Make hir::TyKind::TraitObject use tagged ptr

compiler/rustc_ast/src/ast.rs

@@ -28,6 +28,7 @@ use rustc_data_structures::packed::Pu128;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_data_structures::sync::Lrc;
+use rustc_data_structures::tagged_ptr::Tag;
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
 pub use rustc_span::AttrId;
 use rustc_span::source_map::{Spanned, respan};
@@ -2269,10 +2270,32 @@ impl TyKind {
 
 /// Syntax used to declare a trait object.
 #[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
+#[repr(u8)]
 pub enum TraitObjectSyntax {
-    Dyn,
-    DynStar,
-    None,
+    // SAFETY: When adding new variants make sure to update the `Tag` impl.
+    Dyn = 0,
+    DynStar = 1,
+    None = 2,
+}
+
+/// SAFETY: `TraitObjectSyntax` only has 3 data-less variants which means
+/// it can be represented with a `u2`. We use `repr(u8)` to guarantee the
+/// discriminants of the variants are no greater than `3`.
+unsafe impl Tag for TraitObjectSyntax {
+    const BITS: u32 = 2;
+
+    fn into_usize(self) -> usize {
+        self as u8 as usize
+    }
+
+    unsafe fn from_usize(tag: usize) -> Self {
+        match tag {
+            0 => TraitObjectSyntax::Dyn,
+            1 => TraitObjectSyntax::DynStar,
+            2 => TraitObjectSyntax::None,
+            _ => unreachable!(),
+        }
+    }
 }
 
 #[derive(Clone, Encodable, Decodable, Debug)]

compiler/rustc_ast_lowering/src/lib.rs

@@ -48,6 +48,7 @@ use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::sorted_map::SortedMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync::Lrc;
+use rustc_data_structures::tagged_ptr::TaggedRef;
 use rustc_errors::{DiagArgFromDisplay, DiagCtxtHandle, StashKey};
 use rustc_hir::def::{DefKind, LifetimeRes, Namespace, PartialRes, PerNS, Res};
 use rustc_hir::def_id::{CRATE_DEF_ID, LOCAL_CRATE, LocalDefId};
@@ -1158,7 +1159,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 let lifetime_bound = this.elided_dyn_bound(t.span);
                 (bounds, lifetime_bound)
             });
-            let kind = hir::TyKind::TraitObject(bounds, lifetime_bound, TraitObjectSyntax::None);
+            let kind = hir::TyKind::TraitObject(
+                bounds,
+                TaggedRef::new(lifetime_bound, TraitObjectSyntax::None),
+            );
             return hir::Ty { kind, span: self.lower_span(t.span), hir_id: self.next_id() };
         }
 
@@ -1309,7 +1313,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         lifetime_bound.unwrap_or_else(|| this.elided_dyn_bound(t.span));
                     (bounds, lifetime_bound)
                 });
-                hir::TyKind::TraitObject(bounds, lifetime_bound, *kind)
+                hir::TyKind::TraitObject(bounds, TaggedRef::new(lifetime_bound, *kind))
             }
             TyKind::ImplTrait(def_node_id, bounds) => {
                 let span = t.span;
@@ -2365,8 +2369,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         hir_id = self.next_id();
                         hir::TyKind::TraitObject(
                             arena_vec![self; principal],
-                            self.elided_dyn_bound(span),
-                            TraitObjectSyntax::None,
+                            TaggedRef::new(self.elided_dyn_bound(span), TraitObjectSyntax::None),
                         )
                     }
                     _ => hir::TyKind::Path(hir::QPath::Resolved(None, path)),

