Make most calls to determinePos() lazy

src/libcmd/installables.cc

@@ -497,7 +497,7 @@ Value * InstallableFlake::getFlakeOutputs(EvalState & state, const flake::Locked
     auto aOutputs = vFlake->attrs->get(state.symbols.create("outputs"));
     assert(aOutputs);
 
-    state.forceValue(*aOutputs->value, aOutputs->value->determinePos(noPos));
+    state.forceValue(*aOutputs->value, [&]() { return aOutputs->value->determinePos(noPos); });
 
     return aOutputs->value;
 }

src/libexpr/eval-inline.hh

@@ -15,12 +15,6 @@ LocalNoInlineNoReturn(void throwEvalError(const Pos & pos, const char * s))
     });
 }
 
-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({
@@ -31,6 +25,13 @@ LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const
 
 
 void EvalState::forceValue(Value & v, const Pos & pos)
+{
+    forceValue(v, [&]() { return pos; });
+}
+
+
+template<typename Callable>
+void EvalState::forceValue(Value & v, Callable getPos)
 {
     if (v.isThunk()) {
         Env * env = v.thunk.env;
@@ -47,15 +48,22 @@ void EvalState::forceValue(Value & v, const Pos & pos)
     else if (v.isApp())
         callFunction(*v.app.left, *v.app.right, v, noPos);
     else if (v.isBlackhole())
-        throwEvalError(pos, "infinite recursion encountered");
+        throwEvalError(getPos(), "infinite recursion encountered");
 }
 
 
 inline void EvalState::forceAttrs(Value & v, const Pos & pos)
 {
-    forceValue(v, pos);
+    forceAttrs(v, [&]() { return pos; });
+}
+
+
+template <typename Callable>
+inline void EvalState::forceAttrs(Value & v, Callable getPos)
+{
+    forceValue(v, getPos);
     if (v.type() != nAttrs)
-        throwTypeError(pos, "value is %1% while a set was expected", v);
+        throwTypeError(getPos(), "value is %1% while a set was expected", v);
 }
 
 

src/libexpr/eval.cc

@@ -219,7 +219,7 @@ string showType(const Value & v)
     }
 }
 
-Pos Value::determinePos(const Pos &pos) const
+Pos Value::determinePos(const Pos & pos) const
 {
     switch (internalType) {
         case tAttrs: return *attrs->pos;
@@ -754,6 +754,11 @@ LocalNoInlineNoReturn(void throwTypeError(const Pos & pos, const char * s, const
     });
 }
 
+LocalNoInlineNoReturn(void throwTypeError(const char * s, const Value & v))
+{
+    throw TypeError(s, showType(v));
+}
+
 LocalNoInlineNoReturn(void throwAssertionError(const Pos & pos, const char * s, const string & s1))
 {
     throw AssertionError({
@@ -1138,7 +1143,7 @@ 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, vOverrides->determinePos(noPos));
+            state.forceAttrs(*vOverrides, [&]() { return vOverrides->determinePos(noPos); });
             Bindings * newBnds = state.allocBindings(v.attrs->capacity() + vOverrides->attrs->size());
             for (auto & i : *v.attrs)
                 newBnds->push_back(i);
@@ -1500,7 +1505,8 @@ void EvalState::incrFunctionCall(ExprLambda * fun)
 
 void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
 {
-    Pos pos = fun.determinePos(noPos);
+    auto pos = fun.determinePos(noPos);
+
     forceValue(fun, pos);
 
     if (fun.type() == nAttrs) {
@@ -1797,7 +1803,7 @@ void EvalState::forceValueDeep(Value & v)
     recurse = [&](Value & v) {
         if (!seen.insert(&v).second) return;
 
-        forceValue(v, v.determinePos(noPos));
+        forceValue(v, [&]() { return v.determinePos(noPos); });
 
         if (v.type() == nAttrs) {
             for (auto & i : *v.attrs)

src/libexpr/eval.hh

@@ -224,6 +224,9 @@ public:
        result.  Otherwise, this is a no-op. */
     inline void forceValue(Value & v, const Pos & pos);
 
