nix edit: Support non-derivation attributes

E.g.

  $ nix edit .#nixosConfigurations.bla

now works.

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/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

@@ -130,7 +130,7 @@ App::App(EvalState & state, Value & vApp)
 
 App Installable::toApp(EvalState & state)
 {
-    return App(state, *toValue(state));
+    return App(state, *toValue(state).first);
 }
 
 struct InstallableStorePath : Installable
@@ -166,7 +166,7 @@ std::vector<flake::EvalCache::Derivation> InstallableValue::toDerivations()
 {
     auto state = cmd.getEvalState();
 
-    auto v = toValue(*state);
+    auto v = toValue(*state).first;
 
     Bindings & autoArgs = *cmd.getAutoArgs(*state);
 
@@ -229,11 +229,11 @@ 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};
     }
 };
 
@@ -248,11 +248,11 @@ struct InstallableAttrPath : InstallableValue
 
     std::string what() override { return attrPath; }
 
-    Value * toValue(EvalState & state) override
+    std::pair<Value *, Pos> toValue(EvalState & state) override
     {
-        auto vRes = findAlongAttrPath(state, attrPath, *cmd.getAutoArgs(state), *v).first;
+        auto [vRes, pos] = findAlongAttrPath(state, attrPath, *cmd.getAutoArgs(state), *v);
         state.forceValue(*vRes);
-        return vRes;
+        return {vRes, pos};
     }
 };
 
@@ -391,7 +391,7 @@ std::vector<flake::EvalCache::Derivation> InstallableFlake::toDerivations()
     return res;
 }
 
-Value * InstallableFlake::toValue(EvalState & state)
+std::pair<Value *, Pos> InstallableFlake::toValue(EvalState & state)
 {
     auto lockedFlake = lockFlake(state, flakeRef, cmd.lockFlags);
 
@@ -401,9 +401,9 @@ Value * InstallableFlake::toValue(EvalState & state)
 
     for (auto & attrPath : getActualAttrPaths()) {
         try {
-            auto * v = findAlongAttrPath(state, attrPath, *emptyArgs, *vOutputs).first;
+            auto [v, pos] = findAlongAttrPath(state, attrPath, *emptyArgs, *vOutputs);
             state.forceValue(*v);
-            return v;
+            return {v, pos};
         } catch (AttrPathNotFound & e) {
         }
     }

src/nix/installables.hh

@@ -3,14 +3,13 @@
 #include "util.hh"
 #include "path.hh"
 #include "flake/eval-cache.hh"
+#include "eval.hh"
 
 #include <optional>
 
 namespace nix {
 
-struct Value;
 struct DrvInfo;
-class EvalState;
 struct SourceExprCommand;
 
 struct Buildable
@@ -45,7 +44,7 @@ struct Installable
 
     App toApp(EvalState & state);
 
-    virtual Value * toValue(EvalState & state)
+    virtual std::pair<Value *, Pos> toValue(EvalState & state)
     {
         throw Error("argument '%s' cannot be evaluated", what());
     }
@@ -91,7 +90,7 @@ struct InstallableFlake : InstallableValue
 
     std::vector<flake::EvalCache::Derivation> toDerivations() override;
 
-    Value * toValue(EvalState & state) override;
+    std::pair<Value *, Pos> toValue(EvalState & state) override;
 };
 
 }