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -887,7 +887,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 if alias_ty.span.desugaring_kind().is_some() {
                     // Skip `async` desugaring `impl Future`.
                 }
-                if let TyKind::TraitObject(_, lt, _) = alias_ty.kind {
+                if let TyKind::TraitObject(_, lt) = alias_ty.kind {
                     if lt.ident.name == kw::Empty {
                         spans_suggs.push((lt.ident.span.shrink_to_hi(), " + 'a".to_string()));
                     } else {

compiler/rustc_hir/src/hir.rs

@@ -15,6 +15,7 @@ pub use rustc_ast::{
 };
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::sorted_map::SortedMap;
+use rustc_data_structures::tagged_ptr::TaggedRef;
 use rustc_index::IndexVec;
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
 use rustc_span::def_id::LocalDefId;
@@ -764,11 +765,8 @@ impl<'hir> Generics<'hir> {
                     && let [.., segment] = trait_ref.path.segments
                     && let Some(ret_ty) = segment.args().paren_sugar_output()
                     && let ret_ty = ret_ty.peel_refs()
-                    && let TyKind::TraitObject(
-                        _,
-                        _,
-                        TraitObjectSyntax::Dyn | TraitObjectSyntax::DynStar,
-                    ) = ret_ty.kind
+                    && let TyKind::TraitObject(_, tagged_ptr) = ret_ty.kind
+                    && let TraitObjectSyntax::Dyn | TraitObjectSyntax::DynStar = tagged_ptr.tag()
                     && ret_ty.span.can_be_used_for_suggestions()
                 {
                     Some(ret_ty.span)
@@ -3230,7 +3228,10 @@ pub enum TyKind<'hir> {
     TraitAscription(GenericBounds<'hir>),
     /// A trait object type `Bound1 + Bound2 + Bound3`
     /// where `Bound` is a trait or a lifetime.
-    TraitObject(&'hir [PolyTraitRef<'hir>], &'hir Lifetime, TraitObjectSyntax),
+    ///
+    /// We use pointer tagging to represent a `&'hir Lifetime` and `TraitObjectSyntax` pair
+    /// as otherwise this type being `repr(C)` would result in `TyKind` increasing in size.
+    TraitObject(&'hir [PolyTraitRef<'hir>], TaggedRef<'hir, Lifetime, TraitObjectSyntax>),
     /// Unused for now.
     Typeof(&'hir AnonConst),
     /// `TyKind::Infer` means the type should be inferred instead of it having been

compiler/rustc_hir/src/intravisit.rs

@@ -922,7 +922,7 @@ pub fn walk_ty<'v, V: Visitor<'v>>(visitor: &mut V, typ: &'v Ty<'v>) -> V::Resul
             try_visit!(visitor.visit_ty(ty));
             try_visit!(visitor.visit_const_arg(length));
         }
-        TyKind::TraitObject(bounds, ref lifetime, _syntax) => {
+        TyKind::TraitObject(bounds, ref lifetime) => {
             for bound in bounds {
                 try_visit!(visitor.visit_poly_trait_ref(bound));
             }

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -810,7 +810,9 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
                     intravisit::walk_ty(this, ty);
                 });
             }
-            hir::TyKind::TraitObject(bounds, lifetime, _) => {
+            hir::TyKind::TraitObject(bounds, lifetime) => {
+                let lifetime = lifetime.pointer();
+
                 debug!(?bounds, ?lifetime, "TraitObject");
                 let scope = Scope::TraitRefBoundary { s: self.scope };
                 self.with(scope, |this| {
@@ -827,7 +829,7 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
                         // use the object lifetime defaulting
                         // rules. So e.g., `Box<dyn Debug>` becomes
                         // `Box<dyn Debug + 'static>`.
-                        self.resolve_object_lifetime_default(lifetime)
+                        self.resolve_object_lifetime_default(&*lifetime)
                     }
                     LifetimeName::Infer => {
                         // If the user writes `'_`, we use the *ordinary* elision
@@ -838,7 +840,7 @@ impl<'a, 'tcx> Visitor<'tcx> for BoundVarContext<'a, 'tcx> {
                     }
                     LifetimeName::Param(..) | LifetimeName::Static => {
                         // If the user wrote an explicit name, use that.
-                        self.visit_lifetime(lifetime);
+                        self.visit_lifetime(&*lifetime);
                     }
                     LifetimeName::Error => {}
                 }

compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs

@@ -23,9 +23,12 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     ) -> Option<ErrorGuaranteed> {
         let tcx = self.tcx();
 
