
-std::string_view EvalState::forceStringNoCtx(Value & v, const Pos & pos)
+std::string_view EvalState::forceStringNoCtx(Value & v, const Pos & pos, const std::string & errorCtx)
 {
-    auto s = forceString(v, pos);
+    auto s = forceString(v, pos, errorCtx);
     if (v.string.context) {
         if (pos)
-            throwEvalError(pos, "the string '%1%' is not allowed to refer to a store path (such as '%2%')",
+            throwEvalError(pos, (errorCtx + ": the string '%1%' is not allowed to refer to a store path (such as '%2%')").c_str(),
                 v.string.s, v.string.context[0]);
         else
-            throwEvalError("the string '%1%' is not allowed to refer to a store path (such as '%2%')",
+            throwEvalError((errorCtx + ": the string '%1%' is not allowed to refer to a store path (such as '%2%')").c_str(),
                 v.string.s, v.string.context[0]);
     }
     return s;

src/libexpr/eval.hh

@@ -234,20 +234,20 @@ public:
     void forceValueDeep(Value & v);
 
     /* Force `v', and then verify that it has the expected type. */
-    NixInt forceInt(Value & v, const Pos & pos);
-    NixFloat forceFloat(Value & v, const Pos & pos);
-    bool forceBool(Value & v, const Pos & pos);
+    NixInt forceInt(Value & v, const Pos & pos, const std::string & errorCtx);
+    NixFloat forceFloat(Value & v, const Pos & pos, const std::string & errorCtx);
+    bool forceBool(Value & v, const Pos & pos, const std::string & errorCtx);
 
-    void forceAttrs(Value & v, const Pos & pos);
+    void forceAttrs(Value & v, const Pos & pos, const std::string & errorCtx);
 
     template <typename Callable>
-    inline void forceAttrs(Value & v, Callable getPos);
+    inline void forceAttrs(Value & v, Callable getPos, const std::string & errorCtx);
 
-    inline void forceList(Value & v, const Pos & pos);
-    void forceFunction(Value & v, const Pos & pos); // either lambda or primop
-    std::string_view forceString(Value & v, const Pos & pos = noPos);
-    std::string_view forceString(Value & v, PathSet & context, const Pos & pos = noPos);
-    std::string_view forceStringNoCtx(Value & v, const Pos & pos = noPos);
+    inline void forceList(Value & v, const Pos & pos, const std::string & errorCtx);
+    void forceFunction(Value & v, const Pos & pos, const std::string & errorCtx); // either lambda or primop
+    std::string_view forceString(Value & v, const Pos & pos, const std::string & errorCtx);
+    std::string_view forceString(Value & v, PathSet & context, const Pos & pos, const std::string & errorCtx);
+    std::string_view forceStringNoCtx(Value & v, const Pos & pos, const std::string & errorCtx);
 
     /* Return true iff the value `v' denotes a derivation (i.e. a
        set with attribute `type = "derivation"'). */
@@ -363,7 +363,7 @@ public:
     void mkThunk_(Value & v, Expr * expr);
     void mkPos(Value & v, ptr<Pos> pos);
 
-    void concatLists(Value & v, size_t nrLists, Value * * lists, const Pos & pos);
+    void concatLists(Value & v, size_t nrLists, Value * * lists, const Pos & pos, const std::string & errorCtx);
 
     /* Print statistics. */
     void printStats();

src/libexpr/flake/flake.cc

@@ -255,7 +255,7 @@ static Flake getFlake(
         for (auto & setting : *nixConfig->value->attrs) {
             forceTrivialValue(state, *setting.value, *setting.pos);
             if (setting.value->type() == nString)
-                flake.config.settings.insert({setting.name, std::string(state.forceStringNoCtx(*setting.value, *setting.pos))});
+                flake.config.settings.insert({setting.name, std::string(state.forceStringNoCtx(*setting.value, *setting.pos, ""))});
             else if (setting.value->type() == nPath) {
                 PathSet emptyContext = {};
                 flake.config.settings.emplace(
@@ -263,16 +263,16 @@ static Flake getFlake(
                     state.coerceToString(*setting.pos, *setting.value, emptyContext, false, true, true) .toOwned());
             }
             else if (setting.value->type() == nInt)
-                flake.config.settings.insert({setting.name, state.forceInt(*setting.value, *setting.pos)});
+                flake.config.settings.insert({setting.name, state.forceInt(*setting.value, *setting.pos, "")});
             else if (setting.value->type() == nBool)
-                flake.config.settings.insert({setting.name, Explicit<bool> { state.forceBool(*setting.value, *setting.pos) }});
+                flake.config.settings.insert({setting.name, Explicit<bool> { state.forceBool(*setting.value, *setting.pos, "") }});
             else if (setting.value->type() == nList) {
                 std::vector<std::string> ss;
                 for (auto elem : setting.value->listItems()) {
                     if (elem->type() != nString)
                         throw TypeError("list element in flake configuration setting '%s' is %s while a string is expected",
                             setting.name, showType(*setting.value));
-                    ss.emplace_back(state.forceStringNoCtx(*elem, *setting.pos));
+                    ss.emplace_back(state.forceStringNoCtx(*elem, *setting.pos, ""));
                 }
                 flake.config.settings.insert({setting.name, ss});
             }
@@ -708,7 +708,7 @@ static void prim_getFlake(EvalState & state, const Pos & pos, Value * * args, Va
 {
     state.requireExperimentalFeatureOnEvaluation(Xp::Flakes, "builtins.getFlake", pos);
 
-    std::string flakeRefS(state.forceStringNoCtx(*args[0], pos));
+    std::string flakeRefS(state.forceStringNoCtx(*args[0], pos, "While evaluating the argument passed to builtins.getFlake"));
     auto flakeRef = parseFlakeRef(flakeRefS, {}, true);
     if (evalSettings.pureEval && !flakeRef.input.isLocked())
         throw Error("cannot call 'getFlake' on unlocked flake reference '%s', at %s (use --impure to override)", flakeRefS, pos);

src/libexpr/get-drvs.cc

@@ -50,7 +50,7 @@ std::string DrvInfo::queryName() const
     if (name == "" && attrs) {
         auto i = attrs->find(state->sName);
         if (i == attrs->end()) throw TypeError("derivation name missing");
-        name = state->forceStringNoCtx(*i->value);
+        name = state->forceStringNoCtx(*i->value, noPos, "While evaluating the name of a DrvInfo");
     }
     return name;
 }
@@ -60,7 +60,7 @@ std::string DrvInfo::querySystem() const
 {
     if (system == "" && attrs) {
         auto i = attrs->find(state->sSystem);
-        system = i == attrs->end() ? "unknown" : state->forceStringNoCtx(*i->value, *i->pos);
+        system = i == attrs->end() ? "unknown" : state->forceStringNoCtx(*i->value, *i->pos, "While evaluating the system of a DrvInfo");
     }
     return system;
 }
@@ -108,15 +108,15 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool onlyOutputsToInstall)
         /* Get the ‘outputs’ list. */
         Bindings::iterator i;
         if (attrs && (i = attrs->find(state->sOutputs)) != attrs->end()) {
-            state->forceList(*i->value, *i->pos);
+            state->forceList(*i->value, *i->pos, "While evaluating the outputs of a DrvInfo");
 
             /* For each output... */
             for (auto elem : i->value->listItems()) {
                 /* Evaluate the corresponding set. */
-                std::string name(state->forceStringNoCtx(*elem, *i->pos));
+                std::string name(state->forceStringNoCtx(*elem, *i->pos, "While evaluating the name of one output of a DrvInfo"));
                 Bindings::iterator out = attrs->find(state->symbols.create(name));
                 if (out == attrs->end()) continue; // FIXME: throw error?
-                state->forceAttrs(*out->value, *i->pos);
+                state->forceAttrs(*out->value, *i->pos, "While evaluating the description of a DrvInfo output");
 
                 /* And evaluate its ‘outPath’ attribute. */
                 Bindings::iterator outPath = out->value->attrs->find(state->sOutPath);
@@ -151,7 +151,7 @@ std::string DrvInfo::queryOutputName() const
 {
     if (outputName == "" && attrs) {
         Bindings::iterator i = attrs->find(state->sOutputName);
-        outputName = i != attrs->end() ? state->forceStringNoCtx(*i->value) : "";
+        outputName = i != attrs->end() ? state->forceStringNoCtx(*i->value, noPos, "While evaluating the output name of a DrvInfo") : "";
     }
     return outputName;
 }
@@ -163,7 +163,7 @@ Bindings * DrvInfo::getMeta()
     if (!attrs) return 0;
     Bindings::iterator a = attrs->find(state->sMeta);
     if (a == attrs->end()) return 0;
-    state->forceAttrs(*a->value, *a->pos);
+    state->forceAttrs(*a->value, *a->pos, "While evaluating the `meta` attribute of a DrvInfo");
     meta = a->value->attrs;
     return meta;
 }
@@ -364,7 +364,7 @@ static void getDerivations(EvalState & state, Value & vIn,
                    `recurseForDerivations = true' attribute. */
                 if (i->value->type() == nAttrs) {
                     Bindings::iterator j = i->value->attrs->find(state.sRecurseForDerivations);
-                    if (j != i->value->attrs->end() && state.forceBool(*j->value, *j->pos))
+                    if (j != i->value->attrs->end() && state.forceBool(*j->value, *j->pos, "While evaluating the attribute `recurseForDerivations`"))
                         getDerivations(state, *i->value, pathPrefix2, autoArgs, drvs, done, ignoreAssertionFailures);
                 }
             }

src/libexpr/nixexpr.hh

@@ -4,7 +4,6 @@
 #include "symbol-table.hh"
 #include "error.hh"
 
-
 namespace nix {
 
 

src/libexpr/parser.y

@@ -396,7 +396,7 @@ expr_function
   ;
 
 expr_if
-  : IF expr THEN expr ELSE expr { $$ = new ExprIf(CUR_POS, $2, $4, $6); }
+  : IF expr THEN expr ELSE expr { $$ = new ExprIf(makeCurPos(@2, data), $2, $4, $6); }
   | expr_op
   ;
 
