Shrink Statement.

This commit reduces the size of `Statement` from 80 bytes to 56 bytes on
64-bit platforms, by boxing the `AscribeUserType` variant of
`StatementKind`.

This change reduces instruction counts on most benchmarks by 1--3%.

src/librustc/mir/mod.rs

@@ -1674,6 +1674,10 @@ impl<'tcx> Statement<'tcx> {
     /// Changes a statement to a nop. This is both faster than deleting instructions and avoids
     /// invalidating statement indices in `Location`s.
     pub fn make_nop(&mut self) {
+        // `Statement` contributes significantly to peak memory usage. Make
+        // sure it doesn't get bigger.
+        static_assert!(STATEMENT_IS_AT_MOST_56_BYTES: mem::size_of::<Statement<'_>>() <= 56);
+
         self.kind = StatementKind::Nop
     }
 
@@ -1737,7 +1741,7 @@ pub enum StatementKind<'tcx> {
     /// - `Contravariant` -- requires that `T_y :> T`
     /// - `Invariant` -- requires that `T_y == T`
     /// - `Bivariant` -- no effect
-    AscribeUserType(Place<'tcx>, ty::Variance, UserTypeAnnotation<'tcx>),
+    AscribeUserType(Place<'tcx>, ty::Variance, Box<UserTypeAnnotation<'tcx>>),
 
     /// No-op. Useful for deleting instructions without affecting statement indices.
     Nop,

src/librustc_mir/borrow_check/nll/type_check/mod.rs

@@ -1308,7 +1308,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
                     );
                 };
             }
-            StatementKind::AscribeUserType(ref place, variance, c_ty) => {
+            StatementKind::AscribeUserType(ref place, variance, box c_ty) => {
                 let place_ty = place.ty(mir, tcx).to_ty(tcx);
                 if let Err(terr) = self.relate_type_and_user_type(
                     place_ty,

src/librustc_mir/build/expr/as_place.rs

@@ -147,7 +147,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                             kind: StatementKind::AscribeUserType(
                                 place.clone(),
                                 Variance::Invariant,
-                                user_ty,
+                                box user_ty,
                             ),
                         },
                     );
@@ -167,7 +167,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                             kind: StatementKind::AscribeUserType(
                                 Place::Local(temp.clone()),
                                 Variance::Invariant,
-                                user_ty,
+                                box user_ty,
                             ),
                         },
                     );

src/librustc_mir/build/matches/mod.rs

@@ -316,7 +316,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                         kind: StatementKind::AscribeUserType(
                             place.clone(),
                             ty::Variance::Invariant,
-                            ascription_user_ty,
+                            box ascription_user_ty,
                         ),
                     },
                 );
@@ -1323,7 +1323,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                     kind: StatementKind::AscribeUserType(
                         ascription.source.clone(),
                         ty::Variance::Covariant,
-                        ascription.user_ty,
+                        box ascription.user_ty,
                     ),
                 },
             );