move fast reject test out of `SelectionContext::match_impl`.

`match_impl` has two call sites. For one of them (within `rematch_impl`)
the fast reject test isn't necessary, because any rejection would
represent a compiler bug.

This commit moves the fast reject test to the other `match_impl` call
site, in `assemble_candidates_from_impls`. This lets us move the fast
reject test outside the `probe` call in that function. This avoids the
taking of useless snapshots when the fast reject test succeeds, which
gives a performance win when compiling the `bitmaps` and `nalgebra`
crates.
Co-authored-by: Nname <n.nethercote@gmail.com>

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

@@ -539,8 +539,16 @@ fn assemble_candidates_from_impls(
             obligation.predicate.def_id(),
             obligation.predicate.skip_binder().trait_ref.self_ty(),
             |impl_def_id| {
+                // Before we create the substitutions and everything, first
+                // consider a "quick reject". This avoids creating more types
+                // and so forth that we need to.
+                let impl_trait_ref = self.tcx().bound_impl_trait_ref(impl_def_id).unwrap();
+                if self.fast_reject_trait_refs(obligation, &impl_trait_ref.0) {
+                    return;
+                }
+
                 self.infcx.probe(|_| {
-                    if let Ok(_substs) = self.match_impl(impl_def_id, obligation) {
+                    if let Ok(_substs) = self.match_impl(impl_def_id, impl_trait_ref, obligation) {
                         candidates.vec.push(ImplCandidate(impl_def_id));
                     }
                 });

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

@@ -2043,7 +2043,8 @@ fn rematch_impl(
         impl_def_id: DefId,
         obligation: &TraitObligation<'tcx>,
     ) -> Normalized<'tcx, SubstsRef<'tcx>> {
-        match self.match_impl(impl_def_id, obligation) {
+        let impl_trait_ref = self.tcx().bound_impl_trait_ref(impl_def_id).unwrap();
+        match self.match_impl(impl_def_id, impl_trait_ref, obligation) {
             Ok(substs) => substs,
             Err(()) => {
                 self.infcx.tcx.sess.delay_span_bug(
@@ -2070,17 +2071,9 @@ fn rematch_impl(
     fn match_impl(
         &mut self,
         impl_def_id: DefId,
+        impl_trait_ref: EarlyBinder<ty::TraitRef<'tcx>>,
         obligation: &TraitObligation<'tcx>,
     ) -> Result<Normalized<'tcx, SubstsRef<'tcx>>, ()> {
-        let impl_trait_ref = self.tcx().bound_impl_trait_ref(impl_def_id).unwrap();
-
-        // Before we create the substitutions and everything, first
-        // consider a "quick reject". This avoids creating more types
-        // and so forth that we need to.
-        if self.fast_reject_trait_refs(obligation, &impl_trait_ref.0) {
-            return Err(());
-        }
-
         let placeholder_obligation =
             self.infcx().replace_bound_vars_with_placeholders(obligation.predicate);
         let placeholder_obligation_trait_ref = placeholder_obligation.trait_ref;