trait_sel: extend fast path with constness

Extends the fast path for `Sized` traits to include `const {Meta,}Sized`.

Author: davidtwco

compiler/rustc_trait_selection/src/traits/util.rs

@@ -548,6 +548,23 @@ pub fn sizedness_fast_path<'tcx>(
         }
     }
 
+    // Likewise, determining if a sizedness trait is implemented const-ly is a trivial
+    // determination that can happen in the fast path.
+    //
+    // NOTE: Keep this in sync with `evaluate_host_effect_for_sizedness_goal` in the old solver,
+    // `const_conditions_for_sizedness` in the new solver.
+    if let ty::PredicateKind::Clause(ty::ClauseKind::HostEffect(host_pred)) =
+        predicate.kind().skip_binder()
+    {
+        let is_sizedness = tcx.is_lang_item(host_pred.def_id(), LangItem::Sized)
+            || tcx.is_lang_item(host_pred.def_id(), LangItem::MetaSized);
+
+        if is_sizedness && !host_pred.self_ty().has_non_const_sizedness() {
+            debug!("fast path -- host effect");
+            return true;
+        }
+    }
+
     false
 }