@@ -405,21 +405,21 @@ expr_op
   | '-' expr_op %prec NEGATE { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__sub")), {new ExprInt(0), $2}); }
   | expr_op EQ expr_op { $$ = new ExprOpEq($1, $3); }
   | expr_op NEQ expr_op { $$ = new ExprOpNEq($1, $3); }
-  | expr_op '<' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__lessThan")), {$1, $3}); }
-  | expr_op LEQ expr_op { $$ = new ExprOpNot(new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__lessThan")), {$3, $1})); }
-  | expr_op '>' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__lessThan")), {$3, $1}); }
-  | expr_op GEQ expr_op { $$ = new ExprOpNot(new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__lessThan")), {$1, $3})); }
-  | expr_op AND expr_op { $$ = new ExprOpAnd(CUR_POS, $1, $3); }
-  | expr_op OR expr_op { $$ = new ExprOpOr(CUR_POS, $1, $3); }
-  | expr_op IMPL expr_op { $$ = new ExprOpImpl(CUR_POS, $1, $3); }
-  | expr_op UPDATE expr_op { $$ = new ExprOpUpdate(CUR_POS, $1, $3); }
+  | expr_op '<' expr_op { $$ = new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__lessThan")), {$1, $3}); }
+  | expr_op LEQ expr_op { $$ = new ExprOpNot(new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__lessThan")), {$3, $1})); }
+  | expr_op '>' expr_op { $$ = new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__lessThan")), {$3, $1}); }
+  | expr_op GEQ expr_op { $$ = new ExprOpNot(new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__lessThan")), {$1, $3})); }
+  | expr_op AND expr_op { $$ = new ExprOpAnd(makeCurPos(@2, data), $1, $3); }
+  | expr_op OR expr_op { $$ = new ExprOpOr(makeCurPos(@2, data), $1, $3); }
+  | expr_op IMPL expr_op { $$ = new ExprOpImpl(makeCurPos(@2, data), $1, $3); }
+  | expr_op UPDATE expr_op { $$ = new ExprOpUpdate(makeCurPos(@2, data), $1, $3); }
   | expr_op '?' attrpath { $$ = new ExprOpHasAttr($1, *$3); }
   | expr_op '+' expr_op
-    { $$ = new ExprConcatStrings(CUR_POS, false, new std::vector<std::pair<Pos, Expr *> >({{makeCurPos(@1, data), $1}, {makeCurPos(@3, data), $3}})); }
-  | expr_op '-' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__sub")), {$1, $3}); }
-  | expr_op '*' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__mul")), {$1, $3}); }
-  | expr_op '/' expr_op { $$ = new ExprCall(CUR_POS, new ExprVar(data->symbols.create("__div")), {$1, $3}); }
-  | expr_op CONCAT expr_op { $$ = new ExprOpConcatLists(CUR_POS, $1, $3); }
+    { $$ = new ExprConcatStrings(makeCurPos(@2, data), false, new std::vector<std::pair<Pos, Expr *> >({{makeCurPos(@1, data), $1}, {makeCurPos(@3, data), $3}})); }
+  | expr_op '-' expr_op { $$ = new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__sub")), {$1, $3}); }
+  | expr_op '*' expr_op { $$ = new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__mul")), {$1, $3}); }
+  | expr_op '/' expr_op { $$ = new ExprCall(makeCurPos(@2, data), new ExprVar(data->symbols.create("__div")), {$1, $3}); }
+  | expr_op CONCAT expr_op { $$ = new ExprOpConcatLists(makeCurPos(@2, data), $1, $3); }
   | expr_app
   ;
 

src/libexpr/primops.cc

@@ -195,9 +195,9 @@ static void import(EvalState & state, const Pos & pos, Value & vPath, Value * vS
                 , "/"), **state.vImportedDrvToDerivation);
         }
 
-        state.forceFunction(**state.vImportedDrvToDerivation, pos);
+        state.forceFunction(**state.vImportedDrvToDerivation, pos, "While evaluating imported-drv-to-derivation.nix.gen.hh");
         v.mkApp(*state.vImportedDrvToDerivation, w);
-        state.forceAttrs(v, pos);
+        state.forceAttrs(v, pos, "While calling imported-drv-to-derivation.nix.gen.hh");
     }
 
     else if (path == corepkgsPrefix + "fetchurl.nix") {
@@ -210,7 +210,7 @@ static void import(EvalState & state, const Pos & pos, Value & vPath, Value * vS
         if (!vScope)
             state.evalFile(path, v);
         else {
-            state.forceAttrs(*vScope, pos);
+            state.forceAttrs(*vScope, pos, "While evaluating the first argument passed to builtins.scopedImport");
 
             Env * env = &state.allocEnv(vScope->attrs->size());
             env->up = &state.baseEnv;
@@ -314,7 +314,7 @@ void prim_importNative(EvalState & state, const Pos & pos, Value * * args, Value
 {
     auto path = realisePath(state, pos, *args[0]);
 
-    std::string sym(state.forceStringNoCtx(*args[1], pos));
+    std::string sym(state.forceStringNoCtx(*args[1], pos, "While evaluating the second argument passed to builtins.importNative"));
 
     void *handle = dlopen(path.c_str(), RTLD_LAZY | RTLD_LOCAL);
     if (!handle)
@@ -340,7 +340,7 @@ void prim_importNative(EvalState & state, const Pos & pos, Value * * args, Value
 /* Execute a program and parse its output */
 void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceList(*args[0], pos);
+    state.forceList(*args[0], pos, "While evaluating the first argument passed to builtins.exec");
     auto elems = args[0]->listElems();
     auto count = args[0]->listSize();
     if (count == 0) {
@@ -381,7 +381,6 @@ void prim_exec(EvalState & state, const Pos & pos, Value * * args, Value & v)
     }
 }
 
-
 /* Return a string representing the type of the expression. */
 static void prim_typeOf(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
@@ -535,8 +534,10 @@ static RegisterPrimOp primop_isPath({
 struct CompareValues
 {
     EvalState & state;
+    const Pos & pos;
+    const std::string errorCtx;
 
-    CompareValues(EvalState & state) : state(state) { };
+    CompareValues(EvalState & state, const Pos & pos, const std::string && errorCtx) : state(state), pos(pos), errorCtx(std::move(errorCtx)) { };
 
     bool operator () (Value * v1, Value * v2) const
     {
@@ -545,7 +546,10 @@ struct CompareValues
         if (v1->type() == nInt && v2->type() == nFloat)
             return v1->integer < v2->fpoint;
         if (v1->type() != v2->type())
-            throw EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2));
+            throw EvalError({
+                .msg = hintfmt("%s: cannot compare %s with %s", errorCtx, showType(*v1), showType(*v2)),
+                .errPos = pos,
+            });
         switch (v1->type()) {
             case nInt:
                 return v1->integer < v2->integer;
@@ -567,7 +571,10 @@ struct CompareValues
                     }
                 }
             default:
-                throw EvalError("cannot compare %1% with %2%", showType(*v1), showType(*v2));
+                throw EvalError({
+                    .msg = hintfmt("%s: cannot compare %s with %s", errorCtx, showType(*v1), showType(*v2)),
+                    .errPos = pos,
+                });
         }
     }
 };
@@ -619,7 +626,7 @@ static Bindings::iterator getAttr(
 
 static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos);
+    state.forceAttrs(*args[0], pos, "While evaluating the first argument passed to builtins.genericClosure");
 
     /* Get the start set. */
     Bindings::iterator startSet = getAttr(
@@ -630,7 +637,7 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
         pos
     );
 
-    state.forceList(*startSet->value, pos);
+    state.forceList(*startSet->value, pos, "While evaluating the `startSet` attribute passed to builtins.genericClosure");
 
     ValueList workSet;
     for (auto elem : startSet->value->listItems())
@@ -645,7 +652,7 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
         pos
     );
 
-    state.forceValue(*op->value, pos);
+    state.forceFunction(*op->value, pos, "While evaluating the `operator` attribute passed to builtins.genericClosure");
 
     /* Construct the closure by applying the operator to element of
        `workSet', adding the result to `workSet', continuing until
@@ -653,19 +660,19 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
     ValueList res;
     // `doneKeys' doesn't need to be a GC root, because its values are
     // reachable from res.
-    auto cmp = CompareValues(state);
+    auto cmp = CompareValues(state, pos, "While comparing the `key` attributes of two genericClosure elements");
     std::set<Value *, decltype(cmp)> doneKeys(cmp);
     while (!workSet.empty()) {
         Value * e = *(workSet.begin());
         workSet.pop_front();
 
-        state.forceAttrs(*e, pos);
+        state.forceAttrs(*e, pos, "While evaluating one item to be part of the genericClosure");
 
         Bindings::iterator key =
             e->attrs->find(state.sKey);
         if (key == e->attrs->end())
             throw EvalError({
-                .msg = hintfmt("attribute 'key' required"),
+                .msg = hintfmt("While evaluating one of the attribute sets to be part of the genericClosure: attribute `key` required"),
                 .errPos = pos
             });
         state.forceValue(*key->value, pos);
@@ -676,7 +683,7 @@ static void prim_genericClosure(EvalState & state, const Pos & pos, Value * * ar
         /* Call the `operator' function with `e' as argument. */
         Value call;
         call.mkApp(op->value, e);
-        state.forceList(call, pos);
+        state.forceList(call, pos, "While evaluating the return value of the `operator` passed to builtins.genericClosure");
 
         /* Add the values returned by the operator to the work set. */
         for (auto elem : call.listItems()) {
@@ -750,7 +757,7 @@ static RegisterPrimOp primop_addErrorContext(RegisterPrimOp::Info {
 
 static void prim_ceil(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    auto value = state.forceFloat(*args[0], args[0]->determinePos(pos));
+    auto value = state.forceFloat(*args[0], args[0]->determinePos(pos), "While evaluating the first argument passed to builtins.ceil");
     v.mkInt(ceil(value));
 }
 
@@ -769,7 +776,7 @@ static RegisterPrimOp primop_ceil({
 
 static void prim_floor(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    auto value = state.forceFloat(*args[0], args[0]->determinePos(pos));
+    auto value = state.forceFloat(*args[0], args[0]->determinePos(pos), "While evaluating the first argument passed to builtins.floor");
     v.mkInt(floor(value));
 }
 
