trait_sel: add builtin impl for PointeeSized

davidtwco · davidtwco · commit cc9699d2f998 · 2025-06-18T09:15:45.000Z
During review of rust-lang#137944, we thought we could remove the builtin impl of `PointeeSized` as it was removed during lowering, but users can still write `dyn PointeeSized` and so we need the builtin impl.
diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
@@ -1779,7 +1779,7 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
-    /// Fast path helper for testing if a type is `Sized` or `MetaSized`.
+    /// Fast path helper for testing if a type is `Sized`, `MetaSized` or `PointeeSized`.
     ///
     /// Returning true means the type is known to implement the sizedness trait. Returning `false`
     /// means nothing -- could be sized, might not be.
@@ -1814,11 +1814,12 @@ impl<'tcx> Ty<'tcx> {
 
             ty::Str | ty::Slice(_) | ty::Dynamic(_, _, ty::Dyn) => match sizedness {
                 SizedTraitKind::Sized => false,
-                SizedTraitKind::MetaSized => true,
+                SizedTraitKind::MetaSized | SizedTraitKind::PointeeSized => true,
             },
 
             ty::Foreign(..) => match sizedness {
                 SizedTraitKind::Sized | SizedTraitKind::MetaSized => false,
+                SizedTraitKind::PointeeSized => true,
             },
 
             ty::Tuple(tys) => tys.last().is_none_or(|ty| ty.has_trivial_sizedness(tcx, sizedness)),
diff --git a/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs b/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
@@ -476,7 +476,11 @@ where
                     G::consider_builtin_sizedness_candidates(self, goal, SizedTraitKind::MetaSized)
                 }
                 Some(TraitSolverLangItem::PointeeSized) => {
-                    unreachable!("`PointeeSized` is removed during lowering");
+                    G::consider_builtin_sizedness_candidates(
+                        self,
+                        goal,
+                        SizedTraitKind::PointeeSized,
+                    )
                 }
                 Some(TraitSolverLangItem::Copy | TraitSolverLangItem::Clone) => {
                     G::consider_builtin_copy_clone_candidate(self, goal)
diff --git a/compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs b/compiler/rustc_next_trait_solver/src/solve/assembly/structural_traits.rs
@@ -115,9 +115,9 @@ where
     I: Interner,
 {
     match ty.kind() {
-        // impl {Meta,}Sized for u*, i*, bool, f*, FnDef, FnPtr, *(const/mut) T, char
-        // impl {Meta,}Sized for &mut? T, [T; N], dyn* Trait, !, Coroutine, CoroutineWitness
-        // impl {Meta,}Sized for Closure, CoroutineClosure
+        // impl {Meta,Pointee,}Sized for u*, i*, bool, f*, FnDef, FnPtr, *(const/mut) T, char
+        // impl {Meta,Pointee,}Sized for &mut? T, [T; N], dyn* Trait, !, Coroutine, CoroutineWitness
+        // impl {Meta,Pointee,}Sized for Closure, CoroutineClosure
         ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
         | ty::Uint(_)
         | ty::Int(_)
@@ -138,14 +138,19 @@ where
         | ty::Dynamic(_, _, ty::DynStar)
         | ty::Error(_) => Ok(ty::Binder::dummy(vec![])),
 
-        // impl {Meta,}Sized for str, [T], dyn Trait
+        // impl {Meta,Pointee,}Sized for str, [T], dyn Trait
         ty::Str | ty::Slice(_) | ty::Dynamic(..) => match sizedness {
             SizedTraitKind::Sized => Err(NoSolution),
-            SizedTraitKind::MetaSized => Ok(ty::Binder::dummy(vec![])),
+            SizedTraitKind::MetaSized | SizedTraitKind::PointeeSized => {
+                Ok(ty::Binder::dummy(vec![]))
+            }
         },
 
-        // impl {} for extern type
-        ty::Foreign(..) => Err(NoSolution),
+        // impl PointeeSized for extern type
+        ty::Foreign(..) => match sizedness {
+            SizedTraitKind::Sized | SizedTraitKind::MetaSized => Err(NoSolution),
+            SizedTraitKind::PointeeSized => Ok(ty::Binder::dummy(vec![])),
+        },
 
         ty::Alias(..) | ty::Param(_) | ty::Placeholder(..) => Err(NoSolution),
 
@@ -156,17 +161,17 @@ where
 
         ty::UnsafeBinder(bound_ty) => Ok(bound_ty.map_bound(|ty| vec![ty])),
 
-        // impl {Meta,}Sized for ()
-        // impl {Meta,}Sized for (T1, T2, .., Tn) where Tn: {Meta,}Sized if n >= 1
+        // impl {Meta,Pointee,}Sized for ()
+        // impl {Meta,Pointee,}Sized for (T1, T2, .., Tn) where Tn: {Meta,}Sized if n >= 1
         ty::Tuple(tys) => Ok(ty::Binder::dummy(tys.last().map_or_else(Vec::new, |ty| vec![ty]))),
 
