nix edit: Support non-derivation attributes

E.g.

  $ nix edit .#nixosConfigurations.bla

now works.

(cherry picked from commit d2032edb2f)

src/libexpr/attr-path.cc

@@ -103,7 +103,7 @@ Pos findDerivationFilename(EvalState & state, Value & v, std::string what)
         auto dummyArgs = state.allocBindings(0);
         v2 = findAlongAttrPath(state, "meta.position", *dummyArgs, v).first;
     } catch (Error &) {
-        throw Error("package '%s' has no source location information", what);
+        throw NoPositionInfo("package '%s' has no source location information", what);
     }
 
     // FIXME: is it possible to extract the Pos object instead of doing this

src/libexpr/attr-path.hh

@@ -8,6 +8,7 @@
 namespace nix {
 
 MakeError(AttrPathNotFound, Error);
+MakeError(NoPositionInfo, Error);
 
 std::pair<Value *, Pos> findAlongAttrPath(EvalState & state, const string & attrPath,
     Bindings & autoArgs, Value & vIn);

src/nix/command.hh

@@ -3,17 +3,12 @@
 #include "args.hh"
 #include "common-eval-args.hh"
 #include "path.hh"
+#include "eval.hh"
 
 namespace nix {
 
 extern std::string programPath;
 
-struct Value;
-class Bindings;
-class EvalState;
-struct Pos;
-class Store;
-
 /* A command that requires a Nix store. */
 struct StoreCommand : virtual Command
 {
@@ -48,7 +43,7 @@ struct Installable
 
     Buildable toBuildable();
 
-    virtual Value * toValue(EvalState & state)
+    virtual std::pair<Value *, Pos> toValue(EvalState & state)
     {
         throw Error("argument '%s' cannot be evaluated", what());
     }

src/nix/edit.cc

@@ -29,9 +29,15 @@ struct CmdEdit : InstallableCommand
     {
         auto state = getEvalState();
 
-        auto v = installable->toValue(*state);
+        auto [v, pos] = installable->toValue(*state);
 
-        Pos pos = findDerivationFilename(*state, *v, installable->what());
+        try {
+            pos = findDerivationFilename(*state, *v, installable->what());
+        } catch (NoPositionInfo &) {
+        }
+
+        if (pos == noPos)
+            throw Error("cannot find position information for '%s", installable->what());
 
         stopProgressBar();
 

src/nix/eval.cc

@@ -52,7 +52,7 @@ struct CmdEval : MixJSON, InstallableCommand
 
         auto state = getEvalState();
 
-        auto v = installable->toValue(*state);
+        auto v = installable->toValue(*state).first;
         PathSet context;
 
         stopProgressBar();

src/nix/installables.cc

@@ -121,7 +121,7 @@ struct InstallableValue : Installable
     {
         auto state = cmd.getEvalState();
 
-        auto v = toValue(*state);
+        auto v = toValue(*state).first;
 
         Bindings & autoArgs = *cmd.getAutoArgs(*state);
 
@@ -169,11 +169,11 @@ struct InstallableExpr : InstallableValue
 
     std::string what() override { return text; }
 
-    Value * toValue(EvalState & state) override
+    std::pair<Value *, Pos> toValue(EvalState & state) override
     {
         auto v = state.allocValue();
         state.eval(state.parseExprFromString(text, absPath(".")), *v);
-        return v;
+        return {v, noPos};
     }
 };
 
@@ -187,7 +187,7 @@ struct InstallableAttrPath : InstallableValue
 
     std::string what() override { return attrPath; }
 
-    Value * toValue(EvalState & state) override
+    std::pair<Value *, Pos> toValue(EvalState & state) override
     {
         auto source = cmd.getSourceExpr(state);
 
@@ -196,7 +196,7 @@ struct InstallableAttrPath : InstallableValue
         auto v = findAlongAttrPath(state, attrPath, autoArgs, *source).first;
         state.forceValue(*v);
 
-        return v;
+        return {v, noPos};
     }
 };