diagnose duplicated attrs at correct path

diagnose attr duplication at the path the duplication was detected, not
at the path the current attribute wanted to place. doing the latter is
only correct if a leaf attribute was duplicated, not if an attrpath was
set to a non-attrset in one binding and a (potentially implied) attrset
in another binding.

fixes #124

Change-Id: Ic4aa9cc12a9874d4e7897c6f64408f10aa36fc82

1 files changed, 22 insertions, 0 deletions

diff --git a/doc/manual/rl-next/dup-attr-errors.md b/doc/manual/rl-next/dup-attr-errors.md
new file mode 100644
index 000000000..127c8ceb1
--- /dev/null
+++ b/doc/manual/rl-next/dup-attr-errors.md
@@ -0,0 +1,22 @@
+---
+synopsis: Duplicate attribute reports are more accurate
+# prs: cl 557
+---
+
+Duplicate attribute errors are now more accurate, showing the path at which an error was detected rather than the full, possibly longer, path that caused the error.
+Error reports are now
+```ShellSession
+$ nix eval --expr '{ a.b = 1; a.b.c.d = 1; }'
+error: attribute 'a.b' already defined at «string»:1:3
+       at «string»:1:12:
+            1| { a.b = 1; a.b.c.d = 1;
+             |            ^
+```
+instead of
+```ShellSession
+$ nix eval --expr '{ a.b = 1; a.b.c.d = 1; }'
+error: attribute 'a.b.c.d' already defined at «string»:1:3
+       at «string»:1:12:
+            1| { a.b = 1; a.b.c.d = 1;
+             |            ^
+```