-        let hir::TyKind::TraitObject([poly_trait_ref, ..], _, TraitObjectSyntax::None) =
+        let poly_trait_ref = if let hir::TyKind::TraitObject([poly_trait_ref, ..], tagged_ptr) =
             self_ty.kind
-        else {
+            && let TraitObjectSyntax::None = tagged_ptr.tag()
+        {
+            poly_trait_ref
+        } else {
             return None;
         };
 
@@ -294,7 +297,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 let (dyn_str, paren_dyn_str) =
                     if borrowed { ("dyn ", "(dyn ") } else { ("&dyn ", "&(dyn ") };
 
-                let sugg = if let hir::TyKind::TraitObject([_, _, ..], _, _) = self_ty.kind {
+                let sugg = if let hir::TyKind::TraitObject([_, _, ..], _) = self_ty.kind {
                     // There are more than one trait bound, we need surrounding parentheses.
                     vec![
                         (self_ty.span.shrink_to_lo(), paren_dyn_str.to_string()),

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -2311,7 +2311,10 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     tcx.late_bound_vars(hir_ty.hir_id),
                 ),
             ),
-            hir::TyKind::TraitObject(bounds, lifetime, repr) => {
+            hir::TyKind::TraitObject(bounds, tagged_ptr) => {
+                let lifetime = tagged_ptr.pointer();
+                let repr = tagged_ptr.tag();
+
                 if let Some(guar) = self.prohibit_or_lint_bare_trait_object_ty(hir_ty) {
                     // Don't continue with type analysis if the `dyn` keyword is missing
                     // It generates confusing errors, especially if the user meant to use another

compiler/rustc_hir_pretty/src/lib.rs

@@ -402,7 +402,8 @@ impl<'a> State<'a> {
                 self.print_bounds("impl", bounds);
             }
             hir::TyKind::Path(ref qpath) => self.print_qpath(qpath, false),
-            hir::TyKind::TraitObject(bounds, lifetime, syntax) => {
+            hir::TyKind::TraitObject(bounds, lifetime) => {
+                let syntax = lifetime.tag();
                 match syntax {
                     ast::TraitObjectSyntax::Dyn => self.word_nbsp("dyn"),
                     ast::TraitObjectSyntax::DynStar => self.word_nbsp("dyn*"),
@@ -421,7 +422,7 @@ impl<'a> State<'a> {
                 if !lifetime.is_elided() {
                     self.nbsp();
                     self.word_space("+");
-                    self.print_lifetime(lifetime);
+                    self.print_lifetime(lifetime.pointer());
                 }
             }
             hir::TyKind::Array(ty, ref length) => {

compiler/rustc_lint/src/traits.rs

@@ -111,7 +111,9 @@ impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints {
     }
 
     fn check_ty(&mut self, cx: &LateContext<'_>, ty: &'tcx hir::Ty<'tcx>) {
-        let hir::TyKind::TraitObject(bounds, _lifetime, _syntax) = &ty.kind else { return };
+        let hir::TyKind::TraitObject(bounds, _lifetime_and_syntax_pointer) = &ty.kind else {
+            return;
+        };
         for bound in &bounds[..] {
             let def_id = bound.trait_ref.trait_def_id();
             if def_id.is_some_and(|def_id| cx.tcx.is_lang_item(def_id, LangItem::Drop)) {

