Auto merge of #113312 - Ddystopia:auto-trait-fun, r=lcnr

discard default auto trait impls if explicit ones exist (rebase of #85048)

Rebase of #85048

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

@@ -1783,6 +1783,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
     /// candidates and prefer where-clause candidates.
     ///
     /// See the comment for "SelectionCandidate" for more details.
+    #[instrument(level = "debug", skip(self))]
     fn candidate_should_be_dropped_in_favor_of(
         &mut self,
         victim: &EvaluatedCandidate<'tcx>,
@@ -1806,13 +1807,6 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
         // This is a fix for #53123 and prevents winnowing from accidentally extending the
         // lifetime of a variable.
         match (&other.candidate, &victim.candidate) {
-            (_, AutoImplCandidate) | (AutoImplCandidate, _) => {
-                bug!(
-                    "default implementations shouldn't be recorded \
-                    when there are other valid candidates"
-                );
-            }
-
             // FIXME(@jswrenn): this should probably be more sophisticated
             (TransmutabilityCandidate, _) | (_, TransmutabilityCandidate) => DropVictim::No,
 
@@ -1854,6 +1848,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             (
                 ParamCandidate(ref other_cand),
                 ImplCandidate(..)
+                | AutoImplCandidate
                 | ClosureCandidate { .. }
                 | GeneratorCandidate
                 | FutureCandidate
@@ -1881,6 +1876,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             }
             (
                 ImplCandidate(_)
+                | AutoImplCandidate
                 | ClosureCandidate { .. }
                 | GeneratorCandidate
                 | FutureCandidate
@@ -1914,6 +1910,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             (
                 ObjectCandidate(_) | ProjectionCandidate(..),
                 ImplCandidate(..)
+                | AutoImplCandidate
                 | ClosureCandidate { .. }
                 | GeneratorCandidate
                 | FutureCandidate
@@ -1927,6 +1924,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
 
             (
                 ImplCandidate(..)
+                | AutoImplCandidate
                 | ClosureCandidate { .. }
                 | GeneratorCandidate
                 | FutureCandidate
@@ -2017,6 +2015,19 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
                 }
             }
 
+            (AutoImplCandidate, ImplCandidate(_)) | (ImplCandidate(_), AutoImplCandidate) => {
+                DropVictim::No
+            }
+
+            (AutoImplCandidate, _) | (_, AutoImplCandidate) => {
+                bug!(
+                    "default implementations shouldn't be recorded \
+                    when there are other global candidates: {:?} {:?}",
+                    other,
+                    victim
+                );
+            }
+
             // Everything else is ambiguous
             (
                 ImplCandidate(_)