w

Author: lcnr

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -474,7 +474,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
         } else {
             let has_non_region_infer = stack.obligation.predicate.has_non_region_infer();
-            if let Some(candidate) = self.winnow_candidates(has_non_region_infer, candidates) {
+            let is_default_auto_trait = self
+                .tcx()
+                .as_lang_item(stack.obligation.predicate.def_id())
+                .is_some_and(|lang_item| matches!(lang_item, LangItem::DefaultTrait1));
+            if let Some(candidate) =
+                self.winnow_candidates(has_non_region_infer, is_default_auto_trait, candidates)
+            {
                 self.filter_reservation_impls(candidate)
             } else {
                 Ok(None)
@@ -1821,15 +1827,19 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
     fn winnow_candidates(
         &mut self,
         has_non_region_infer: bool,
+        is_default_auto_trait: bool,
         mut candidates: Vec<EvaluatedCandidate<'tcx>>,
     ) -> Option<SelectionCandidate<'tcx>> {
         if candidates.len() == 1 {
             return Some(candidates.pop().unwrap().candidate);
         }
 
         // We prefer `Sized` candidates over everything.
-        let mut sized_candidates =
-            candidates.iter().filter(|c| matches!(c.candidate, SizedCandidate));
+        let mut sized_candidates = candidates.iter().filter(|c| {
+            matches!(c.candidate, SizedCandidate)
+                || (is_default_auto_trait
+                    && matches!(c.candidate, AutoImplCandidate | ImplCandidate(..)))
+        });
         if let Some(sized_candidate) = sized_candidates.next() {
             // There should only ever be a single sized candidate
             // as they would otherwise overlap.