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/src/def.rs

@@ -314,89 +314,95 @@ pub enum Res<Id = hir::HirId> {
     /// **Belongs to the type namespace.**
     PrimTy(hir::PrimTy),
 
-    /// The `Self` type, optionally with the [`DefId`] of the trait it belongs to and
-    /// optionally with the [`DefId`] of the item introducing the `Self` type alias.
+    /// The `Self` type, as used within a trait.
+    ///
+    /// **Belongs to the type namespace.**
+    ///
+    /// See the examples on [`Res::SelfTyAlias`] for details.
+    SelfTyParam {
+        /// The trait this `Self` is a generic parameter for.
+        trait_: DefId,
+    },
+
+    /// The `Self` type, as used somewhere other than within a trait.
     ///
     /// **Belongs to the type namespace.**
     ///
     /// Examples:
     /// ```
-    /// struct Bar(Box<Self>);
-    /// // `Res::SelfTy { trait_: None, alias_of: Some(Bar) }`
+    /// struct Bar(Box<Self>); // SelfTyAlias
     ///
     /// trait Foo {
-    ///     fn foo() -> Box<Self>;
-    ///     // `Res::SelfTy { trait_: Some(Foo), alias_of: None }`
+    ///     fn foo() -> Box<Self>; // SelfTyParam
     /// }
     ///
     /// impl Bar {
     ///     fn blah() {
-    ///         let _: Self;
-    ///         // `Res::SelfTy { trait_: None, alias_of: Some(::{impl#0}) }`
+    ///         let _: Self; // SelfTyAlias
     ///     }
     /// }
     ///
     /// impl Foo for Bar {
-    ///     fn foo() -> Box<Self> {
-    ///     // `Res::SelfTy { trait_: Some(Foo), alias_of: Some(::{impl#1}) }`
-    ///         let _: Self;
-    ///         // `Res::SelfTy { trait_: Some(Foo), alias_of: Some(::{impl#1}) }`
+    ///     fn foo() -> Box<Self> { // SelfTyAlias
+    ///         let _: Self;        // SelfTyAlias
     ///
     ///         todo!()
     ///     }
     /// }
     /// ```
-    ///
     /// *See also [`Res::SelfCtor`].*
     ///
-    /// -----
-    ///
-    /// HACK(min_const_generics): self types also have an optional requirement to **not** mention
-    /// any generic parameters to allow the following with `min_const_generics`:
-    /// ```
-    /// # struct Foo;
-    /// impl Foo { fn test() -> [u8; std::mem::size_of::<Self>()] { todo!() } }
-    ///
-    /// struct Bar([u8; baz::<Self>()]);
-    /// const fn baz<T>() -> usize { 10 }
-    /// ```
-    /// We do however allow `Self` in repeat expression even if it is generic to not break code
-    /// which already works on stable while causing the `const_evaluatable_unchecked` future compat
-    /// lint:
-    /// ```
-    /// fn foo<T>() {
-    ///     let _bar = [1_u8; std::mem::size_of::<*mut T>()];
-    /// }
-    /// ```
-    // FIXME(generic_const_exprs): Remove this bodge once that feature is stable.
-    SelfTy {
-        /// The trait this `Self` is a generic arg for.
-        trait_: Option<DefId>,
+    SelfTyAlias {
         /// The item introducing the `Self` type alias. Can be used in the `type_of` query
-        /// to get the underlying type. Additionally whether the `Self` type is disallowed
-        /// from mentioning generics (i.e. when used in an anonymous constant).
-        alias_to: Option<(DefId, bool)>,
-    },
+        /// to get the underlying type.
+        alias_to: DefId,
 
-    /// A tool attribute module; e.g., the `rustfmt` in `#[rustfmt::skip]`.
-    ///
-    /// **Belongs to the type namespace.**
-    ToolMod,
+        /// Whether the `Self` type is disallowed from mentioning generics (i.e. when used in an
+        /// anonymous constant).
+        ///
+        /// HACK(min_const_generics): self types also have an optional requirement to **not**
+        /// mention any generic parameters to allow the following with `min_const_generics`:
+        /// ```
+        /// # struct Foo;
+        /// impl Foo { fn test() -> [u8; std::mem::size_of::<Self>()] { todo!() } }
+        ///
+        /// struct Bar([u8; baz::<Self>()]);
+        /// const fn baz<T>() -> usize { 10 }
+        /// ```
+        /// We do however allow `Self` in repeat expression even if it is generic to not break code
+        /// which already works on stable while causing the `const_evaluatable_unchecked` future
+        /// compat lint:
+        /// ```
+        /// fn foo<T>() {
+        ///     let _bar = [1_u8; std::mem::size_of::<*mut T>()];
+        /// }
+        /// ```
+        // FIXME(generic_const_exprs): Remove this bodge once that feature is stable.
+        forbid_generic: bool,
+
+        /// Is this within an `impl Foo for bar`?
+        is_trait_impl: bool,
+    },
 
