Inline and remove SelectionContext::fast_reject_trait_refs.

Because it has a single call site, and it lets us move a small amount of
the work outside the loop.

Author: nnethercote

Diff for: compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -11,7 +11,7 @@ use hir::LangItem;
 use rustc_hir as hir;
 use rustc_infer::traits::ObligationCause;
 use rustc_infer::traits::{Obligation, SelectionError, TraitObligation};
-use rustc_middle::ty::fast_reject::TreatProjections;
+use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams, TreatProjections};
 use rustc_middle::ty::{self, Ty, TypeVisitableExt};
 
 use crate::traits;
@@ -344,6 +344,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return;
         }
 
+        let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::ForLookup };
+        let obligation_substs = obligation.predicate.skip_binder().trait_ref.substs;
         self.tcx().for_each_relevant_impl(
             obligation.predicate.def_id(),
             obligation.predicate.skip_binder().trait_ref.self_ty(),
@@ -352,7 +354,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 // consider a "quick reject". This avoids creating more types
                 // and so forth that we need to.
                 let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
-                if self.fast_reject_trait_refs(obligation, &impl_trait_ref.0) {
+                if !drcx.substs_refs_may_unify(obligation_substs, impl_trait_ref.0.substs) {
                     return;
                 }
                 if self.reject_fn_ptr_impls(

Diff for: compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -45,7 +45,6 @@ use rustc_infer::traits::TraitEngineExt;
 use rustc_middle::dep_graph::{DepKind, DepNodeIndex};
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
-use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
 use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::relate::TypeRelation;
 use rustc_middle::ty::SubstsRef;
@@ -2533,21 +2532,6 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
         Ok(Normalized { value: impl_substs, obligations: nested_obligations })
     }
 
-    fn fast_reject_trait_refs(
-        &mut self,
-        obligation: &TraitObligation<'tcx>,
-        impl_trait_ref: &ty::TraitRef<'tcx>,
-    ) -> bool {
-        // We can avoid creating type variables and doing the full
-        // substitution if we find that any of the input types, when
-        // simplified, do not match.
-        let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::ForLookup };
-        !drcx.substs_refs_may_unify(
-            obligation.predicate.skip_binder().trait_ref.substs,
-            impl_trait_ref.substs,
-        )
-    }
-
     /// Normalize `where_clause_trait_ref` and try to match it against
     /// `obligation`. If successful, return any predicates that
     /// result from the normalization.