Auto merge of #138249 - compiler-errors:auto-self, r=lcnr

Do not register `Self: AutoTrait` when confirming auto trait (in old solver)

Every built-in auto impl for a trait goal like `Ty: Auto` immediately registers another obligation of `Ty: Auto` as one of its nested obligations, leading to us stressing the cycle detection machinery a lot more than we need to. This is because all traits have a `Self: Trait` predicate.

To fix this, remove the call to `impl_or_trait_obligations` in `vtable_auto_impl`, since auto traits do not have where clauses.

r? lcnr

1 files changed, 1 insertions, 15 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 6db97fc321a..4cd6781ab89 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -463,18 +463,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             let cause = obligation.derived_cause(ObligationCauseCode::BuiltinDerived);
 
             assert_eq!(obligation.predicate.polarity(), ty::PredicatePolarity::Positive);
-            let trait_ref =
-                self.infcx.enter_forall_and_leak_universe(obligation.predicate).trait_ref;
-            let trait_obligations = self.impl_or_trait_obligations(
-                &cause,
-                obligation.recursion_depth + 1,
-                obligation.param_env,
-                trait_def_id,
-                trait_ref.args,
-                obligation.predicate,
-            );
 
-            let mut obligations = self.collect_predicates_for_types(
+            let obligations = self.collect_predicates_for_types(
                 obligation.param_env,
                 cause,
                 obligation.recursion_depth + 1,
@@ -482,10 +472,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 nested,
             );
 
-            // Adds the predicates from the trait. Note that this contains a `Self: Trait`
-            // predicate as usual. It won't have any effect since auto traits are coinductive.
-            obligations.extend(trait_obligations);
-
             debug!(?obligations, "vtable_auto_impl");
 
             obligations