nix::parser::State::addAttr: Deep attribute merging

Closes #845

Change-Id: Ie14d0e5a7a9fb778325c4ad30d1e1bd73c60b4f9

tests/functional2/lang/parser-merge-attrs/eval-okay-deep.out.exp

@@ -0,0 +1 @@
+{ foo = { bar = { baz = 1; qux = 2; }; }; }

tests/functional2/lang/parser-merge-attrs/in-deep.nix

@@ -0,0 +1,20 @@
+# Deep recursive attr merges
+# Regression test for https://git.lix.systems/lix-project/lix/issues/845 / https://github.com/NixOS/nix/issues/11268
+
+# Also this is an eval test and not a parser test because the printed value is easier to inspect than the AST,
+# though we are primarily testing parser functionality here
+let
+  reference = { a.b.c = 1; a.b.d = 2; };
+in
+# Test cases courtesy of @rhendric
+assert { a = { b = { c = 1; }; }; a = { b = { d = 2; }; }; } == reference;
+assert { a.b = { c = 1; }; a.b = { d = 2; }; } == reference;
+assert { a = { b.c = 1; }; a = { b.d = 2; }; } == reference;
+assert { a = { b = { c = 1; }; }; a.b.d = 2; } == reference;
+assert { a.b.c = 1; a = { b = { d = 2; }; }; } == reference;
+{
+  foo.bar.baz = 1;
+  foo = {
+    bar.qux = 2;
+  };
+}

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

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