nix::parser::State::addAttr: Improve error message

For some reason that eludes me, it used to only print the last attribute
instead of the full chain.

Change-Id: I2f5f6e85e2b1ab3ab20a9c99bf06d1e45a4d09bb

lix/libexpr/parser/state.hh

@@ -217,7 +217,8 @@ inline void State::mergeAttrs(AttrPath & attrPath, ExprSet * source, ExprSet * t
         if (auto collision = target->attrs.find(insertKey);
             collision != target->attrs.end()) // Attr already defined in target, error.
         {
-            return dupAttr(insertKey, insertDef.pos, collision->second.pos);
+            attrPath.push_back(AttrName(insertDef.pos, insertKey));
+            return dupAttr(attrPath, insertDef.pos, collision->second.pos);
         }
         if (insertDef.kind == ExprAttrs::AttrDef::Kind::InheritedFrom) {
             auto & sel = dynamic_cast<ExprSelect &>(*insertDef.e);

tests/functional2/lang/parser-merge-attrs/parse-fail-err1.err.exp

@@ -1,4 +1,4 @@
-error: attribute 'z' already defined at /pwd/in.nix:2:3
+error: attribute 'x.z' already defined at /pwd/in.nix:2:3
        at /pwd/in.nix:3:16:
             2|   x.z = 3;
             3|   x = { y = 3; z = 3; };

tests/functional2/lang/parser-merge-attrs/parse-fail-err2.err.exp

@@ -1,4 +1,4 @@
-error: attribute 'y' already defined at /pwd/in.nix:2:3
+error: attribute 'x.y' already defined at /pwd/in.nix:2:3
        at /pwd/in.nix:3:9:
             2|   x.y.y = 3;
             3|   x = { y.y= 3; z = 3; };