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -572,16 +572,16 @@ pub struct TraitObjectVisitor<'tcx>(pub Vec<&'tcx hir::Ty<'tcx>>, pub crate::hir
 impl<'v> hir::intravisit::Visitor<'v> for TraitObjectVisitor<'v> {
     fn visit_ty(&mut self, ty: &'v hir::Ty<'v>) {
         match ty.kind {
-            hir::TyKind::TraitObject(
-                _,
-                hir::Lifetime {
+            hir::TyKind::TraitObject(_, tagged_ptr)
+                if let hir::Lifetime {
                     res:
                         hir::LifetimeName::ImplicitObjectLifetimeDefault | hir::LifetimeName::Static,
                     ..
-                },
-                _,
-            )
-            | hir::TyKind::OpaqueDef(..) => self.0.push(ty),
+                } = tagged_ptr.pointer() =>
+            {
+                self.0.push(ty)
+            }
+            hir::TyKind::OpaqueDef(..) => self.0.push(ty),
             _ => {}
         }
         hir::intravisit::walk_ty(self, ty);

compiler/rustc_trait_selection/src/error_reporting/infer/nice_region_error/static_impl_trait.rs

@@ -374,7 +374,7 @@ pub fn suggest_new_region_bound(
                     }
                 }
             }
-            TyKind::TraitObject(_, lt, _) => {
+            TyKind::TraitObject(_, lt) => {
                 if let LifetimeName::ImplicitObjectLifetimeDefault = lt.res {
                     err.span_suggestion_verbose(
                         fn_return.span.shrink_to_hi(),
@@ -592,11 +592,9 @@ pub struct HirTraitObjectVisitor<'a>(pub &'a mut Vec<Span>, pub DefId);
 
 impl<'a, 'tcx> Visitor<'tcx> for HirTraitObjectVisitor<'a> {
     fn visit_ty(&mut self, t: &'tcx hir::Ty<'tcx>) {
-        if let TyKind::TraitObject(
-            poly_trait_refs,
-            Lifetime { res: LifetimeName::ImplicitObjectLifetimeDefault, .. },
-            _,
-        ) = t.kind
+        if let TyKind::TraitObject(poly_trait_refs, lifetime_ptr) = t.kind
+            && let Lifetime { res: LifetimeName::ImplicitObjectLifetimeDefault, .. } =
+                lifetime_ptr.pointer()
         {
             for ptr in poly_trait_refs {
                 if Some(self.1) == ptr.trait_ref.trait_def_id() {

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -580,8 +580,8 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             self.tcx.hir_node_by_def_id(obligation.cause.body_id)
                             && let hir::ItemKind::Impl(impl_) = item.kind
                             && let None = impl_.of_trait
-                            && let hir::TyKind::TraitObject(_, _, syntax) = impl_.self_ty.kind
-                            && let TraitObjectSyntax::None = syntax
+                            && let hir::TyKind::TraitObject(_, tagged_ptr) = impl_.self_ty.kind
+                            && let TraitObjectSyntax::None = tagged_ptr.tag()
                             && impl_.self_ty.span.edition().at_least_rust_2021()
                         {
                             // Silence the dyn-compatibility error in favor of the missing dyn on

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -3074,7 +3074,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 }
                 if let Some(ty) = ty {
                     match ty.kind {
-                        hir::TyKind::TraitObject(traits, _, _) => {
+                        hir::TyKind::TraitObject(traits, _) => {
                             let (span, kw) = match traits {
                                 [first, ..] if first.span.lo() == ty.span.lo() => {
                                     // Missing `dyn` in front of trait object.

