Auto merge of rust-lang#140978 - davidtwco:deep-reject-in-match-norm-trait-ref, r=<try>

trait_sel: deep reject `match_normalize_trait_ref`

Spotted during an in-person review of rust-lang#137944 at RustWeek: `match_normalize_trait_ref` could be using `DeepRejectCtxt` to exit early as an optimisation for projection candidates, like is done with param candidates.

r? `@lcnr`
cc `@oli-obk`

Author: bors

compiler/rustc_middle/src/ty/mod.rs

@@ -52,6 +52,7 @@ pub use rustc_session::lint::RegisteredTools;
 use rustc_span::hygiene::MacroKind;
 use rustc_span::{DUMMY_SP, ExpnId, ExpnKind, Ident, Span, Symbol, kw, sym};
 pub use rustc_type_ir::data_structures::{DelayedMap, DelayedSet};
+pub use rustc_type_ir::fast_reject::DeepRejectCtxt;
 #[allow(
     hidden_glob_reexports,
     rustc::usage_of_type_ir_inherent,

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

@@ -27,8 +27,8 @@ use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::error::TypeErrorToStringExt;
 use rustc_middle::ty::print::{PrintTraitRefExt as _, with_no_trimmed_paths};
 use rustc_middle::ty::{
-    self, GenericArgsRef, PolyProjectionPredicate, Ty, TyCtxt, TypeFoldable, TypeVisitableExt,
-    TypingMode, Upcast, elaborate,
+    self, DeepRejectCtxt, GenericArgsRef, PolyProjectionPredicate, Ty, TyCtxt, TypeFoldable,
+    TypeVisitableExt, TypingMode, Upcast, elaborate,
 };
 use rustc_span::{Symbol, sym};
 use tracing::{debug, instrument, trace};
@@ -1669,6 +1669,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return Err(());
         }
 
+        let drcx = DeepRejectCtxt::relate_rigid_rigid(self.infcx.tcx);
+        let obligation_args = obligation.predicate.skip_binder().trait_ref.args;
+        if !drcx.args_may_unify(obligation_args, trait_bound.skip_binder().args) {
+            return Err(());
+        }
+
         let trait_bound = self.infcx.instantiate_binder_with_fresh_vars(
             obligation.cause.span,
             HigherRankedType,

compiler/rustc_type_ir/src/fast_reject.rs

@@ -232,6 +232,9 @@ impl<I: Interner, const INSTANTIATE_LHS_WITH_INFER: bool, const INSTANTIATE_RHS_
     }
 
     pub fn types_may_unify(self, lhs: I::Ty, rhs: I::Ty) -> bool {
+        if lhs == rhs {
+            return true;
+        }
         self.types_may_unify_inner(lhs, rhs, Self::STARTING_DEPTH)
     }