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

Closes #845

Change-Id: Ie14d0e5a7a9fb778325c4ad30d1e1bd73c60b4f9

lix/libexpr/parser/state.hh

@@ -214,11 +214,20 @@ inline void State::mergeAttrs(AttrPath & attrPath, ExprSet * source, ExprSet * t
         target->inheritFromExprs = std::make_unique<std::list<std::unique_ptr<Expr>>>();
     }
     for (auto & [insertKey, insertDef] : source->attrs) {
-        if (auto collision = target->attrs.find(insertKey);
-            collision != target->attrs.end()) // Attr already defined in target, error.
-        {
+        if (auto collision = target->attrs.find(insertKey); collision != target->attrs.end()) {
+            // Attr already defined in target, recurse merge if possible otherwise error.
+            auto * collisionInsert = dynamic_cast<ExprSet *>(insertDef.e.get());
+            auto * collisionTarget = dynamic_cast<ExprSet *>(collision->second.e.get());
+            if (!collisionInsert || !collisionTarget) {
+                attrPath.push_back(AttrName(insertDef.pos, insertKey));
+                return dupAttr(attrPath, insertDef.pos, collision->second.pos);
+            }
+
+            // Push insertKey to the attrPath for error propagation (pop afterwards), then recurse
+            // merge
             attrPath.push_back(AttrName(insertDef.pos, insertKey));
-            return dupAttr(attrPath, insertDef.pos, collision->second.pos);
+            mergeAttrs(attrPath, collisionInsert, collisionTarget);
+            attrPath.pop_back();
         }
         if (insertDef.kind == ExprAttrs::AttrDef::Kind::InheritedFrom) {
             auto & sel = dynamic_cast<ExprSelect &>(*insertDef.e);

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; };