     // Value namespace
     /// The `Self` constructor, along with the [`DefId`]
     /// of the impl it is associated with.
     ///
     /// **Belongs to the value namespace.**
     ///
-    /// *See also [`Res::SelfTy`].*
+    /// *See also [`Res::SelfTyParam`] and [`Res::SelfTyAlias`].*
     SelfCtor(DefId),
 
     /// A local variable or function parameter.
     ///
     /// **Belongs to the value namespace.**
     Local(Id),
 
+    /// A tool attribute module; e.g., the `rustfmt` in `#[rustfmt::skip]`.
+    ///
+    /// **Belongs to the type namespace.**
+    ToolMod,
+
     // Macro namespace
     /// An attribute that is *not* implemented via macro.
     /// E.g., `#[inline]` and `#[rustfmt::skip]`, which are essentially directives,
@@ -606,7 +612,8 @@ impl<Id> Res<Id> {
 
             Res::Local(..)
             | Res::PrimTy(..)
-            | Res::SelfTy { .. }
+            | Res::SelfTyParam { .. }
+            | Res::SelfTyAlias { .. }
             | Res::SelfCtor(..)
             | Res::ToolMod
             | Res::NonMacroAttr(..)
@@ -629,7 +636,7 @@ impl<Id> Res<Id> {
             Res::SelfCtor(..) => "self constructor",
             Res::PrimTy(..) => "builtin type",
             Res::Local(..) => "local variable",
-            Res::SelfTy { .. } => "self type",
+            Res::SelfTyParam { .. } | Res::SelfTyAlias { .. } => "self type",
             Res::ToolMod => "tool module",
             Res::NonMacroAttr(attr_kind) => attr_kind.descr(),
             Res::Err => "unresolved item",
@@ -652,7 +659,10 @@ impl<Id> Res<Id> {
             Res::SelfCtor(id) => Res::SelfCtor(id),
             Res::PrimTy(id) => Res::PrimTy(id),
             Res::Local(id) => Res::Local(map(id)),
-            Res::SelfTy { trait_, alias_to } => Res::SelfTy { trait_, alias_to },
+            Res::SelfTyParam { trait_ } => Res::SelfTyParam { trait_ },
+            Res::SelfTyAlias { alias_to, forbid_generic, is_trait_impl } => {
+                Res::SelfTyAlias { alias_to, forbid_generic, is_trait_impl }
+            }
             Res::ToolMod => Res::ToolMod,
             Res::NonMacroAttr(attr_kind) => Res::NonMacroAttr(attr_kind),
             Res::Err => Res::Err,
@@ -665,7 +675,10 @@ impl<Id> Res<Id> {
             Res::SelfCtor(id) => Res::SelfCtor(id),
             Res::PrimTy(id) => Res::PrimTy(id),
             Res::Local(id) => Res::Local(map(id)?),
-            Res::SelfTy { trait_, alias_to } => Res::SelfTy { trait_, alias_to },
+            Res::SelfTyParam { trait_ } => Res::SelfTyParam { trait_ },
+            Res::SelfTyAlias { alias_to, forbid_generic, is_trait_impl } => {
+                Res::SelfTyAlias { alias_to, forbid_generic, is_trait_impl }
+            }
             Res::ToolMod => Res::ToolMod,
             Res::NonMacroAttr(attr_kind) => Res::NonMacroAttr(attr_kind),
             Res::Err => Res::Err,
@@ -692,7 +705,9 @@ impl<Id> Res<Id> {
     pub fn ns(&self) -> Option<Namespace> {
         match self {
             Res::Def(kind, ..) => kind.ns(),
-            Res::PrimTy(..) | Res::SelfTy { .. } | Res::ToolMod => Some(Namespace::TypeNS),
+            Res::PrimTy(..) | Res::SelfTyParam { .. } | Res::SelfTyAlias { .. } | Res::ToolMod => {
+                Some(Namespace::TypeNS)
+            }
             Res::SelfCtor(..) | Res::Local(..) => Some(Namespace::ValueNS),
             Res::NonMacroAttr(..) => Some(Namespace::MacroNS),
             Res::Err => None,