-        // impl {Meta,}Sized for Adt<Args...>
+        // impl {Meta,Pointee,}Sized for Adt<Args...>
         //   where {meta,pointee,}sized_constraint(Adt)<Args...>: {Meta,}Sized
         //
         //   `{meta,pointee,}sized_constraint(Adt)` is the deepest struct trail that can be
         //   determined by the definition of `Adt`, independent of the generic args.
         //
-        // impl {Meta,}Sized for Adt<Args...>
+        // impl {Meta,Pointee,}Sized for Adt<Args...>
         //   if {meta,pointee,}sized_constraint(Adt) == None
         //
         //   As a performance optimization, `{meta,pointee,}sized_constraint(Adt)` can return `None`
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/ambiguity.rs
@@ -201,7 +201,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 // begin with in those cases.
                 if matches!(
                     self.tcx.as_lang_item(trait_pred.def_id()),
-                    Some(LangItem::Sized | LangItem::MetaSized)
+                    Some(LangItem::Sized | LangItem::MetaSized | LangItem::PointeeSized)
                 ) {
                     match self.tainted_by_errors() {
                         None => {
diff --git a/compiler/rustc_trait_selection/src/solve/delegate.rs b/compiler/rustc_trait_selection/src/solve/delegate.rs
@@ -90,6 +90,13 @@ impl<'tcx> rustc_next_trait_solver::delegate::SolverDelegate for SolverDelegate<
                     {
                         return Some(Certainty::Yes);
                     }
+                    Some(LangItem::PointeeSized)
+                        if self
+                            .resolve_vars_if_possible(trait_pred.self_ty().skip_binder())
+                            .has_trivial_sizedness(self.0.tcx, SizedTraitKind::PointeeSized) =>
+                    {
+                        return Some(Certainty::Yes);
+                    }
                     Some(LangItem::Copy | LangItem::Clone) => {
                         let self_ty =
                             self.resolve_vars_if_possible(trait_pred.self_ty().skip_binder());
diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -101,7 +101,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     );
                 }
                 Some(LangItem::PointeeSized) => {
-                    bug!("`PointeeSized` is removed during lowering");
+                    self.assemble_builtin_sized_candidate(
+                        obligation,
+                        &mut candidates,
+                        SizedTraitKind::PointeeSized,
+                    );
                 }
                 Some(LangItem::Unsize) => {
                     self.assemble_candidates_for_unsizing(obligation, &mut candidates);
@@ -1113,7 +1117,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
-    /// Assembles the `Sized` and `MetaSized` traits which are built-in to the language itself.
+    /// Assembles the `Sized`, `MetaSized` and `PointeeSized` traits which are built-in to the
+    /// language itself.
     #[instrument(level = "debug", skip(self, candidates))]
     fn assemble_builtin_sized_candidate(
         &mut self,
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -270,7 +270,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     self.sizedness_conditions(obligation, SizedTraitKind::MetaSized)
                 }
                 Some(LangItem::PointeeSized) => {
-                    bug!("`PointeeSized` is removing during lowering");
+                    self.sizedness_conditions(obligation, SizedTraitKind::PointeeSized)
                 }
                 Some(LangItem::Copy | LangItem::Clone) => self.copy_clone_conditions(obligation),
                 Some(LangItem::FusedIterator) => self.fused_iterator_conditions(obligation),
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -2129,10 +2129,15 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
 
             ty::Str | ty::Slice(_) | ty::Dynamic(..) => match sizedness {
                 SizedTraitKind::Sized => None,
-                SizedTraitKind::MetaSized => Where(ty::Binder::dummy(Vec::new())),
+                SizedTraitKind::MetaSized | SizedTraitKind::PointeeSized => {
+                    Where(ty::Binder::dummy(Vec::new()))
+                }
             },
 
