try to speed up try_fast_path sizedness check

Author: estebank

compiler/rustc_trait_selection/src/traits/query/type_op/prove_predicate.rs

@@ -16,6 +16,9 @@ impl<'tcx> super::QueryTypeOp<'tcx> for ProvePredicate<'tcx> {
         // we have to prove. No need to canonicalize and all that for
         // such cases.
         if let ty::PredicateKind::Trait(trait_ref) = key.value.predicate.kind().skip_binder() {
+            if trait_ref.implicit == ty::ImplicitBound::Yes {
+                return Some(());
+            }
             if let Some(sized_def_id) = tcx.lang_items().sized_trait() {
                 if trait_ref.def_id() == sized_def_id {
                     if trait_ref.self_ty().is_trivially_sized(tcx) {