trait_sel: skip elaboration of sizedness supertrait

As a performance optimization, skip elaborating the supertraits of
`Sized`, and if a `MetaSized` obligation is being checked, then look for
a `Sized` predicate in the parameter environment. This makes the
`ParamEnv` smaller which should improve compiler performance as it avoids
all the iteration over the larger `ParamEnv`.

1 files changed, 13 insertions, 3 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index b2d2f1188c3..80f71c78993 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -166,10 +166,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             )
             .break_value()
             .expect("expected to index into clause that exists");
-        let candidate = candidate_predicate
+        let candidate_predicate = candidate_predicate
             .as_trait_clause()
-            .expect("projection candidate is not a trait predicate")
-            .map_bound(|t| t.trait_ref);
+            .expect("projection candidate is not a trait predicate");
+        let candidate_predicate =
+            util::lazily_elaborate_sizedness_candidate(self.infcx, obligation, candidate_predicate);
+
+        let candidate = candidate_predicate.map_bound(|t| t.trait_ref);
 
         let candidate = self.infcx.instantiate_binder_with_fresh_vars(
             obligation.cause.span,
@@ -226,6 +229,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) -> PredicateObligations<'tcx> {
         debug!(?obligation, ?param, "confirm_param_candidate");
 
+        let param = util::lazily_elaborate_sizedness_candidate(
+            self.infcx,
+            obligation,
+            param.upcast(self.infcx.tcx),
+        )
+        .map_bound(|p| p.trait_ref);
+
         // During evaluation, we already checked that this
         // where-clause trait-ref could be unified with the obligation
         // trait-ref. Repeat that unification now without any