-            ty::Foreign(..) => None,
+            ty::Foreign(..) => match sizedness {
+                SizedTraitKind::Sized | SizedTraitKind::MetaSized => None,
+                SizedTraitKind::PointeeSized => Where(ty::Binder::dummy(Vec::new())),
+            },
 
             ty::Tuple(tys) => Where(
                 obligation.predicate.rebind(tys.last().map_or_else(Vec::new, |&last| vec![last])),
diff --git a/compiler/rustc_trait_selection/src/traits/util.rs b/compiler/rustc_trait_selection/src/traits/util.rs
@@ -374,6 +374,7 @@ pub fn sizedness_fast_path<'tcx>(tcx: TyCtxt<'tcx>, predicate: ty::Predicate<'tc
         let sizedness = match tcx.as_lang_item(trait_ref.def_id()) {
             Some(LangItem::Sized) => SizedTraitKind::Sized,
             Some(LangItem::MetaSized) => SizedTraitKind::MetaSized,
+            Some(LangItem::PointeeSized) => SizedTraitKind::PointeeSized,
             _ => return false,
         };
 
diff --git a/compiler/rustc_ty_utils/src/ty.rs b/compiler/rustc_ty_utils/src/ty.rs
@@ -23,7 +23,7 @@ fn sizedness_constraint_for_ty<'tcx>(
     ty: Ty<'tcx>,
 ) -> Option<Ty<'tcx>> {
     match ty.kind() {
-        // Always `Sized` or `MetaSized`
+        // Always `{Meta,Pointee,}Sized`
         ty::Bool
         | ty::Char
         | ty::Int(..)
@@ -44,11 +44,11 @@ fn sizedness_constraint_for_ty<'tcx>(
         ty::Str | ty::Slice(..) | ty::Dynamic(_, _, ty::Dyn) => match sizedness {
             // Never `Sized`
             SizedTraitKind::Sized => Some(ty),
-            // Always `MetaSized`
-            SizedTraitKind::MetaSized => None,
+            // Always `{Meta,Pointee}Sized`
+            SizedTraitKind::MetaSized | SizedTraitKind::PointeeSized => None,
         },
 
-        // Maybe `Sized` or `MetaSized`
+        // Maybe `{Meta,Pointee,}Sized`
         ty::Param(..) | ty::Alias(..) | ty::Error(_) => Some(ty),
 
         // We cannot instantiate the binder, so just return the *original* type back,
@@ -58,8 +58,11 @@ fn sizedness_constraint_for_ty<'tcx>(
             sizedness_constraint_for_ty(tcx, sizedness, inner_ty.skip_binder()).map(|_| ty)
         }
 
-        // Never `MetaSized` or `Sized`
-        ty::Foreign(..) => Some(ty),
+        // Never `{Meta,}Sized`
+        ty::Foreign(..) => match sizedness {
+            SizedTraitKind::Sized | SizedTraitKind::MetaSized => Some(ty),
+            SizedTraitKind::PointeeSized => None,
+        },
 
         // Recursive cases
         ty::Pat(ty, _) => sizedness_constraint_for_ty(tcx, sizedness, *ty),
diff --git a/compiler/rustc_type_ir/src/solve/mod.rs b/compiler/rustc_type_ir/src/solve/mod.rs
@@ -377,6 +377,8 @@ pub enum SizedTraitKind {
     Sized,
     /// `MetaSized` trait
     MetaSized,
+    /// `PointeeSized` trait
+    PointeeSized,
 }
 
 impl SizedTraitKind {
@@ -385,6 +387,7 @@ impl SizedTraitKind {
         cx.require_lang_item(match self {
             SizedTraitKind::Sized => TraitSolverLangItem::Sized,
             SizedTraitKind::MetaSized => TraitSolverLangItem::MetaSized,
+            SizedTraitKind::PointeeSized => TraitSolverLangItem::PointeeSized,
         })
     }
 }
diff --git a/tests/ui/sized-hierarchy/dyn-pointeesized-issue-142652-1.rs b/tests/ui/sized-hierarchy/dyn-pointeesized-issue-142652-1.rs
@@ -0,0 +1,12 @@
+//@ check-pass
+#![feature(sized_hierarchy)]
+
+use std::marker::PointeeSized;
+
+type Foo = dyn PointeeSized;
+
+fn foo(f: &Foo) {}
+
+fn main() {
+    foo(&());
+}
diff --git a/tests/ui/sized-hierarchy/dyn-pointeesized-issue-142652.rs b/tests/ui/sized-hierarchy/dyn-pointeesized-issue-142652.rs
@@ -0,0 +1,9 @@
+#![feature(sized_hierarchy)]
+
+use std::marker::PointeeSized;
+
+fn main() {
+      let x = main;
+      let y: Box<dyn PointeeSized> = x;
+//~^ ERROR mismatched types
+}
diff --git a/tests/ui/sized-hierarchy/dyn-pointeesized-issue-142652.stderr b/tests/ui/sized-hierarchy/dyn-pointeesized-issue-142652.stderr
@@ -0,0 +1,19 @@
+error[E0308]: mismatched types
+  --> $DIR/dyn-pointeesized-issue-142652.rs:7:38
+   |
+LL |       let y: Box<dyn PointeeSized> = x;
+   |              ---------------------   ^ expected `Box<dyn PointeeSized>`, found fn item
+   |              |
+   |              expected due to this
+   |
+   = note: expected struct `Box<dyn PointeeSized>`
+             found fn item `fn() {main}`
+   = note: for more on the distinction between the stack and the heap, read https://doc.rust-lang.org/book/ch15-01-box.html, https://doc.rust-lang.org/rust-by-example/std/box.html, and https://doc.rust-lang.org/std/boxed/index.html
+help: store this in the heap by calling `Box::new`
+   |
+LL |       let y: Box<dyn PointeeSized> = Box::new(x);
+   |                                      +++++++++ +
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