+    template <typename Callable>
+    inline void forceValue(Value & v, Callable getPos);
+
     /* Force a value, then recursively force list elements and
        attributes. */
     void forceValueDeep(Value & v);
@@ -232,7 +235,12 @@ public:
     NixInt forceInt(Value & v, const Pos & pos);
     NixFloat forceFloat(Value & v, const Pos & pos);
     bool forceBool(Value & v, const Pos & pos);
-    inline void forceAttrs(Value & v, const Pos & pos);
+
+    void forceAttrs(Value & v, const Pos & pos);
+
+    template <typename Callable>
+    inline void forceAttrs(Value & v, Callable getPos);
+
     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);

src/libexpr/get-drvs.cc

@@ -172,7 +172,7 @@ StringSet DrvInfo::queryMetaNames()
 
 bool DrvInfo::checkMeta(Value & v)
 {
-    state->forceValue(v, v.determinePos(noPos));
+    state->forceValue(v, [&]() { return v.determinePos(noPos); });
     if (v.type() == nList) {
         for (auto elem : v.listItems())
             if (!checkMeta(*elem)) return false;
@@ -278,7 +278,7 @@ static bool getDerivation(EvalState & state, Value & v,
     bool ignoreAssertionFailures)
 {
     try {
-        state.forceValue(v, v.determinePos(noPos));
+        state.forceValue(v, [&]() { return v.determinePos(noPos); });
         if (!state.isDerivation(v)) return true;
 
         /* Remove spurious duplicates (e.g., a set like `rec { x =

src/libexpr/value.hh

@@ -361,7 +361,7 @@ public:
         return internalType == tList1 ? 1 : internalType == tList2 ? 2 : bigList.size;
     }
 
-    Pos determinePos(const Pos &pos) const;
+    Pos determinePos(const Pos & pos) const;
 
     /* Check whether forcing this value requires a trivial amount of
        computation. In particular, function applications are

src/nix-build/nix-build.cc

@@ -318,7 +318,7 @@ static void main_nix_build(int argc, char * * argv)
 
         for (auto & i : attrPaths) {
             Value & v(*findAlongAttrPath(*state, i, *autoArgs, vRoot).first);
-            state->forceValue(v, v.determinePos(noPos));
+            state->forceValue(v, [&]() { return v.determinePos(noPos); });
             getDerivations(*state, v, "", *autoArgs, drvs, false);
         }
     }

src/nix-env/user-env.cc

@@ -129,7 +129,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
 
     /* Evaluate it. */
     debug("evaluating user environment builder");
-    state.forceValue(topLevel, topLevel.determinePos(noPos));
+    state.forceValue(topLevel, [&]() { return topLevel.determinePos(noPos); });
     PathSet context;
     Attr & aDrvPath(*topLevel.attrs->find(state.sDrvPath));
     auto topLevelDrv = state.store->parseStorePath(state.coerceToPath(*aDrvPath.pos, *aDrvPath.value, context));

src/nix-instantiate/nix-instantiate.cc

@@ -40,7 +40,7 @@ void processExpr(EvalState & state, const Strings & attrPaths,
 
     for (auto & i : attrPaths) {
         Value & v(*findAlongAttrPath(state, i, autoArgs, vRoot).first);
-        state.forceValue(v, v.determinePos(noPos));
+        state.forceValue(v, [&]() { return v.determinePos(noPos); });
 
         PathSet context;
         if (evalOnly) {

src/nix/repl.cc

@@ -673,7 +673,7 @@ void NixRepl::reloadFiles()
 
 void NixRepl::addAttrsToScope(Value & attrs)
 {
-    state->forceAttrs(attrs, attrs.determinePos(noPos));
+    state->forceAttrs(attrs, [&]() { return attrs.determinePos(noPos); });
     if (displ + attrs.attrs->size() >= envSize)
         throw Error("environment full; cannot add more variables");
 
@@ -712,7 +712,7 @@ void NixRepl::evalString(string s, Value & v)
 {
     Expr * e = parseString(s);
     e->eval(*state, *env, v);
-    state->forceValue(v, v.determinePos(noPos));
+    state->forceValue(v, [&]() { return v.determinePos(noPos); });
 }
 
 
@@ -742,7 +742,7 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
     str.flush();
     checkInterrupt();
 
-    state->forceValue(v, v.determinePos(noPos));
+    state->forceValue(v, [&]() { return v.determinePos(noPos); });
 
     switch (v.type()) {