parser: merge nested dynamic attributes

Fixes https://github.com/NixOS/nix/issues/7115

doc/manual/src/release-notes/rl-next.md

@@ -6,3 +6,22 @@
 
 - Nix now allows unprivileged/[`allowed-users`](../command-ref/conf-file.md#conf-allowed-users) to sign paths.
   Previously, only [`trusted-users`](../command-ref/conf-file.md#conf-trusted-users) users could sign paths.
+
+- Nested dynamic attributes are now merged correctly by the parser. For example:
+
+  ```nix
+  {
+    nested = { foo = 1; };
+    nested = { ${"ba" + "r"} = 2; };
+  }
+  ```
+
+  This used to silently discard `nested.bar`, but now behaves as one would expect and evaluates to:
+
+  ```nix
+  { nested = { bar = 2; foo = 1; }; }
+  ```
+
+  Note that the feature of merging multiple attribute set declarations is of questionable value.
+  It allows writing expressions that are very hard to read, for instance when there are many lines of code between two declarations of the same attribute.
+  This has been around for a long time and is therefore supported for backwards compatibility, but should not be relied upon.

src/libexpr/parser.y

@@ -137,6 +137,7 @@ static void addAttr(ExprAttrs * attrs, AttrPath && attrPath,
                         dupAttr(state, ad.first, j2->second.pos, ad.second.pos);
                     jAttrs->attrs.emplace(ad.first, ad.second);
                 }
+                jAttrs->dynamicAttrs.insert(jAttrs->dynamicAttrs.end(), ae->dynamicAttrs.begin(), ae->dynamicAttrs.end());
             } else {
                 dupAttr(state, attrPath, pos, j->second.pos);
             }

tests/lang/eval-fail-dup-dynamic-attrs.err.exp

@@ -0,0 +1,8 @@
+error: dynamic attribute 'b' already defined at /pwd/lang/eval-fail-dup-dynamic-attrs.nix:2:11
+
+       at /pwd/lang/eval-fail-dup-dynamic-attrs.nix:3:11:
+
+            2|   set = { "${"" + "b"}" = 1; };
+            3|   set = { "${"b" + ""}" = 2; };
+             |           ^
+            4| }

tests/lang/eval-fail-dup-dynamic-attrs.nix

@@ -0,0 +1,4 @@
+{
+  set = { "${"" + "b"}" = 1; };
+  set = { "${"b" + ""}" = 2; };
+}

tests/lang/eval-okay-merge-dynamic-attrs.exp

@@ -0,0 +1 @@
+{ set1 = { a = 1; b = 2; }; set2 = { a = 1; b = 2; }; set3 = { a = 1; b = 2; }; set4 = { a = 1; b = 2; }; }

tests/lang/eval-okay-merge-dynamic-attrs.nix

@@ -0,0 +1,13 @@
+{
+  set1 = { a = 1; };
+  set1 = { "${"b" + ""}" = 2; };
+
+  set2 = { "${"b" + ""}" = 2; };
+  set2 = { a = 1; };
+
+  set3.a = 1;
+  set3."${"b" + ""}" = 2;
+
+  set4."${"b" + ""}" = 2;
+  set4.a = 1;
+}