@@ -826,7 +833,7 @@ static RegisterPrimOp primop_tryEval({
 /* Return an environment variable.  Use with care. */
 static void prim_getEnv(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    std::string name(state.forceStringNoCtx(*args[0], pos));
+    std::string name(state.forceStringNoCtx(*args[0], pos, "While evaluating the first argument passed to builtins.getEnv"));
     v.mkString(evalSettings.restrictEval || evalSettings.pureEval ? "" : getEnv(name).value_or(""));
 }
 
@@ -924,7 +931,7 @@ static RegisterPrimOp primop_trace({
    derivation. */
 static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos);
+    state.forceAttrs(*args[0], pos, "While evaluating the argument passed to builtins.derivationStrict");
 
     /* Figure out the name first (for stack backtraces). */
     Bindings::iterator attr = getAttr(
@@ -938,7 +945,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
     std::string drvName;
     Pos & posDrvName(*attr->pos);
     try {
-        drvName = state.forceStringNoCtx(*attr->value, pos);
+        drvName = state.forceStringNoCtx(*attr->value, pos, "While evaluating the `name` attribute passed to builtins.derivationStrict");
     } catch (Error & e) {
         e.addTrace(posDrvName, "while evaluating the derivation attribute 'name'");
         throw;
@@ -948,14 +955,14 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
     std::ostringstream jsonBuf;
     std::unique_ptr<JSONObject> jsonObject;
     attr = args[0]->attrs->find(state.sStructuredAttrs);
-    if (attr != args[0]->attrs->end() && state.forceBool(*attr->value, pos))
+    if (attr != args[0]->attrs->end() && state.forceBool(*attr->value, pos, "While evaluating the `__structuredAttrs` attribute passed to builtins.derivationStrict"))
         jsonObject = std::make_unique<JSONObject>(jsonBuf);
 
     /* Check whether null attributes should be ignored. */
     bool ignoreNulls = false;
     attr = args[0]->attrs->find(state.sIgnoreNulls);
     if (attr != args[0]->attrs->end())
-        ignoreNulls = state.forceBool(*attr->value, pos);
+        ignoreNulls = state.forceBool(*attr->value, pos, "While evaluating the `__ignoreNulls` attribute passed to builtins.derivationStrict");
 
     /* Build the derivation expression by processing the attributes. */
     Derivation drv;
@@ -1021,7 +1028,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             }
 
             if (i->name == state.sContentAddressed) {
-                contentAddressed = state.forceBool(*i->value, pos);
+                contentAddressed = state.forceBool(*i->value, pos, "While evaluating the `__contentAddressed` attribute passed to builtins.derivationStrict");
                 if (contentAddressed)
                     settings.requireExperimentalFeature(Xp::CaDerivations);
             }
@@ -1029,7 +1036,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
             /* The `args' attribute is special: it supplies the
                command-line arguments to the builder. */
             else if (i->name == state.sArgs) {
-                state.forceList(*i->value, pos);
+                state.forceList(*i->value, pos, "While evaluating the `args` attribute passed to builtins.derivationStrict");
                 for (auto elem : i->value->listItems()) {
                     auto s = state.coerceToString(posDrvName, *elem, context, true).toOwned();
                     drv.args.push_back(s);
@@ -1048,21 +1055,21 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
                     printValueAsJSON(state, true, *i->value, pos, placeholder, context);
 
                     if (i->name == state.sBuilder)
-                        drv.builder = state.forceString(*i->value, context, posDrvName);
+                        drv.builder = state.forceString(*i->value, context, posDrvName, "While evaluating the `builder` attribute passed to builtins.derivationStrict");
                     else if (i->name == state.sSystem)
-                        drv.platform = state.forceStringNoCtx(*i->value, posDrvName);
+                        drv.platform = state.forceStringNoCtx(*i->value, posDrvName, "While evaluating the `system` attribute passed to builtins.derivationStrict");
                     else if (i->name == state.sOutputHash)
-                        outputHash = state.forceStringNoCtx(*i->value, posDrvName);
+                        outputHash = state.forceStringNoCtx(*i->value, posDrvName, "While evaluating the `outputHash` attribute passed to builtins.derivationStrict");
                     else if (i->name == state.sOutputHashAlgo)
-                        outputHashAlgo = state.forceStringNoCtx(*i->value, posDrvName);
+                        outputHashAlgo = state.forceStringNoCtx(*i->value, posDrvName, "While evaluating the `outputHashAlgo` attribute passed to builtins.derivationStrict");
                     else if (i->name == state.sOutputHashMode)
-                        handleHashMode(state.forceStringNoCtx(*i->value, posDrvName));
+                        handleHashMode(state.forceStringNoCtx(*i->value, posDrvName, "While evaluating the `outputHashMode` attribute passed to builtins.derivationStrict"));
                     else if (i->name == state.sOutputs) {
                         /* Require ‘outputs’ to be a list of strings. */
-                        state.forceList(*i->value, posDrvName);
+                        state.forceList(*i->value, posDrvName, "While evaluating the `outputs` attribute passed to builtins.derivationStrict");
                         Strings ss;
                         for (auto elem : i->value->listItems())
-                            ss.emplace_back(state.forceStringNoCtx(*elem, posDrvName));
+                            ss.emplace_back(state.forceStringNoCtx(*elem, posDrvName, "While evaluating an element of the `outputs` attribute passed to builtins.derivationStrict"));
                         handleOutputs(ss);
                     }
 
@@ -1275,7 +1282,7 @@ static RegisterPrimOp primop_derivationStrict(RegisterPrimOp::Info {
    ‘out’. */
 static void prim_placeholder(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    v.mkString(hashPlaceholder(state.forceStringNoCtx(*args[0], pos)));
+    v.mkString(hashPlaceholder(state.forceStringNoCtx(*args[0], pos, "While evaluating the first argument passed to builtins.placeholder")));
 }
 
 static RegisterPrimOp primop_placeholder({
@@ -1467,17 +1474,17 @@ static RegisterPrimOp primop_readFile({
    which are desugared to 'findFile __nixPath "x"'. */
 static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceList(*args[0], pos);
+    state.forceList(*args[0], pos, "While evaluating the first argument passed to builtins.findFile");
 
     SearchPath searchPath;
 
     for (auto v2 : args[0]->listItems()) {
-        state.forceAttrs(*v2, pos);
+        state.forceAttrs(*v2, pos, "While evaluating an element of the list passed to builtins.findFile");
 
         std::string prefix;
         Bindings::iterator i = v2->attrs->find(state.sPrefix);
         if (i != v2->attrs->end())
-            prefix = state.forceStringNoCtx(*i->value, pos);
+            prefix = state.forceStringNoCtx(*i->value, pos, "While evaluating the `prefix` attribute passed to an element of the list passed to builtins.findFile");
 
         i = getAttr(
             state,
@@ -1504,7 +1511,7 @@ static void prim_findFile(EvalState & state, const Pos & pos, Value * * args, Va
         searchPath.emplace_back(prefix, path);
     }
 
-    auto path = state.forceStringNoCtx(*args[1], pos);
+    auto path = state.forceStringNoCtx(*args[1], pos, "While evaluating the second argument passed to builtins.findFile");
 
     v.mkPath(state.checkSourcePath(state.findFile(searchPath, path, pos)));
 }
@@ -1518,7 +1525,7 @@ static RegisterPrimOp primop_findFile(RegisterPrimOp::Info {
 /* Return the cryptographic hash of a file in base-16. */
 static void prim_hashFile(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    auto type = state.forceStringNoCtx(*args[0], pos);
+    auto type = state.forceStringNoCtx(*args[0], pos, "While evaluating the first argument passed to builtins.hashFile");
     std::optional<HashType> ht = parseHashType(type);
     if (!ht)
         throw Error({
@@ -1725,7 +1732,7 @@ static RegisterPrimOp primop_toJSON({
 /* Parse a JSON string to a value. */
 static void prim_fromJSON(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    auto s = state.forceStringNoCtx(*args[0], pos);
+    auto s = state.forceStringNoCtx(*args[0], pos, "While evaluating the first argument passed to builtins.fromJSON");
     try {
         parseJSON(state, s, v);
     } catch (JSONParseError &e) {
@@ -1754,8 +1761,8 @@ static RegisterPrimOp primop_fromJSON({
 static void prim_toFile(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     PathSet context;
-    std::string name(state.forceStringNoCtx(*args[0], pos));
-    std::string contents(state.forceString(*args[1], context, pos));
+    std::string name(state.forceStringNoCtx(*args[0], pos, "While evaluating the first argument passed to builtins.toFile"));
+    std::string contents(state.forceString(*args[1], context, pos, "While evaluating the second argument passed to builtins.toFile"));
 
     StorePathSet refs;
 
@@ -1912,7 +1919,7 @@ static void addPath(
             Value res;
             state.callFunction(*filterFun, 2, args, res, pos);
 
-            return state.forceBool(res, pos);
+            return state.forceBool(res, pos, "While evaluating the return value of the path filter function");
         }) : defaultPathFilter;
 
         std::optional<StorePath> expectedStorePath;
@@ -2009,7 +2016,7 @@ static RegisterPrimOp primop_filterSource({
 
 static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos);
+    state.forceAttrs(*args[0], pos, "While evaluating the argument passed to builtins.path");
     Path path;
     std::string name;
     Value * filterFun = nullptr;
@@ -2022,14 +2029,14 @@ static void prim_path(EvalState & state, const Pos & pos, Value * * args, Value
         if (n == "path")
             path = state.coerceToPath(*attr.pos, *attr.value, context);
         else if (attr.name == state.sName)
-            name = state.forceStringNoCtx(*attr.value, *attr.pos);
+            name = state.forceStringNoCtx(*attr.value, *attr.pos, "while evaluating the `name` attribute passed to builtins.path");
         else if (n == "filter") {
             state.forceValue(*attr.value, pos);
             filterFun = attr.value;
         } else if (n == "recursive")
-            method = FileIngestionMethod { state.forceBool(*attr.value, *attr.pos) };
+            method = FileIngestionMethod { state.forceBool(*attr.value, *attr.pos, "While evaluating the `recursive` attribute passed to builtins.path") };
         else if (n == "sha256")
-            expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA256);
+            expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, *attr.pos, "While evaluating the `sha256` attribute passed to builtins.path"), htSHA256);
         else
             throw EvalError({
                 .msg = hintfmt("unsupported argument '%1%' to 'addPath'", attr.name),
@@ -2092,7 +2099,7 @@ static RegisterPrimOp primop_path({
    strings. */
 static void prim_attrNames(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos);
+    state.forceAttrs(*args[0], pos, "While evaluating the argument passed to builtins.attrNames");
 
     state.mkList(v, args[0]->attrs->size());
 
