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.

clippy_lints/src/trait_bounds.rs

@@ -128,7 +128,7 @@ impl<'tcx> LateLintPass<'tcx> for TraitBounds {
                 if !bound_predicate.span.from_expansion();
                 if let TyKind::Path(QPath::Resolved(_, Path { segments, .. })) = bound_predicate.bounded_ty.kind;
                 if let Some(PathSegment {
-                    res: Res::SelfTy{ trait_: Some(def_id), alias_to: _ }, ..
+                    res: Res::SelfTyParam { trait_: def_id }, ..
                 }) = segments.first();
                 if let Some(
                     Node::Item(

clippy_lints/src/use_self.rs

@@ -206,7 +206,12 @@ impl<'tcx> LateLintPass<'tcx> for UseSelf {
                 ref types_to_skip,
             }) = self.stack.last();
             if let TyKind::Path(QPath::Resolved(_, path)) = hir_ty.kind;
-            if !matches!(path.res, Res::SelfTy { .. } | Res::Def(DefKind::TyParam, _));
+            if !matches!(
+                path.res,
+                Res::SelfTyParam { .. }
+                | Res::SelfTyAlias { .. }
+                | Res::Def(DefKind::TyParam, _)
+            );
             if !types_to_skip.contains(&hir_ty.hir_id);
             let ty = if in_body > 0 {
                 cx.typeck_results().node_type(hir_ty.hir_id)
@@ -230,7 +235,7 @@ impl<'tcx> LateLintPass<'tcx> for UseSelf {
         }
         match expr.kind {
             ExprKind::Struct(QPath::Resolved(_, path), ..) => match path.res {
-                Res::SelfTy { .. } => (),
+                Res::SelfTyParam { .. } | Res::SelfTyAlias { .. } => (),
                 Res::Def(DefKind::Variant, _) => lint_path_to_variant(cx, path),
                 _ => span_lint(cx, path.span),
             },

clippy_utils/src/lib.rs

@@ -1535,7 +1535,7 @@ pub fn is_self(slf: &Param<'_>) -> bool {
 
 pub fn is_self_ty(slf: &hir::Ty<'_>) -> bool {
     if let TyKind::Path(QPath::Resolved(None, path)) = slf.kind {
-        if let Res::SelfTy { .. } = path.res {
+        if let Res::SelfTyParam { .. } | Res::SelfTyAlias { .. } = path.res {
             return true;
         }
     }