forceValue: make pos mandatory

- Make passing the position to `forceValue` mandatory,
  this way we remember people that the position is
  important for better error messages
- Add pos to all `forceValue` calls

src/libexpr/attr-path.cc

@@ -58,7 +58,7 @@ std::pair<Value *, Pos> findAlongAttrPath(EvalState & state, const string & attr
         Value * vNew = state.allocValue();
         state.autoCallFunction(autoArgs, *v, *vNew);
         v = vNew;
-        state.forceValue(*v);
+        state.forceValue(*v, v->determinePos(vIn.determinePos(noPos)));
 
         /* It should evaluate to either a set or an expression,
            according to what is specified in the attrPath. */

src/libexpr/eval-cache.cc

@@ -381,7 +381,7 @@ Value & AttrCursor::forceValue()
     auto & v = getValue();
 
     try {
-        root->state.forceValue(v);
+        root->state.forceValue(v, v.determinePos(noPos));
     } catch (EvalError &) {
         debug("setting '%s' to failed", getAttrPathStr());
         if (root->db)

src/libexpr/eval-inline.hh

@@ -53,7 +53,7 @@ void EvalState::forceValue(Value & v, const Pos & pos)
 
 inline void EvalState::forceAttrs(Value & v)
 {
-    forceValue(v);
+    forceValue(v, v.determinePos(noPos));
     if (v.type() != nAttrs)
         throwTypeError("value is %1% while a set was expected", v);
 }
@@ -69,7 +69,7 @@ inline void EvalState::forceAttrs(Value & v, const Pos & pos)
 
 inline void EvalState::forceList(Value & v)
 {
-    forceValue(v);
+    forceValue(v, v.determinePos(noPos));
     if (!v.isList())
         throwTypeError("value is %1% while a list was expected", v);
 }

src/libexpr/eval.cc

@@ -1280,7 +1280,7 @@ void ExprOpHasAttr::eval(EvalState & state, Env & env, Value & v)
     e->eval(state, env, vTmp);
 
     for (auto & i : attrPath) {
-        state.forceValue(*vAttrs);
+        state.forceValue(*vAttrs, vAttrs->determinePos(noPos));
         Bindings::iterator j;
         Symbol name = getName(i, state, env);
         if (vAttrs->type() != nAttrs ||
@@ -1494,14 +1494,15 @@ void EvalState::incrFunctionCall(ExprLambda * fun)
 
 void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
 {
-    forceValue(fun);
+    Pos pos = fun.determinePos(noPos);
+    forceValue(fun, pos);
 
     if (fun.type() == nAttrs) {
         auto found = fun.attrs->find(sFunctor);
         if (found != fun.attrs->end()) {
             Value * v = allocValue();
-            callFunction(*found->value, fun, *v, noPos);
-            forceValue(*v);
+            callFunction(*found->value, fun, *v, pos);
+            forceValue(*v, pos);
             return autoCallFunction(args, *v, res);
         }
     }
@@ -1787,7 +1788,7 @@ void EvalState::forceValueDeep(Value & v)
     recurse = [&](Value & v) {
         if (!seen.insert(&v).second) return;
 
-        forceValue(v);
+        forceValue(v, v.determinePos(noPos));
 
         if (v.type() == nAttrs) {
             for (auto & i : *v.attrs)
@@ -1924,7 +1925,7 @@ bool EvalState::isDerivation(Value & v)
     if (v.type() != nAttrs) return false;
     Bindings::iterator i = v.attrs->find(sType);
     if (i == v.attrs->end()) return false;
-    forceValue(*i->value);
+    forceValue(*i->value, *i->pos);
     if (i->value->type() != nString) return false;
     return strcmp(i->value->string.s, "derivation") == 0;
 }
@@ -2036,8 +2037,8 @@ Path EvalState::coerceToPath(const Pos & pos, Value & v, PathSet & context)
 
 bool EvalState::eqValues(Value & v1, Value & v2)
 {
-    forceValue(v1);
-    forceValue(v2);
+    forceValue(v1, v1.determinePos(noPos));
+    forceValue(v2, v2.determinePos(noPos));
 
     /* !!! Hack to support some old broken code that relies on pointer
        equality tests between sets.  (Specifically, builderDefs calls

src/libexpr/eval.hh

@@ -221,7 +221,7 @@ public:
        of the evaluation of the thunk.  If `v' is a delayed function
        application, call the function and overwrite `v' with the
        result.  Otherwise, this is a no-op. */
-    inline void forceValue(Value & v, const Pos & pos = noPos);
+    inline void forceValue(Value & v, const Pos & pos);
 
     /* Force a value, then recursively force list elements and
        attributes. */

src/libexpr/get-drvs.cc

@@ -172,7 +172,7 @@ StringSet DrvInfo::queryMetaNames()
 
 bool DrvInfo::checkMeta(Value & v)
 {
-    state->forceValue(v);
+    state->forceValue(v, 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);
+        state.forceValue(v, v.determinePos(noPos));
         if (!state.isDerivation(v)) return true;
 
         /* Remove spurious duplicates (e.g., a set like `rec { x =