@@ -2119,7 +2126,7 @@ static RegisterPrimOp primop_attrNames({
    order as attrNames. */
 static void prim_attrValues(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos);
+    state.forceAttrs(*args[0], pos, "While evaluating the argument passed to builtins.attrValues");
 
     state.mkList(v, args[0]->attrs->size());
 
@@ -2150,8 +2157,8 @@ static RegisterPrimOp primop_attrValues({
 /* Dynamic version of the `.' operator. */
 void prim_getAttr(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    auto attr = state.forceStringNoCtx(*args[0], pos);
-    state.forceAttrs(*args[1], pos);
+    auto attr = state.forceStringNoCtx(*args[0], pos, "While evaluating the first argument passed to builtins.getAttr");
+    state.forceAttrs(*args[1], pos, "While evaluating the second argument passed to builtins.getAttr");
     Bindings::iterator i = getAttr(
         state,
         "getAttr",
@@ -2180,8 +2187,8 @@ static RegisterPrimOp primop_getAttr({
 /* Return position information of the specified attribute. */
 static void prim_unsafeGetAttrPos(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    auto attr = state.forceStringNoCtx(*args[0], pos);
-    state.forceAttrs(*args[1], pos);
+    auto attr = state.forceStringNoCtx(*args[0], pos, "While evaluating the first argument passed to builtins.unsafeGetAttrPos");
+    state.forceAttrs(*args[1], pos, "While evaluating the second argument passed to builtins.unsafeGetAttrPos");
     Bindings::iterator i = args[1]->attrs->find(state.symbols.create(attr));
     if (i == args[1]->attrs->end())
         v.mkNull();
@@ -2198,8 +2205,8 @@ static RegisterPrimOp primop_unsafeGetAttrPos(RegisterPrimOp::Info {
 /* Dynamic version of the `?' operator. */
 static void prim_hasAttr(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    auto attr = state.forceStringNoCtx(*args[0], pos);
-    state.forceAttrs(*args[1], pos);
+    auto attr = state.forceStringNoCtx(*args[0], pos, "While evaluating the first argument passed to builtins.hasAttr");
+    state.forceAttrs(*args[1], pos, "While evaluating the second argument passed to builtins.hasAttr");
     v.mkBool(args[1]->attrs->find(state.symbols.create(attr)) != args[1]->attrs->end());
 }
 
@@ -2232,8 +2239,8 @@ static RegisterPrimOp primop_isAttrs({
 
 static void prim_removeAttrs(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceAttrs(*args[0], pos, "While evaluating the first argument passed to builtins.removeAttrs");
+    state.forceList(*args[1], pos, "While evaluating the second argument passed to builtins.removeAttrs");
 
     /* Get the attribute names to be removed.
        We keep them as Attrs instead of Symbols so std::set_difference
@@ -2241,7 +2248,7 @@ static void prim_removeAttrs(EvalState & state, const Pos & pos, Value * * args,
     boost::container::small_vector<Attr, 64> names;
     names.reserve(args[1]->listSize());
     for (auto elem : args[1]->listItems()) {
-        state.forceStringNoCtx(*elem, pos);
+        state.forceStringNoCtx(*elem, pos, "While evaluating the values of the second argument passed to builtins.removeAttrs");
         names.emplace_back(state.symbols.create(elem->string.s), nullptr);
     }
     std::sort(names.begin(), names.end());
@@ -2280,14 +2287,14 @@ static RegisterPrimOp primop_removeAttrs({
    name, the first takes precedence. */
 static void prim_listToAttrs(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceList(*args[0], pos);
+    state.forceList(*args[0], pos, "While evaluating the argument passed to builtins.listToAttrs");
 
     auto attrs = state.buildBindings(args[0]->listSize());
 
     std::set<Symbol> seen;
 
     for (auto v2 : args[0]->listItems()) {
-        state.forceAttrs(*v2, pos);
+        state.forceAttrs(*v2, pos, "While evaluating an element of the list passed to builtins.listToAttrs");
 
         Bindings::iterator j = getAttr(
             state,
@@ -2297,7 +2304,7 @@ static void prim_listToAttrs(EvalState & state, const Pos & pos, Value * * args,
             pos
         );
 
-        auto name = state.forceStringNoCtx(*j->value, *j->pos);
+        auto name = state.forceStringNoCtx(*j->value, *j->pos, "While evaluating the `name` attribute of an element of the list passed to builtins.listToAttrs");
 
         Symbol sym = state.symbols.create(name);
         if (seen.insert(sym).second) {
@@ -2342,8 +2349,8 @@ static RegisterPrimOp primop_listToAttrs({
 
 static void prim_intersectAttrs(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[0], pos);
-    state.forceAttrs(*args[1], pos);
+    state.forceAttrs(*args[0], pos, "While evaluating the first argument passed to builtins.intersectAttrs");
+    state.forceAttrs(*args[1], pos, "While evaluating the second argument passed to builtins.intersectAttrs");
 
     auto attrs = state.buildBindings(std::min(args[0]->attrs->size(), args[1]->attrs->size()));
 
@@ -2368,14 +2375,14 @@ static RegisterPrimOp primop_intersectAttrs({
 
 static void prim_catAttrs(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    Symbol attrName = state.symbols.create(state.forceStringNoCtx(*args[0], pos));
-    state.forceList(*args[1], pos);
+    Symbol attrName = state.symbols.create(state.forceStringNoCtx(*args[0], pos, "While evaluating the first argument passed to builtins.catAttrs"));
+    state.forceList(*args[1], pos, "While evaluating the second argument passed to builtins.catAttrs");
 
     Value * res[args[1]->listSize()];
     unsigned int found = 0;
 
     for (auto v2 : args[1]->listItems()) {
-        state.forceAttrs(*v2, pos);
+        state.forceAttrs(*v2, pos, "While evaluating an element in the list passed as second argument to builtins.catAttrs");
         Bindings::iterator i = v2->attrs->find(attrName);
         if (i != v2->attrs->end())
             res[found++] = i->value;
@@ -2448,7 +2455,7 @@ static RegisterPrimOp primop_functionArgs({
 /*  */
 static void prim_mapAttrs(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceAttrs(*args[1], pos);
+    state.forceAttrs(*args[1], pos, "While evaluating the second argument passed to builtins.mapAttrs");
 
     auto attrs = state.buildBindings(args[1]->attrs->size());
 
@@ -2489,15 +2496,15 @@ static void prim_zipAttrsWith(EvalState & state, const Pos & pos, Value * * args
 
     std::map<Symbol, std::pair<size_t, Value * *>> attrsSeen;
 
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceFunction(*args[0], pos, "While evaluating the first argument passed to builtins.zipAttrsWith");
+    state.forceList(*args[1], pos, "While evaluating the second argument passed to builtins.zipAttrsWith");
     const auto listSize = args[1]->listSize();
     const auto listElems = args[1]->listElems();
 
     for (unsigned int n = 0; n < listSize; ++n) {
         Value * vElem = listElems[n];
         try {
-            state.forceAttrs(*vElem, noPos);
+            state.forceAttrs(*vElem, noPos, "While evaluating a value of the list passed as second argument to builtins.zipAttrsWith");
             for (auto & attr : *vElem->attrs)
                 attrsSeen[attr.name].first++;
         } catch (TypeError & e) {
@@ -2587,7 +2594,7 @@ static RegisterPrimOp primop_isList({
 
 static void elemAt(EvalState & state, const Pos & pos, Value & list, int n, Value & v)
 {
-    state.forceList(list, pos);
+    state.forceList(list, pos, "While evaluating the first argument passed to builtins.elemAt");
     if (n < 0 || (unsigned int) n >= list.listSize())
         throw Error({
             .msg = hintfmt("list index %1% is out of bounds", n),
@@ -2600,7 +2607,7 @@ static void elemAt(EvalState & state, const Pos & pos, Value & list, int n, Valu
 /* Return the n-1'th element of a list. */
 static void prim_elemAt(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    elemAt(state, pos, *args[0], state.forceInt(*args[1], pos), v);
+    elemAt(state, pos, *args[0], state.forceInt(*args[1], pos, "While evaluating the second argument passed to builtins.elemAt"), v);
 }
 
 static RegisterPrimOp primop_elemAt({
@@ -2635,7 +2642,7 @@ static RegisterPrimOp primop_head({
    don't want to use it!  */
 static void prim_tail(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceList(*args[0], pos);
+    state.forceList(*args[0], pos, "While evaluating the first argument passed to builtins.tail");
     if (args[0]->listSize() == 0)
         throw Error({
             .msg = hintfmt("'tail' called on an empty list"),
@@ -2666,13 +2673,17 @@ static RegisterPrimOp primop_tail({
 /* Apply a function to every element of a list. */
 static void prim_map(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceList(*args[1], pos);
+    state.forceList(*args[1], pos, "While evaluating the second argument passed to builtins.map");
 
     state.mkList(v, args[1]->listSize());
 