compiler/rustc_hir/src/hir.rs

@@ -2404,8 +2404,9 @@ impl<'hir> Ty<'hir> {
             return None;
         };
         match path.res {
-            Res::Def(DefKind::TyParam, def_id)
-            | Res::SelfTy { trait_: Some(def_id), alias_to: None } => Some((def_id, segment.ident)),
+            Res::Def(DefKind::TyParam, def_id) | Res::SelfTyParam { trait_: def_id } => {
+                Some((def_id, segment.ident))
+            }
             _ => None,
         }
     }
@@ -3533,9 +3534,10 @@ mod size_asserts {
     static_assert_size!(Param<'_>, 32);
     static_assert_size!(Pat<'_>, 72);
     static_assert_size!(PatKind<'_>, 48);
-    static_assert_size!(Path<'_>, 48);
-    static_assert_size!(PathSegment<'_>, 56);
+    static_assert_size!(Path<'_>, 40);
+    static_assert_size!(PathSegment<'_>, 48);
     static_assert_size!(QPath<'_>, 24);
+    static_assert_size!(Res, 12);
     static_assert_size!(Stmt<'_>, 32);
     static_assert_size!(StmtKind<'_>, 16);
     static_assert_size!(TraitItem<'_>, 88);

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -1902,7 +1902,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         // Find the type of the associated item, and the trait where the associated
         // item is declared.
         let bound = match (&qself_ty.kind(), qself_res) {
-            (_, Res::SelfTy { trait_: Some(_), alias_to: Some((impl_def_id, _)) }) => {
+            (_, Res::SelfTyAlias { alias_to: impl_def_id, is_trait_impl: true, .. }) => {
                 // `Self` in an impl of a trait -- we have a concrete self type and a
                 // trait reference.
                 let Some(trait_ref) = tcx.impl_trait_ref(impl_def_id) else {
@@ -1921,8 +1921,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             }
             (
                 &ty::Param(_),
-                Res::SelfTy { trait_: Some(param_did), alias_to: None }
-                | Res::Def(DefKind::TyParam, param_did),
+                Res::SelfTyParam { trait_: param_did } | Res::Def(DefKind::TyParam, param_did),
             ) => self.find_bound_for_assoc_item(param_did.expect_local(), assoc_ident, span)?,
             _ => {
                 let reported = if variant_resolution.is_some() {
@@ -2417,7 +2416,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 let index = generics.param_def_id_to_index[&def_id.to_def_id()];
                 tcx.mk_ty_param(index, tcx.hir().ty_param_name(def_id))
             }
-            Res::SelfTy { trait_: Some(_), alias_to: None } => {
+            Res::SelfTyParam { .. } => {
                 // `Self` in trait or type alias.
                 assert_eq!(opt_self_ty, None);
                 self.prohibit_generics(path.segments.iter(), |err| {
@@ -2432,7 +2431,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 });
                 tcx.types.self_param
             }
-            Res::SelfTy { trait_: _, alias_to: Some((def_id, forbid_generic)) } => {
+            Res::SelfTyAlias { alias_to: def_id, forbid_generic, .. } => {
                 // `Self` in impl (we know the concrete type).
                 assert_eq!(opt_self_ty, None);
                 // Try to evaluate any array length constants.

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))
                 }

compiler/rustc_hir_analysis/src/mem_categorization.rs

@@ -560,7 +560,8 @@ impl<'a, 'tcx> MemCategorizationContext<'a, 'tcx> {
             Res::Def(DefKind::Ctor(CtorOf::Struct, ..), _)
             | Res::Def(DefKind::Struct | DefKind::Union | DefKind::TyAlias | DefKind::AssocTy, _)
             | Res::SelfCtor(..)
-            | Res::SelfTy { .. } => {
+            | Res::SelfTyParam { .. }
+            | Res::SelfTyAlias { .. } => {
                 // Structs and Unions have only have one variant.
                 Ok(VariantIdx::new(0))
             }

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -1021,7 +1021,7 @@ impl<'a, 'tcx> FindInferSourceVisitor<'a, 'tcx> {
                                     }
                                     // There cannot be inference variables in the self type,
                                     // so there's nothing for us to do here.
-                                    Res::SelfTy { .. } => {}
+                                    Res::SelfTyParam { .. } | Res::SelfTyAlias { .. } => {}
                                     _ => warn!(
                                         "unexpected path: def={:?} substs={:?} path={:?}",
                                         def, substs, path,

