fix(3p/nix/expr): use traceable_allocator for btree_map r/1530

The prior use of gc_allocator meant that the btree nodes themselves were being collected. Additionally, have Attr (contains a Value) and Bindings explicitly inherit from gc, even though Bindings is always allocated under `new (GC)`.

Detected by running under GC_ENABLE_INCREMENTAL=1.

Change-Id: Iacf13b34b5aa12e417ea87c9b46e2bf9199fdb26
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1544
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

1 files changed, 3 insertions, 3 deletions

diff --git a/third_party/nix/src/libexpr/attr-set.hh b/third_party/nix/src/libexpr/attr-set.hh
index b918bf30ff..a9db0db856 100644
--- a/third_party/nix/src/libexpr/attr-set.hh
+++ b/third_party/nix/src/libexpr/attr-set.hh
@@ -14,7 +14,7 @@ class EvalState;
 struct Value;
 
 /* Map one attribute name to its value. */
-struct Attr {
+struct Attr : public gc {
   Symbol name;
   Value* value;  // TODO(tazjin): Who owns this?
   Pos* pos;      // TODO(tazjin): Who owns this?
@@ -26,9 +26,9 @@ struct Attr {
 // allocator explicitly specified.
 using AttributeMap =
     absl::btree_map<Symbol, Attr, std::less<Symbol>,
-                    gc_allocator<std::pair<const Symbol, Attr>>>;
+                    traceable_allocator<std::pair<const Symbol, Attr>>>;
 
-class Bindings {
+class Bindings : public gc {
  public:
   typedef AttributeMap::iterator iterator;