-    for (unsigned int n = 0; n < v.listSize(); ++n)
-        (v.listElems()[n] = state.allocValue())->mkApp(
-            args[0], args[1]->listElems()[n]);
+    if (args[1]->listSize() > 0) {
+        state.forceFunction(*args[0], pos, "While evaluating the first argument passed to builtins.map");
+
+        for (unsigned int n = 0; n < v.listSize(); ++n)
+            (v.listElems()[n] = state.allocValue())->mkApp(
+                args[0], args[1]->listElems()[n]);
+    };
 }
 
 static RegisterPrimOp primop_map({
@@ -2696,8 +2707,14 @@ static RegisterPrimOp primop_map({
    returns true. */
 static void prim_filter(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceList(*args[1], pos, "While evaluating the second argument passed to builtins.filter");
+
+    if (args[1]->listSize() == 0) {
+        v = *args[1];
+        return;
+    }
+
+    state.forceFunction(*args[0], pos, "While evaluating the first argument passed to builtins.filter");
 
     // FIXME: putting this on the stack is risky.
     Value * vs[args[1]->listSize()];
@@ -2707,7 +2724,7 @@ static void prim_filter(EvalState & state, const Pos & pos, Value * * args, Valu
     for (unsigned int n = 0; n < args[1]->listSize(); ++n) {
         Value res;
         state.callFunction(*args[0], *args[1]->listElems()[n], res, noPos);
-        if (state.forceBool(res, pos))
+        if (state.forceBool(res, pos, "While evaluating the return value of the filtering function passed to builtins.filter"))
             vs[k++] = args[1]->listElems()[n];
         else
             same = false;
@@ -2735,7 +2752,7 @@ static RegisterPrimOp primop_filter({
 static void prim_elem(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     bool res = false;
-    state.forceList(*args[1], pos);
+    state.forceList(*args[1], pos, "While evaluating the second argument passed to builtins.elem");
     for (auto elem : args[1]->listItems())
         if (state.eqValues(*args[0], *elem)) {
             res = true;
@@ -2757,8 +2774,8 @@ static RegisterPrimOp primop_elem({
 /* Concatenate a list of lists. */
 static void prim_concatLists(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceList(*args[0], pos);
-    state.concatLists(v, args[0]->listSize(), args[0]->listElems(), pos);
+    state.forceList(*args[0], pos, "While evaluating the first argument passed to builtins.concatLists");
+    state.concatLists(v, args[0]->listSize(), args[0]->listElems(), pos, "While evaluating a value of the list passed to builtins.concatLists");
 }
 
 static RegisterPrimOp primop_concatLists({
@@ -2773,7 +2790,7 @@ static RegisterPrimOp primop_concatLists({
 /* Return the length of a list.  This is an O(1) time operation. */
 static void prim_length(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceList(*args[0], pos);
+    state.forceList(*args[0], pos, "While evaluating the first argument passed to builtins.length");
     v.mkInt(args[0]->listSize());
 }
 
@@ -2790,8 +2807,8 @@ static RegisterPrimOp primop_length({
    right. The operator is applied strictly. */
 static void prim_foldlStrict(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[2], pos);
+    state.forceFunction(*args[0], pos, "While evaluating the first argument passed to builtins.foldlStrict");
+    state.forceList(*args[2], pos, "While evaluating the third argument passed to builtins.foldlStrict");
 
     if (args[2]->listSize()) {
         Value * vCur = args[1];
@@ -2823,13 +2840,13 @@ static RegisterPrimOp primop_foldlStrict({
 
 static void anyOrAll(bool any, EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceFunction(*args[0], pos, std::string("While evaluating the first argument passed to builtins.") + (any ? "any" : "all"));
+    state.forceList(*args[1], pos, std::string("While evaluating the second argument passed to builtins.") + (any ? "any" : "all"));
 
     Value vTmp;
     for (auto elem : args[1]->listItems()) {
         state.callFunction(*args[0], *elem, vTmp, pos);
-        bool res = state.forceBool(vTmp, pos);
+        bool res = state.forceBool(vTmp, pos, std::string("While evaluating the return value of the function passed to builtins.") + (any ? "any" : "all"));
         if (res == any) {
             v.mkBool(any);
             return;
@@ -2872,7 +2889,7 @@ static RegisterPrimOp primop_all({
 
 static void prim_genList(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    auto len = state.forceInt(*args[1], pos);
+    auto len = state.forceInt(*args[1], pos, "While evaluating the second argument passed to builtins.genList");
 
     if (len < 0)
         throw EvalError({
@@ -2910,10 +2927,16 @@ static void prim_lessThan(EvalState & state, const Pos & pos, Value * * args, Va
 
 static void prim_sort(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceList(*args[1], pos, "While evaluating the second argument passed to builtins.sort");
 
     auto len = args[1]->listSize();
+    if (len == 0) {
+        v = *args[1];
+        return;
+    }
+
+    state.forceFunction(*args[0], pos, "While evaluating the first argument passed to builtins.sort");
+
     state.mkList(v, len);
     for (unsigned int n = 0; n < len; ++n) {
         state.forceValue(*args[1]->listElems()[n], pos);
@@ -2924,12 +2947,12 @@ static void prim_sort(EvalState & state, const Pos & pos, Value * * args, Value
         /* Optimization: if the comparator is lessThan, bypass
            callFunction. */
         if (args[0]->isPrimOp() && args[0]->primOp->fun == prim_lessThan)
-            return CompareValues(state)(a, b);
+            return CompareValues(state, pos, "While evaluating the ordering function passed to builtins.sort")(a, b);
 
         Value * vs[] = {a, b};
         Value vBool;
         state.callFunction(*args[0], 2, vs, vBool, pos);
-        return state.forceBool(vBool, pos);
+        return state.forceBool(vBool, pos, "While evaluating the return value of the sorting function passed to builtins.sort");
     };
 
     /* FIXME: std::sort can segfault if the comparator is not a strict
@@ -2961,8 +2984,8 @@ static RegisterPrimOp primop_sort({
 
 static void prim_partition(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceFunction(*args[0], pos, "While evaluating the first argument passed to builtins.partition");
+    state.forceList(*args[1], pos, "While evaluating the second argument passed to builtins.partition");
 
     auto len = args[1]->listSize();
 
@@ -2973,7 +2996,7 @@ static void prim_partition(EvalState & state, const Pos & pos, Value * * args, V
         state.forceValue(*vElem, pos);
         Value res;
         state.callFunction(*args[0], *vElem, res, pos);
-        if (state.forceBool(res, pos))
+        if (state.forceBool(res, pos, "While evaluating the return value of the partition function passed to builtins.partition"))
             right.push_back(vElem);
         else
             wrong.push_back(vElem);
@@ -3021,15 +3044,15 @@ static RegisterPrimOp primop_partition({
 
 static void prim_groupBy(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceFunction(*args[0], pos, "While evaluating the first argument passed to builtins.groupBy");
+    state.forceList(*args[1], pos, "While evaluating the second argument passed to builtins.groupBy");
 
     ValueVectorMap attrs;
 
     for (auto vElem : args[1]->listItems()) {
         Value res;
         state.callFunction(*args[0], *vElem, res, pos);
-        auto name = state.forceStringNoCtx(res, pos);
+        auto name = state.forceStringNoCtx(res, pos, "While evaluating the return value of the grouping function passed to builtins.groupBy");
         Symbol sym = state.symbols.create(name);
         auto vector = attrs.try_emplace(sym, ValueVector()).first;
         vector->second.push_back(vElem);
@@ -3073,8 +3096,8 @@ static RegisterPrimOp primop_groupBy({
 
 static void prim_concatMap(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceFunction(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceFunction(*args[0], pos, "While evaluating the first argument passed to builtins.concatMap");
+    state.forceList(*args[1], pos, "While evaluating the second argument passed to builtins.concatMap");
     auto nrLists = args[1]->listSize();
 
     Value lists[nrLists];
@@ -3084,7 +3107,7 @@ static void prim_concatMap(EvalState & state, const Pos & pos, Value * * args, V
         Value * vElem = args[1]->listElems()[n];
         state.callFunction(*args[0], *vElem, lists[n], pos);
         try {
-            state.forceList(lists[n], lists[n].determinePos(args[0]->determinePos(pos)));
+            state.forceList(lists[n], lists[n].determinePos(args[0]->determinePos(pos)), "While evaluating the return value of the function passed to buitlins.concatMap");
         } catch (TypeError &e) {
             e.addTrace(pos, hintfmt("while invoking '%s'", "concatMap"));
             throw;
@@ -3123,9 +3146,11 @@ static void prim_add(EvalState & state, const Pos & pos, Value * * args, Value &
     state.forceValue(*args[0], pos);
     state.forceValue(*args[1], pos);
     if (args[0]->type() == nFloat || args[1]->type() == nFloat)
-        v.mkFloat(state.forceFloat(*args[0], pos) + state.forceFloat(*args[1], pos));
+        v.mkFloat(state.forceFloat(*args[0], pos, "While evaluating the first argument of the addition")
+                + state.forceFloat(*args[1], pos, "While evaluating the second argument of the addition"));
     else
-        v.mkInt(state.forceInt(*args[0], pos) + state.forceInt(*args[1], pos));
+        v.mkInt(  state.forceInt(*args[0], pos, "While evaluating the first argument of the addition")
+                + state.forceInt(*args[1], pos, "While evaluating the second argument of the addition"));
 }
 
 static RegisterPrimOp primop_add({
@@ -3142,9 +3167,11 @@ static void prim_sub(EvalState & state, const Pos & pos, Value * * args, Value &
     state.forceValue(*args[0], pos);
     state.forceValue(*args[1], pos);
     if (args[0]->type() == nFloat || args[1]->type() == nFloat)
-        v.mkFloat(state.forceFloat(*args[0], pos) - state.forceFloat(*args[1], pos));
+        v.mkFloat(state.forceFloat(*args[0], pos, "While evaluating the first argument of the subtraction")
+                - state.forceFloat(*args[1], pos, "While evaluating the second argument of the subtraction"));
     else
-        v.mkInt(state.forceInt(*args[0], pos) - state.forceInt(*args[1], pos));
+        v.mkInt(  state.forceInt(*args[0], pos, "While evaluating the first argument of the subtraction")
+                - state.forceInt(*args[1], pos, "While evaluating the second argument of the subtraction"));
 }
 
 static RegisterPrimOp primop_sub({
@@ -3161,9 +3188,11 @@ static void prim_mul(EvalState & state, const Pos & pos, Value * * args, Value &
     state.forceValue(*args[0], pos);
     state.forceValue(*args[1], pos);
     if (args[0]->type() == nFloat || args[1]->type() == nFloat)
-        v.mkFloat(state.forceFloat(*args[0], pos) * state.forceFloat(*args[1], pos));
+        v.mkFloat(state.forceFloat(*args[0], pos, "While evaluating the first of the multiplication")
+                * state.forceFloat(*args[1], pos, "While evaluating the second argument of the multiplication"));
     else
-        v.mkInt(state.forceInt(*args[0], pos) * state.forceInt(*args[1], pos));
+        v.mkInt(  state.forceInt(*args[0], pos, "While evaluating the first argument of the multiplication")
+                * state.forceInt(*args[1], pos, "While evaluating the second argument of the multiplication"));
 }
 
 static RegisterPrimOp primop_mul({
@@ -3180,7 +3209,7 @@ static void prim_div(EvalState & state, const Pos & pos, Value * * args, Value &
     state.forceValue(*args[0], pos);
     state.forceValue(*args[1], pos);
 
