Deprecate the FxHashMap() and FxHashSet() constructor function hack

src/librustc_mir/transform/elaborate_drops.rs

@@ -75,7 +75,7 @@ impl MirPass for ElaborateDrops {
                 env: &env,
                 flow_inits,
                 flow_uninits,
-                drop_flags: FxHashMap(),
+                drop_flags: FxHashMap::default(),
                 patch: MirPatch::new(mir),
             }.elaborate()
         };