trait_sel: don't require predicates of aliases hold

No longer require that we prove that the predicates of aliases hold when
checking the well-formedness of the alias. This permits more uses of GATs
and changes the output of yet more tests.

Author: davidtwco

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

@@ -144,8 +144,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         obligation: &PolyTraitObligation<'tcx>,
         idx: usize,
     ) -> Result<PredicateObligations<'tcx>, SelectionError<'tcx>> {
-        let tcx = self.tcx();
-
         let placeholder_trait_predicate =
             self.infcx.enter_forall_and_leak_universe(obligation.predicate).trait_ref;
         let placeholder_self_ty = self.infcx.shallow_resolve(placeholder_trait_predicate.self_ty());
@@ -194,28 +192,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 .map_err(|_| SelectionError::Unimplemented)?,
         );
 
-        // FIXME(compiler-errors): I don't think this is needed.
-        if let ty::Alias(ty::Projection, alias_ty) = placeholder_self_ty.kind() {
-            let predicates = tcx.predicates_of(alias_ty.def_id).instantiate_own(tcx, alias_ty.args);
-            for (predicate, _) in predicates {
-                let normalized = normalize_with_depth_to(
-                    self,
-                    obligation.param_env,
-                    obligation.cause.clone(),
-                    obligation.recursion_depth + 1,
-                    predicate,
-                    &mut obligations,
-                );
-                obligations.push(Obligation::with_depth(
-                    self.tcx(),
-                    obligation.cause.clone(),
-                    obligation.recursion_depth + 1,
-                    obligation.param_env,
-                    normalized,
-                ));
-            }
-        }
-
         Ok(obligations)
     }
 

tests/ui/async-await/higher-ranked-auto-trait-14.no_assumptions.stderr

@@ -3,7 +3,7 @@
 //@ revisions: assumptions no_assumptions
 //@[assumptions] compile-flags: -Zhigher-ranked-assumptions
 //@[assumptions] check-pass
-//@[no_assumptions] known-bug: #110338
+//@[no_assumptions] check-pass
 
 use std::collections::HashSet;
 use std::future::Future;

tests/ui/async-await/higher-ranked-auto-trait-14.rs

@@ -3,7 +3,7 @@
 //@ revisions: assumptions no_assumptions
 //@[assumptions] compile-flags: -Zhigher-ranked-assumptions
 //@[assumptions] check-pass
-//@[no_assumptions] known-bug: #110338
+//@[no_assumptions] check-pass
 
 use std::future::Future;