-    NixFloat f2 = state.forceFloat(*args[1], pos);
+    NixFloat f2 = state.forceFloat(*args[1], pos, "While evaluating the second operand of the division");
     if (f2 == 0)
         throw EvalError({
             .msg = hintfmt("division by zero"),
@@ -3188,10 +3217,10 @@ static void prim_div(EvalState & state, const Pos & pos, Value * * args, Value &
         });
 
     if (args[0]->type() == nFloat || args[1]->type() == nFloat) {
-        v.mkFloat(state.forceFloat(*args[0], pos) / state.forceFloat(*args[1], pos));
+        v.mkFloat(state.forceFloat(*args[0], pos, "While evaluating the first operand of the division") / f2);
     } else {
-        NixInt i1 = state.forceInt(*args[0], pos);
-        NixInt i2 = state.forceInt(*args[1], pos);
+        NixInt i1 = state.forceInt(*args[0], pos, "While evaluating the first operand of the division");
+        NixInt i2 = state.forceInt(*args[1], pos, "While evaluating the second operand of the division");
         /* Avoid division overflow as it might raise SIGFPE. */
         if (i1 == std::numeric_limits<NixInt>::min() && i2 == -1)
             throw EvalError({
@@ -3214,7 +3243,8 @@ static RegisterPrimOp primop_div({
 
 static void prim_bitAnd(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    v.mkInt(state.forceInt(*args[0], pos) & state.forceInt(*args[1], pos));
+    v.mkInt(state.forceInt(*args[0], pos, "While evaluating the first argument passed to builtins.bitAnd")
+            & state.forceInt(*args[1], pos, "While evaluating the second argument passed to builtins.bitAnd"));
 }
 
 static RegisterPrimOp primop_bitAnd({
@@ -3228,7 +3258,8 @@ static RegisterPrimOp primop_bitAnd({
 
 static void prim_bitOr(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    v.mkInt(state.forceInt(*args[0], pos) | state.forceInt(*args[1], pos));
+    v.mkInt(state.forceInt(*args[0], pos, "While evaluating the first argument passed to builtins.bitOr")
+            | state.forceInt(*args[1], pos, "While evaluating the second argument passed to builtins.bitOr"));
 }
 
 static RegisterPrimOp primop_bitOr({
@@ -3242,7 +3273,8 @@ static RegisterPrimOp primop_bitOr({
 
 static void prim_bitXor(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    v.mkInt(state.forceInt(*args[0], pos) ^ state.forceInt(*args[1], pos));
+    v.mkInt(state.forceInt(*args[0], pos, "While evaluating the first argument passed to builtins.bitXor")
+            ^ state.forceInt(*args[1], pos, "While evaluating the second argument passed to builtins.bitXor"));
 }
 
 static RegisterPrimOp primop_bitXor({
@@ -3258,7 +3290,8 @@ static void prim_lessThan(EvalState & state, const Pos & pos, Value * * args, Va
 {
     state.forceValue(*args[0], pos);
     state.forceValue(*args[1], pos);
-    CompareValues comp{state};
+    // pos is exact here, no need for a message.
+    CompareValues comp(state, pos, "");
     v.mkBool(comp(args[0], args[1]));
 }
 
@@ -3319,8 +3352,8 @@ static RegisterPrimOp primop_toString({
    non-negative. */
 static void prim_substring(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    int start = state.forceInt(*args[0], pos);
-    int len = state.forceInt(*args[1], pos);
+    int start = state.forceInt(*args[0], pos, "While evaluating the first argument passed to builtins.substring");
+    int len = state.forceInt(*args[1], pos, "While evaluating the second argument passed to builtins.substring");
     PathSet context;
     auto s = state.coerceToString(pos, *args[2], context);
 
@@ -3373,7 +3406,7 @@ static RegisterPrimOp primop_stringLength({
 /* Return the cryptographic hash of a string in base-16. */
 static void prim_hashString(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    auto type = state.forceStringNoCtx(*args[0], pos);
+    auto type = state.forceStringNoCtx(*args[0], pos, "While evaluating the first argument passed to builtins.hashString");
     std::optional<HashType> ht = parseHashType(type);
     if (!ht)
         throw Error({
@@ -3382,7 +3415,7 @@ static void prim_hashString(EvalState & state, const Pos & pos, Value * * args,
         });
 
     PathSet context; // discarded
-    auto s = state.forceString(*args[1], context, pos);
+    auto s = state.forceString(*args[1], context, pos, "While evaluating the second argument passed to builtins.hashString");
 
     v.mkString(hashString(*ht, s).to_string(Base16, false));
 }
@@ -3421,14 +3454,14 @@ std::shared_ptr<RegexCache> makeRegexCache()
 
 void prim_match(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    auto re = state.forceStringNoCtx(*args[0], pos);
+    auto re = state.forceStringNoCtx(*args[0], pos, "While evaluating the first argument passed to builtins.match");
 
     try {
 
         auto regex = state.regexCache->get(re);
 
         PathSet context;
-        const auto str = state.forceString(*args[1], context, pos);
+        const auto str = state.forceString(*args[1], context, pos, "While evaluating the second argument passed to builtins.match");
 
         std::cmatch match;
         if (!std::regex_match(str.begin(), str.end(), match, regex)) {
@@ -3502,14 +3535,14 @@ static RegisterPrimOp primop_match({
    non-matching parts interleaved by the lists of the matching groups. */
 void prim_split(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    auto re = state.forceStringNoCtx(*args[0], pos);
+    auto re = state.forceStringNoCtx(*args[0], pos, "While evaluating the first argument passed to builtins.split");
 
     try {
 
         auto regex = state.regexCache->get(re);
 
         PathSet context;
-        const auto str = state.forceString(*args[1], context, pos);
+        const auto str = state.forceString(*args[1], context, pos, "While evaluating the second argument passed to builtins.split");
 
         auto begin = std::cregex_iterator(str.begin(), str.end(), regex);
         auto end = std::cregex_iterator();
@@ -3608,8 +3641,8 @@ static void prim_concatStringsSep(EvalState & state, const Pos & pos, Value * *
 {
     PathSet context;
 
-    auto sep = state.forceString(*args[0], context, pos);
-    state.forceList(*args[1], pos);
+    auto sep = state.forceString(*args[0], context, pos, "While evaluating the first argument passed to builtins.concatStringsSep");
+    state.forceList(*args[1], pos, "While evaluating the second argument passed to builtins.concatStringsSep");
 
     std::string res;
     res.reserve((args[1]->listSize() + 32) * sep.size());
@@ -3636,8 +3669,8 @@ static RegisterPrimOp primop_concatStringsSep({
 
 static void prim_replaceStrings(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.forceList(*args[0], pos);
-    state.forceList(*args[1], pos);
+    state.forceList(*args[0], pos, "While evaluating the first argument passed to builtins.replaceStrings");
+    state.forceList(*args[1], pos, "While evaluating the second argument passed to builtins.replaceStrings");
     if (args[0]->listSize() != args[1]->listSize())
         throw EvalError({
             .msg = hintfmt("'from' and 'to' arguments to 'replaceStrings' have different lengths"),
@@ -3647,18 +3680,18 @@ static void prim_replaceStrings(EvalState & state, const Pos & pos, Value * * ar
     std::vector<std::string> from;
     from.reserve(args[0]->listSize());
     for (auto elem : args[0]->listItems())
-        from.emplace_back(state.forceString(*elem, pos));
+        from.emplace_back(state.forceString(*elem, pos, "While evaluating one of the strings to replace in builtins.replaceStrings"));
 
     std::vector<std::pair<std::string, PathSet>> to;
     to.reserve(args[1]->listSize());
     for (auto elem : args[1]->listItems()) {
         PathSet ctx;
-        auto s = state.forceString(*elem, ctx, pos);
+        auto s = state.forceString(*elem, ctx, pos, "While evaluating one of the replacement strings of builtins.replaceStrings");
         to.emplace_back(s, std::move(ctx));
     }
 
     PathSet context;
-    auto s = state.forceString(*args[2], context, pos);
+    auto s = state.forceString(*args[2], context, pos, "While evaluating the third argument passed to builtins.replaceStrings");
 
     std::string res;
     // Loops one past last character to handle the case where 'from' contains an empty string.
@@ -3716,7 +3749,7 @@ static RegisterPrimOp primop_replaceStrings({
 
 static void prim_parseDrvName(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    auto name = state.forceStringNoCtx(*args[0], pos);
+    auto name = state.forceStringNoCtx(*args[0], pos, "While evaluating the first argument passed to builtins.parseDrvName");
     DrvName parsed(name);
     auto attrs = state.buildBindings(2);
     attrs.alloc(state.sName).mkString(parsed.name);
@@ -3740,8 +3773,8 @@ static RegisterPrimOp primop_parseDrvName({
 
 static void prim_compareVersions(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    auto version1 = state.forceStringNoCtx(*args[0], pos);
-    auto version2 = state.forceStringNoCtx(*args[1], pos);
+    auto version1 = state.forceStringNoCtx(*args[0], pos, "While evaluating the first argument passed to builtins.compareVersions");
+    auto version2 = state.forceStringNoCtx(*args[1], pos, "While evaluating the second argument passed to builtins.compareVersions");
     v.mkInt(compareVersions(version1, version2));
 }
 
