Shrink hir::def::Res.

`Res::SelfTy` currently has two `Option`s. When the second one is `Some`
the first one is never consulted. So we can split it into two variants,
`Res::SelfTyParam` and `Res::SelfTyAlias`, reducing the size of `Res`
from 24 bytes to 12. This then shrinks `hir::Path` and
`hir::PathSegment`, which are the HIR types that take up the most space.

compiler/rustc_hir_analysis/src/check/check.rs

@@ -609,9 +609,12 @@ pub(super) fn check_opaque_for_inheriting_lifetimes<'tcx>(
         fn visit_ty(&mut self, arg: &'tcx hir::Ty<'tcx>) {
             match arg.kind {
                 hir::TyKind::Path(hir::QPath::Resolved(None, path)) => match &path.segments {
-                    [PathSegment { res: Res::SelfTy { trait_: _, alias_to: impl_ref }, .. }] => {
-                        let impl_ty_name =
-                            impl_ref.map(|(def_id, _)| self.tcx.def_path_str(def_id));
+                    [PathSegment { res: Res::SelfTyParam { .. }, .. }] => {
+                        let impl_ty_name = None;
+                        self.selftys.push((path.span, impl_ty_name));
+                    }
+                    [PathSegment { res: Res::SelfTyAlias { alias_to: def_id, .. }, .. }] => {
+                        let impl_ty_name = Some(self.tcx.def_path_str(*def_id));
                         self.selftys.push((path.span, impl_ty_name));
                     }
                     _ => {}

compiler/rustc_hir_analysis/src/check/fn_ctxt/checks.rs

@@ -1199,7 +1199,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 _ => bug!("unexpected type: {:?}", ty),
             },
             Res::Def(DefKind::Struct | DefKind::Union | DefKind::TyAlias | DefKind::AssocTy, _)
-            | Res::SelfTy { .. } => match ty.kind() {
+            | Res::SelfTyParam { .. }
+            | Res::SelfTyAlias { .. } => match ty.kind() {
                 ty::Adt(adt, substs) if !adt.is_enum() => {
                     Some((adt.non_enum_variant(), adt.did(), substs))
                 }