src/librustdoc/clean/mod.rs

@@ -1843,10 +1843,10 @@ pub(crate) fn clean_ty<'tcx>(ty: &hir::Ty<'tcx>, cx: &mut DocContext<'tcx>) -> T
             ImplTrait(ty.bounds.iter().filter_map(|x| clean_generic_bound(x, cx)).collect())
         }
         TyKind::Path(_) => clean_qpath(ty, cx),
-        TyKind::TraitObject(bounds, lifetime, _) => {
+        TyKind::TraitObject(bounds, lifetime) => {
             let bounds = bounds.iter().map(|bound| clean_poly_trait_ref(bound, cx)).collect();
             let lifetime =
-                if !lifetime.is_elided() { Some(clean_lifetime(lifetime, cx)) } else { None };
+                if !lifetime.is_elided() { Some(clean_lifetime(lifetime.pointer(), cx)) } else { None };
             DynTrait(bounds, lifetime)
         }
         TyKind::BareFn(barefn) => BareFunction(Box::new(clean_bare_fn_ty(barefn, cx))),

src/tools/clippy/clippy_lints/src/lifetimes.rs

@@ -433,7 +433,7 @@ impl<'tcx> Visitor<'tcx> for RefVisitor<'_, 'tcx> {
                 sub_visitor.visit_fn_decl(decl);
                 self.nested_elision_site_lts.append(&mut sub_visitor.all_lts());
             },
-            TyKind::TraitObject(bounds, lt, _) => {
+            TyKind::TraitObject(bounds, lt) => {
                 if !lt.is_elided() {
                     self.unelided_trait_object_lifetime = true;
                 }

src/tools/clippy/clippy_lints/src/types/borrowed_box.rs

@@ -47,7 +47,7 @@ pub(super) fn check(cx: &LateContext<'_>, hir_ty: &hir::Ty<'_>, lt: &Lifetime, m
                 // Originally reported as the issue #3128.
                 let inner_snippet = snippet(cx, inner.span, "..");
                 let suggestion = match &inner.kind {
-                    TyKind::TraitObject(bounds, lt_bound, _) if bounds.len() > 1 || !lt_bound.is_elided() => {
+                    TyKind::TraitObject(bounds, lt_bound) if bounds.len() > 1 || !lt_bound.is_elided() => {
                         format!("&{ltopt}({inner_snippet})")
                     },
                     TyKind::Path(qpath)

src/tools/clippy/clippy_lints/src/types/type_complexity.rs

@@ -52,7 +52,7 @@ impl<'tcx> Visitor<'tcx> for TypeComplexityVisitor {
             // function types bring a lot of overhead
             TyKind::BareFn(bare) if bare.abi == Abi::Rust => (50 * self.nest, 1),
 
-            TyKind::TraitObject(param_bounds, _, _) => {
+            TyKind::TraitObject(param_bounds, _) => {
                 let has_lifetime_parameters = param_bounds.iter().any(|bound| {
                     bound
                         .bound_generic_params

src/tools/clippy/clippy_utils/src/check_proc_macro.rs

@@ -400,7 +400,7 @@ fn ty_search_pat(ty: &Ty<'_>) -> (Pat, Pat) {
         TyKind::OpaqueDef(..) => (Pat::Str("impl"), Pat::Str("")),
         TyKind::Path(qpath) => qpath_search_pat(&qpath),
         TyKind::Infer => (Pat::Str("_"), Pat::Str("_")),
-        TyKind::TraitObject(_, _, TraitObjectSyntax::Dyn) => (Pat::Str("dyn"), Pat::Str("")),
+        TyKind::TraitObject(_, tagged_ptr) if let TraitObjectSyntax::Dyn = tagged_ptr.tag() => (Pat::Str("dyn"), Pat::Str("")),
         // NOTE: `TraitObject` is incomplete. It will always return true then.
         _ => (Pat::Str(""), Pat::Str("")),
     }

src/tools/clippy/clippy_utils/src/hir_utils.rs

@@ -1281,7 +1281,7 @@ impl<'a, 'tcx> SpanlessHash<'a, 'tcx> {
                 }
             },
             TyKind::Path(qpath) => self.hash_qpath(qpath),
-            TyKind::TraitObject(_, lifetime, _) => {
+            TyKind::TraitObject(_, lifetime) => {
                 self.hash_lifetime(lifetime);
             },
             TyKind::Typeof(anon_const) => {