@@ -3760,7 +3793,7 @@ static RegisterPrimOp primop_compareVersions({
 
 static void prim_splitVersion(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    auto version = state.forceStringNoCtx(*args[0], pos);
+    auto version = state.forceStringNoCtx(*args[0], pos, "While evaluating the first argument passed to builtins.splitVersion");
     auto iter = version.cbegin();
     Strings components;
     while (iter != version.cend()) {

src/libexpr/primops/context.cc

@@ -17,7 +17,7 @@ static RegisterPrimOp primop_unsafeDiscardStringContext("__unsafeDiscardStringCo
 static void prim_hasContext(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     PathSet context;
-    state.forceString(*args[0], context, pos);
+    state.forceString(*args[0], context, pos, "While evaluating the argument passed to builtins.hasContext");
     v.mkBool(!context.empty());
 }
 
@@ -72,7 +72,7 @@ static void prim_getContext(EvalState & state, const Pos & pos, Value * * args,
         Strings outputs;
     };
     PathSet context;
-    state.forceString(*args[0], context, pos);
+    state.forceString(*args[0], context, pos, "While evaluating the argument passed to builtins.getContext");
     auto contextInfos = std::map<Path, ContextInfo>();
     for (const auto & p : context) {
         Path drv;
@@ -136,9 +136,9 @@ static RegisterPrimOp primop_getContext("__getContext", 1, prim_getContext);
 static void prim_appendContext(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
     PathSet context;
-    auto orig = state.forceString(*args[0], context, pos);
+    auto orig = state.forceString(*args[0], context, pos, "while evaluating the first argument passed to builtins.appendContext");
 
-    state.forceAttrs(*args[1], pos);
+    state.forceAttrs(*args[1], pos, "While evaluating the second argument passed to builtins.appendContext");
 
     auto sPath = state.symbols.create("path");
     auto sAllOutputs = state.symbols.create("allOutputs");
@@ -150,16 +150,16 @@ static void prim_appendContext(EvalState & state, const Pos & pos, Value * * arg
             });
         if (!settings.readOnlyMode)
             state.store->ensurePath(state.store->parseStorePath(i.name));
-        state.forceAttrs(*i.value, *i.pos);
+        state.forceAttrs(*i.value, *i.pos, "While evaluating the value of a string context");
         auto iter = i.value->attrs->find(sPath);
         if (iter != i.value->attrs->end()) {
-            if (state.forceBool(*iter->value, *iter->pos))
+            if (state.forceBool(*iter->value, *iter->pos, "While evaluating the `path` attribute of a string context"))
                 context.insert(i.name);
         }
 
         iter = i.value->attrs->find(sAllOutputs);
         if (iter != i.value->attrs->end()) {
-            if (state.forceBool(*iter->value, *iter->pos)) {
+            if (state.forceBool(*iter->value, *iter->pos, "While evaluating the `allOutputs` attribute of a string context")) {
                 if (!isDerivation(i.name)) {
                     throw EvalError({
                         .msg = hintfmt("Tried to add all-outputs context of %s, which is not a derivation, to a string", i.name),
@@ -172,7 +172,7 @@ static void prim_appendContext(EvalState & state, const Pos & pos, Value * * arg
 
         iter = i.value->attrs->find(state.sOutputs);
         if (iter != i.value->attrs->end()) {
-            state.forceList(*iter->value, *iter->pos);
+            state.forceList(*iter->value, *iter->pos, "While evaluating the `outputs` attribute of a string context");
             if (iter->value->listSize() && !isDerivation(i.name)) {
                 throw EvalError({
                     .msg = hintfmt("Tried to add derivation output context of %s, which is not a derivation, to a string", i.name),
@@ -180,7 +180,7 @@ static void prim_appendContext(EvalState & state, const Pos & pos, Value * * arg
                 });
             }
             for (auto elem : iter->value->listItems()) {
-                auto name = state.forceStringNoCtx(*elem, *iter->pos);
+                auto name = state.forceStringNoCtx(*elem, *iter->pos, "While evaluating an output name within a string context");
                 context.insert(concatStrings("!", name, "!", i.name));
             }
         }

src/libexpr/primops/fetchMercurial.cc

@@ -19,8 +19,6 @@ static void prim_fetchMercurial(EvalState & state, const Pos & pos, Value * * ar
 
     if (args[0]->type() == nAttrs) {
 
-        state.forceAttrs(*args[0], pos);
-
         for (auto & attr : *args[0]->attrs) {
             std::string_view n(attr.name);
             if (n == "url")
@@ -28,14 +26,14 @@ static void prim_fetchMercurial(EvalState & state, const Pos & pos, Value * * ar
             else if (n == "rev") {
                 // Ugly: unlike fetchGit, here the "rev" attribute can
                 // be both a revision or a branch/tag name.
-                auto value = state.forceStringNoCtx(*attr.value, *attr.pos);
+                auto value = state.forceStringNoCtx(*attr.value, *attr.pos, "While evaluating the `rev` attribute passed to builtins.fetchMercurial");
                 if (std::regex_match(value.begin(), value.end(), revRegex))
                     rev = Hash::parseAny(value, htSHA1);
                 else
                     ref = value;
             }
             else if (n == "name")
-                name = state.forceStringNoCtx(*attr.value, *attr.pos);
+                name = state.forceStringNoCtx(*attr.value, *attr.pos, "While evaluating the `name` attribute passed to builtins.fetchMercurial");
             else
                 throw EvalError({
                     .msg = hintfmt("unsupported argument '%s' to 'fetchMercurial'", attr.name),

src/libexpr/primops/fetchTree.cc

@@ -102,7 +102,7 @@ static void fetchTree(
     state.forceValue(*args[0], pos);
 
     if (args[0]->type() == nAttrs) {
-        state.forceAttrs(*args[0], pos);
+        state.forceAttrs(*args[0], pos, "While evaluating the argument passed to builtins.fetchTree");
 
         fetchers::Attrs attrs;
 
@@ -112,7 +112,7 @@ static void fetchTree(
                     .msg = hintfmt("unexpected attribute 'type'"),
                     .errPos = pos
                 });
-            type = state.forceStringNoCtx(*aType->value, *aType->pos);
+            type = state.forceStringNoCtx(*aType->value, *aType->pos, "While evaluating the `type` attribute passed to builtins.fetchTree");
         } else if (!type)
             throw Error({
                 .msg = hintfmt("attribute 'type' is missing in call to 'fetchTree'"),
@@ -195,16 +195,14 @@ static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
 
     if (args[0]->type() == nAttrs) {
 
-        state.forceAttrs(*args[0], pos);
-
         for (auto & attr : *args[0]->attrs) {
             std::string n(attr.name);
             if (n == "url")
-                url = state.forceStringNoCtx(*attr.value, *attr.pos);
+                url = state.forceStringNoCtx(*attr.value, *attr.pos, "While evaluating the url we should fetch");
             else if (n == "sha256")
-                expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, *attr.pos), htSHA256);
+                expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, *attr.pos, "While evaluating the sha256 of the content we should fetch"), htSHA256);
             else if (n == "name")
-                name = state.forceStringNoCtx(*attr.value, *attr.pos);
+                name = state.forceStringNoCtx(*attr.value, *attr.pos, "While evaluating the name of the content we should fetch");
             else
                 throw EvalError({
                     .msg = hintfmt("unsupported argument '%s' to '%s'", attr.name, who),
@@ -218,7 +216,7 @@ static void fetch(EvalState & state, const Pos & pos, Value * * args, Value & v,
                 .errPos = pos
             });
     } else
-        url = state.forceStringNoCtx(*args[0], pos);
+        url = state.forceStringNoCtx(*args[0], pos, "While evaluating the url we should fetch");
 
     url = resolveUri(*url);
 

src/libexpr/primops/fromTOML.cc

@@ -7,7 +7,7 @@ namespace nix {
 
 static void prim_fromTOML(EvalState & state, const Pos & pos, Value * * args, Value & val)
 {
-    auto toml = state.forceStringNoCtx(*args[0], pos);
+    auto toml = state.forceStringNoCtx(*args[0], pos, "While evaluating the argument passed to builtins.fromTOML");
 
     std::istringstream tomlStream(std::string{toml});
 

src/nix/bundle.cc

@@ -111,7 +111,7 @@ struct CmdBundle : InstallableCommand
 
         if (!outLink) {
             auto &attr = vRes->attrs->need(evalState->sName);
-            outLink = evalState->forceStringNoCtx(*attr.value,*attr.pos);
+            outLink = evalState->forceStringNoCtx(*attr.value, *attr.pos, "");
         }
 
         // TODO: will crash if not a localFSStore?

src/nix/flake.cc

@@ -126,12 +126,12 @@ static void enumerateOutputs(EvalState & state, Value & vFlake,
     std::function<void(const std::string & name, Value & vProvide, const Pos & pos)> callback)
 {
     auto pos = vFlake.determinePos(noPos);
-    state.forceAttrs(vFlake, pos);
+    state.forceAttrs(vFlake, pos, "While evaluating a flake to get its outputs");
 
     auto aOutputs = vFlake.attrs->get(state.symbols.create("outputs"));
     assert(aOutputs);
 
-    state.forceAttrs(*aOutputs->value, pos);
+    state.forceAttrs(*aOutputs->value, pos, "While evaluating the outputs of a flake");
 
     auto sHydraJobs = state.symbols.create("hydraJobs");
 