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/trait_impl_difference.rs

@@ -156,7 +156,8 @@ impl<'tcx> Visitor<'tcx> for TypeParamSpanVisitor<'tcx> {
                 [segment]
                     if matches!(
                         segment.res,
-                        Res::SelfTy { trait_: _, alias_to: _ }
+                        Res::SelfTyParam { .. }
+                            | Res::SelfTyAlias { .. }
                             | Res::Def(hir::def::DefKind::TyParam, _)
                     ) =>
                 {

compiler/rustc_lint/src/internal.rs

@@ -244,7 +244,7 @@ fn is_ty_or_ty_ctxt(cx: &LateContext<'_>, path: &Path<'_>) -> Option<String> {
             }
         }
         // Only lint on `&Ty` and `&TyCtxt` if it is used outside of a trait.
-        Res::SelfTy { trait_: None, alias_to: Some((did, _)) } => {
+        Res::SelfTyAlias { alias_to: did, is_trait_impl: false, .. } => {
             if let ty::Adt(adt, substs) = cx.tcx.type_of(did).kind() {
                 if let Some(name @ (sym::Ty | sym::TyCtxt)) = cx.tcx.get_diagnostic_name(adt.did())
                 {

compiler/rustc_lint/src/pass_by_value.rs

@@ -56,7 +56,7 @@ fn path_for_pass_by_value(cx: &LateContext<'_>, ty: &hir::Ty<'_>) -> Option<Stri
                 let path_segment = path.segments.last().unwrap();
                 return Some(format!("{}{}", name, gen_args(cx, path_segment)));
             }
-            Res::SelfTy { trait_: None, alias_to: Some((did, _)) } => {
+            Res::SelfTyAlias { alias_to: did, is_trait_impl: false, .. } => {
                 if let ty::Adt(adt, substs) = cx.tcx.type_of(did).kind() {
                     if cx.tcx.has_attr(adt.did(), sym::rustc_pass_by_value) {
                         return Some(cx.tcx.def_path_str_with_substs(adt.did(), substs));

compiler/rustc_middle/src/ty/adt.rs

@@ -438,7 +438,8 @@ impl<'tcx> AdtDef<'tcx> {
             | Res::Def(DefKind::Union, _)
             | Res::Def(DefKind::TyAlias, _)
             | Res::Def(DefKind::AssocTy, _)
-            | Res::SelfTy { .. }
+            | Res::SelfTyParam { .. }
+            | Res::SelfTyAlias { .. }
             | Res::SelfCtor(..) => self.non_enum_variant(),
             _ => bug!("unexpected res {:?} in variant_of_res", res),
         }

compiler/rustc_mir_build/src/thir/pattern/mod.rs

@@ -433,7 +433,8 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
                 | DefKind::AssocTy,
                 _,
             )
-            | Res::SelfTy { .. }
+            | Res::SelfTyParam { .. }
+            | Res::SelfTyAlias { .. }
             | Res::SelfCtor(..) => PatKind::Leaf { subpatterns },
             _ => {
                 let pattern_error = match res {

compiler/rustc_passes/src/dead.rs

@@ -102,14 +102,8 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
                 }
             }
             Res::Def(_, def_id) => self.check_def_id(def_id),
-            Res::SelfTy { trait_: t, alias_to: i } => {
-                if let Some(t) = t {
-                    self.check_def_id(t);
-                }
-                if let Some((i, _)) = i {
-                    self.check_def_id(i);
-                }
-            }
+            Res::SelfTyParam { trait_: t } => self.check_def_id(t),
+            Res::SelfTyAlias { alias_to: i, .. } => self.check_def_id(i),
             Res::ToolMod | Res::NonMacroAttr(..) | Res::Err => {}
         }
     }

compiler/rustc_privacy/src/lib.rs

@@ -1422,7 +1422,9 @@ struct ObsoleteCheckTypeForPrivatenessVisitor<'a, 'b, 'tcx> {
 impl<'a, 'tcx> ObsoleteVisiblePrivateTypesVisitor<'a, 'tcx> {
     fn path_is_private_type(&self, path: &hir::Path<'_>) -> bool {
         let did = match path.res {
-            Res::PrimTy(..) | Res::SelfTy { .. } | Res::Err => return false,
+            Res::PrimTy(..) | Res::SelfTyParam { .. } | Res::SelfTyAlias { .. } | Res::Err => {
+                return false;
+            }
             res => res.def_id(),
         };
 

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -1010,7 +1010,8 @@ impl<'a, 'b> BuildReducedGraphVisitor<'a, 'b> {
                 _,
             )
             | Res::Local(..)
-            | Res::SelfTy { .. }
+            | Res::SelfTyParam { .. }
+            | Res::SelfTyAlias { .. }
             | Res::SelfCtor(..)
             | Res::Err => bug!("unexpected resolution: {:?}", res),
         }