Original file line number	Diff line number	Diff line change
`@@ -476,7 +476,11 @@ where`
`476`	`476`	`G::consider_builtin_sizedness_candidates(self, goal, SizedTraitKind::MetaSized)`
`477`	`477`	`}`
`478`	`478`	`Some(TraitSolverLangItem::PointeeSized) => {`
`479`		- unreachable!("`PointeeSized` is removed during lowering");
	`479`	`+ G::consider_builtin_sizedness_candidates(`
	`480`	`+ self,`
	`481`	`+ goal,`
	`482`	`+ SizedTraitKind::PointeeSized,`
	`483`	`+ )`
`480`	`484`	`}`
`481`	`485`	`Some(TraitSolverLangItem::Copy \| TraitSolverLangItem::Clone) => {`
`482`	`486`	`G::consider_builtin_copy_clone_candidate(self, goal)`
Original file line number	Diff line number	Diff line change
`@@ -90,6 +90,13 @@ impl<'tcx> rustc_next_trait_solver::delegate::SolverDelegate for SolverDelegate<`
`90`	`90`	`{`
`91`	`91`	`return Some(Certainty::Yes);`
`92`	`92`	`}`
	`93`	`+ Some(LangItem::PointeeSized)`
	`94`	`+ if self`
	`95`	`+ .resolve_vars_if_possible(trait_pred.self_ty().skip_binder())`
	`96`	`+ .has_trivial_sizedness(self.0.tcx, SizedTraitKind::PointeeSized) =>`
	`97`	`+ {`
	`98`	`+ return Some(Certainty::Yes);`
	`99`	`+ }`
`93`	`100`	`Some(LangItem::Copy \| LangItem::Clone) => {`
`94`	`101`	`let self_ty =`
`95`	`102`	`self.resolve_vars_if_possible(trait_pred.self_ty().skip_binder());`
Original file line number	Diff line number	Diff line change
`@@ -101,7 +101,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {`
`101`	`101`	`);`
`102`	`102`	`}`
`103`	`103`	`Some(LangItem::PointeeSized) => {`
`104`		- bug!("`PointeeSized` is removed during lowering");
	`104`	`+ self.assemble_builtin_sized_candidate(`
	`105`	`+ obligation,`
	`106`	`+ &mut candidates,`
	`107`	`+ SizedTraitKind::PointeeSized,`
	`108`	`+ );`
`105`	`109`	`}`
`106`	`110`	`Some(LangItem::Unsize) => {`
`107`	`111`	`self.assemble_candidates_for_unsizing(obligation, &mut candidates);`
`@@ -1113,7 +1117,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {`
`1113`	`1117`	`}`
`1114`	`1118`	`}`
`1115`	`1119`
`1116`		- /// Assembles the `Sized` and `MetaSized` traits which are built-in to the language itself.
	`1120`	+ /// Assembles the `Sized`, `MetaSized` and `PointeeSized` traits which are built-in to the
	`1121`	`+ /// language itself.`
`1117`	`1122`	`#[instrument(level = "debug", skip(self, candidates))]`
`1118`	`1123`	`fn assemble_builtin_sized_candidate(`
`1119`	`1124`	`&mut self,`
Original file line number	Diff line number	Diff line change
`@@ -270,7 +270,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {`
`270`	`270`	`self.sizedness_conditions(obligation, SizedTraitKind::MetaSized)`
`271`	`271`	`}`
`272`	`272`	`Some(LangItem::PointeeSized) => {`
`273`		- bug!("`PointeeSized` is removing during lowering");
	`273`	`+ self.sizedness_conditions(obligation, SizedTraitKind::PointeeSized)`
`274`	`274`	`}`
`275`	`275`	`Some(LangItem::Copy \| LangItem::Clone) => self.copy_clone_conditions(obligation),`
`276`	`276`	`Some(LangItem::FusedIterator) => self.fused_iterator_conditions(obligation),`