@@ -401,13 +401,13 @@ struct CmdFlakeCheck : FlakeCommand
 
         checkHydraJobs = [&](const std::string & attrPath, Value & v, const Pos & pos) {
             try {
-                state->forceAttrs(v, pos);
+                state->forceAttrs(v, pos, "");
 
                 if (state->isDerivation(v))
                     throw Error("jobset should not be a derivation at top-level");
 
                 for (auto & attr : *v.attrs) {
-                    state->forceAttrs(*attr.value, *attr.pos);
+                    state->forceAttrs(*attr.value, *attr.pos, "");
                     auto attrPath2 = attrPath + "." + (std::string) attr.name;
                     if (state->isDerivation(*attr.value)) {
                         Activity act(*logger, lvlChatty, actUnknown,
@@ -429,7 +429,7 @@ struct CmdFlakeCheck : FlakeCommand
                     fmt("checking NixOS configuration '%s'", attrPath));
                 Bindings & bindings(*state->allocBindings(0));
                 auto vToplevel = findAlongAttrPath(*state, "config.system.build.toplevel", bindings, v).first;
-                state->forceAttrs(*vToplevel, pos);
+                state->forceValue(*vToplevel, pos);
                 if (!state->isDerivation(*vToplevel))
                     throw Error("attribute 'config.system.build.toplevel' is not a derivation");
             } catch (Error & e) {
@@ -443,7 +443,7 @@ struct CmdFlakeCheck : FlakeCommand
                 Activity act(*logger, lvlChatty, actUnknown,
                     fmt("checking template '%s'", attrPath));
 
-                state->forceAttrs(v, pos);
+                state->forceAttrs(v, pos, "");
 
                 if (auto attr = v.attrs->get(state->symbols.create("path"))) {
                     if (attr->name == state->symbols.create("path")) {
@@ -457,7 +457,7 @@ struct CmdFlakeCheck : FlakeCommand
                     throw Error("template '%s' lacks attribute 'path'", attrPath);
 
                 if (auto attr = v.attrs->get(state->symbols.create("description")))
-                    state->forceStringNoCtx(*attr->value, *attr->pos);
+                    state->forceStringNoCtx(*attr->value, *attr->pos, "");
                 else
                     throw Error("template '%s' lacks attribute 'description'", attrPath);
 
@@ -513,10 +513,10 @@ struct CmdFlakeCheck : FlakeCommand
                             warn("flake output attribute '%s' is deprecated; use '%s' instead", name, replacement);
 
                         if (name == "checks") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 checkSystemName(attr.name, *attr.pos);
-                                state->forceAttrs(*attr.value, *attr.pos);
+                                state->forceAttrs(*attr.value, *attr.pos, "");
                                 for (auto & attr2 : *attr.value->attrs) {
                                     auto drvPath = checkDerivation(
                                         fmt("%s.%s.%s", name, attr.name, attr2.name),
@@ -528,10 +528,10 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "packages" || name == "devShells") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 checkSystemName(attr.name, *attr.pos);
-                                state->forceAttrs(*attr.value, *attr.pos);
+                                state->forceAttrs(*attr.value, *attr.pos, "");
                                 for (auto & attr2 : *attr.value->attrs)
                                     checkDerivation(
                                         fmt("%s.%s.%s", name, attr.name, attr2.name),
@@ -540,10 +540,10 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "apps") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 checkSystemName(attr.name, *attr.pos);
-                                state->forceAttrs(*attr.value, *attr.pos);
+                                state->forceAttrs(*attr.value, *attr.pos, "");
                                 for (auto & attr2 : *attr.value->attrs)
                                     checkApp(
                                         fmt("%s.%s.%s", name, attr.name, attr2.name),
@@ -552,7 +552,7 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "defaultPackage" || name == "devShell") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 checkSystemName(attr.name, *attr.pos);
                                 checkDerivation(
@@ -562,7 +562,7 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "defaultApp") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 checkSystemName(attr.name, *attr.pos);
                                 checkApp(
@@ -572,7 +572,7 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "legacyPackages") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 checkSystemName(attr.name, *attr.pos);
                                 // FIXME: do getDerivations?
@@ -583,7 +583,7 @@ struct CmdFlakeCheck : FlakeCommand
                             checkOverlay(name, vOutput, pos);
 
                         else if (name == "overlays") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs)
                                 checkOverlay(fmt("%s.%s", name, attr.name),
                                     *attr.value, *attr.pos);
@@ -593,14 +593,14 @@ struct CmdFlakeCheck : FlakeCommand
                             checkModule(name, vOutput, pos);
 
                         else if (name == "nixosModules") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs)
                                 checkModule(fmt("%s.%s", name, attr.name),
                                     *attr.value, *attr.pos);
                         }
 
                         else if (name == "nixosConfigurations") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs)
                                 checkNixOSConfiguration(fmt("%s.%s", name, attr.name),
                                     *attr.value, *attr.pos);
@@ -613,14 +613,14 @@ struct CmdFlakeCheck : FlakeCommand
                             checkTemplate(name, vOutput, pos);
 
                         else if (name == "templates") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs)
                                 checkTemplate(fmt("%s.%s", name, attr.name),
                                     *attr.value, *attr.pos);
                         }
 
                         else if (name == "defaultBundler") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 checkSystemName(attr.name, *attr.pos);
                                 checkBundler(
@@ -630,10 +630,10 @@ struct CmdFlakeCheck : FlakeCommand
                         }
 
                         else if (name == "bundlers") {
-                            state->forceAttrs(vOutput, pos);
+                            state->forceAttrs(vOutput, pos, "");
                             for (auto & attr : *vOutput.attrs) {
                                 checkSystemName(attr.name, *attr.pos);
-                                state->forceAttrs(*attr.value, *attr.pos);
+                                state->forceAttrs(*attr.value, *attr.pos, "");
                                 for (auto & attr2 : *attr.value->attrs) {
                                     checkBundler(
                                         fmt("%s.%s.%s", name, attr.name, attr2.name),

src/nix/main.cc

@@ -196,7 +196,7 @@ static void showHelp(std::vector<std::string> subcommand, MultiCommand & topleve
     if (!attr)
         throw UsageError("Nix has no subcommand '%s'", concatStringsSep("", subcommand));
 
-    auto markdown = state.forceString(*attr->value);
+    auto markdown = state.forceString(*attr->value, noPos, "While evaluating the lowdown help text");
 
     RunPager pager;
     std::cout << renderMarkdownToTerminal(markdown) << "\n";

src/nix/prefetch.cc

@@ -28,17 +28,17 @@ std::string resolveMirrorUrl(EvalState & state, const std::string & url)
     Value vMirrors;
     // FIXME: use nixpkgs flake
     state.eval(state.parseExprFromString("import <nixpkgs/pkgs/build-support/fetchurl/mirrors.nix>", "."), vMirrors);
-    state.forceAttrs(vMirrors, noPos);
+    state.forceAttrs(vMirrors, noPos, "While evaluating the set of all mirrors");
 
     auto mirrorList = vMirrors.attrs->find(state.symbols.create(mirrorName));
     if (mirrorList == vMirrors.attrs->end())
         throw Error("unknown mirror name '%s'", mirrorName);
-    state.forceList(*mirrorList->value, noPos);
+    state.forceList(*mirrorList->value, noPos, "While evaluating this mirror configuration");
 
     if (mirrorList->value->listSize() < 1)
         throw Error("mirror URL '%s' did not expand to anything", url);
 
-    std::string mirror(state.forceString(*mirrorList->value->listElems()[0]));
+    std::string mirror(state.forceString(*mirrorList->value->listElems()[0], noPos, "While evaluating the first available mirror"));
     return mirror + (hasSuffix(mirror, "/") ? "" : "/") + s.substr(p + 1);
 }
 
@@ -196,27 +196,27 @@ static int main_nix_prefetch_url(int argc, char * * argv)
             Value vRoot;
             state->evalFile(path, vRoot);
             Value & v(*findAlongAttrPath(*state, attrPath, autoArgs, vRoot).first);
-            state->forceAttrs(v, noPos);
+            state->forceAttrs(v, noPos, "While evaluating the source attribute to prefetch");
 
             /* Extract the URL. */
             auto & attr = v.attrs->need(state->symbols.create("urls"));
-            state->forceList(*attr.value, noPos);
+            state->forceList(*attr.value, noPos, "While evaluating the urls to prefetch");
             if (attr.value->listSize() < 1)
                 throw Error("'urls' list is empty");
-            url = state->forceString(*attr.value->listElems()[0]);
+            url = state->forceString(*attr.value->listElems()[0], noPos, "While evaluating the first url from the urls list");
 
             /* Extract the hash mode. */
             auto attr2 = v.attrs->get(state->symbols.create("outputHashMode"));
             if (!attr2)
                 printInfo("warning: this does not look like a fetchurl call");
             else
-                unpack = state->forceString(*attr2->value) == "recursive";
+                unpack = state->forceString(*attr2->value, noPos, "While evaluating the outputHashMode of the source to prefetch") == "recursive";
 
             /* Extract the name. */
             if (!name) {
                 auto attr3 = v.attrs->get(state->symbols.create("name"));
                 if (!attr3)
-                    name = state->forceString(*attr3->value);
+                    name = state->forceString(*attr3->value, noPos, "While evaluating the name of the source to prefetch");
             }
         }
 

src/nix/repl.cc

@@ -342,7 +342,7 @@ StringSet NixRepl::completePrefix(const std::string & prefix)
             Expr * e = parseString(expr);
             Value v;
             e->eval(*state, *env, v);
-            state->forceAttrs(v, noPos);
+            state->forceAttrs(v, noPos, "nevermind, it is ignored anyway");
 
             for (auto & i : *v.attrs) {
                 std::string name = i.name;
@@ -675,7 +675,7 @@ void NixRepl::reloadFiles()
 
 void NixRepl::addAttrsToScope(Value & attrs)
 {
-    state->forceAttrs(attrs, [&]() { return attrs.determinePos(noPos); });
+    state->forceAttrs(attrs, [&]() { return attrs.determinePos(noPos); }, "While evaluating an attribute set to be merged in the global scope");
     if (displ + attrs.attrs->size() >= envSize)
         throw Error("environment full; cannot add more variables");
 

src/nix/upgrade-nix.cc

@@ -144,7 +144,7 @@ struct CmdUpgradeNix : MixDryRun, StoreCommand
         Bindings & bindings(*state->allocBindings(0));
         auto v2 = findAlongAttrPath(*state, settings.thisSystem, bindings, *v).first;
 
-        return store->parseStorePath(state->forceString(*v2));
+        return store->parseStorePath(state->forceString(*v2, noPos, "While evaluating the path tho latest nix version"));
     }
 };