Add rustc_do_not_implement_via_object

compiler/rustc_feature/src/builtin_attrs.rs

@@ -708,6 +708,11 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         rustc_deny_explicit_impl, AttributeType::Normal, template!(Word), ErrorFollowing, @only_local: false,
         "#[rustc_deny_explicit_impl] enforces that a trait can have no user-provided impls"
     ),
+    rustc_attr!(
+        rustc_do_not_implement_via_object, AttributeType::Normal, template!(Word), ErrorFollowing, @only_local: true,
+        "#[rustc_do_not_implement_via_object] marks a trait so that `dyn Trait` does not \
+        implement `Trait` (without requiring `Sized` as a supertrait)"
+    ),
     rustc_attr!(
         rustc_has_incoherent_inherent_impls, AttributeType::Normal, template!(Word), ErrorFollowing,
         "#[rustc_has_incoherent_inherent_impls] allows the addition of incoherent inherent impls for \

compiler/rustc_hir_analysis/src/collect.rs

@@ -985,6 +985,7 @@ fn trait_def(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::TraitDef {
 
             no_dups.then_some(list)
         });
+    let do_not_implement_via_object = tcx.has_attr(def_id, sym::rustc_do_not_implement_via_object);
 
     ty::TraitDef {
         def_id: def_id.to_def_id(),
@@ -996,6 +997,7 @@ fn trait_def(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::TraitDef {
         skip_array_during_method_dispatch,
         specialization_kind,
         must_implement_one_of,
+        do_not_implement_via_object,
     }
 }
 

compiler/rustc_middle/src/ty/trait_def.rs

@@ -52,6 +52,11 @@ pub struct TraitDef {
     /// List of functions from `#[rustc_must_implement_one_of]` attribute one of which
     /// must be implemented.
     pub must_implement_one_of: Option<Box<[Ident]>>,
+
+    /// Whether a type's built-in `dyn Trait: Trait` implementation is explicitly
+    /// denied. This only applies to built-in trait, and is marked via
+    /// `#[rustc_do_not_implement_via_object]`.
+    pub do_not_implement_via_object: bool,
 }
 
 /// Whether this trait is treated specially by the standard library

compiler/rustc_span/src/symbol.rs

@@ -1270,6 +1270,7 @@ symbols! {
         rustc_diagnostic_macros,
         rustc_dirty,
         rustc_do_not_const_check,
+        rustc_do_not_implement_via_object,
         rustc_doc_primitive,
         rustc_dummy,
         rustc_dump_env_program_clauses,

compiler/rustc_trait_selection/src/solve/assembly/mod.rs

@@ -631,6 +631,11 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         goal: Goal<'tcx, G>,
         candidates: &mut Vec<Candidate<'tcx>>,
     ) {
+        let tcx = self.tcx();
+        if !tcx.trait_def(goal.predicate.trait_def_id(tcx)).implement_via_object {
+            return;
+        }
+
         let self_ty = goal.predicate.self_ty();
         let bounds = match *self_ty.kind() {
             ty::Bool
@@ -663,7 +668,6 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             ty::Dynamic(bounds, ..) => bounds,
         };
 
-        let tcx = self.tcx();
         let own_bounds: FxIndexSet<_> =
             bounds.iter().map(|bound| bound.with_self_ty(tcx, self_ty)).collect();
         for assumption in elaborate(tcx, own_bounds.iter().copied())

library/core/src/marker.rs

@@ -174,6 +174,7 @@ pub trait Sized {
 #[unstable(feature = "unsize", issue = "18598")]
 #[lang = "unsize"]
 #[rustc_deny_explicit_impl]
+#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
 pub trait Unsize<T: ?Sized> {
     // Empty.
 }
@@ -855,6 +856,7 @@ impl<T: ?Sized> StructuralEq for PhantomData<T> {}
 )]
 #[lang = "discriminant_kind"]
 #[rustc_deny_explicit_impl]
+#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
 pub trait DiscriminantKind {
     /// The type of the discriminant, which must satisfy the trait
     /// bounds required by `mem::Discriminant`.
@@ -960,6 +962,7 @@ marker_impls! {
 #[lang = "destruct"]
 #[rustc_on_unimplemented(message = "can't drop `{Self}`", append_const_msg)]
 #[rustc_deny_explicit_impl]
+#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
 #[const_trait]
 pub trait Destruct {}
 
@@ -971,6 +974,7 @@ pub trait Destruct {}
 #[lang = "tuple_trait"]
 #[rustc_on_unimplemented(message = "`{Self}` is not a tuple")]
 #[rustc_deny_explicit_impl]
+#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
 pub trait Tuple {}
 
 /// A marker for pointer-like types.

library/core/src/mem/transmutability.rs

@@ -7,6 +7,7 @@ use crate::marker::ConstParamTy;
 /// notwithstanding whatever safety checks you have asked the compiler to [`Assume`] are satisfied.
 #[unstable(feature = "transmutability", issue = "99571")]
 #[lang = "transmute_trait"]
+#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
 pub unsafe trait BikeshedIntrinsicFrom<Src, Context, const ASSUME: Assume = { Assume::NOTHING }>
 where
     Src: ?Sized,

library/core/src/ptr/metadata.rs

@@ -51,6 +51,7 @@ use crate::hash::{Hash, Hasher};
 /// [`to_raw_parts`]: *const::to_raw_parts
 #[lang = "pointee_trait"]
 #[rustc_deny_explicit_impl]
+#[cfg_attr(not(bootstrap), rustc_do_not_implement_via_object)]
 pub trait Pointee {
     /// The type for metadata in pointers and references to `Self`.
     #[lang = "metadata_type"]

tests/ui/unsized/issue-71659.current.stderr

@@ -1,11 +1,11 @@
 error[E0277]: the trait bound `dyn Foo: CastTo<[i32]>` is not satisfied
-  --> $DIR/issue-71659.rs:30:15
+  --> $DIR/issue-71659.rs:33:15
    |
 LL |     let x = x.cast::<[i32]>();
    |               ^^^^ the trait `CastTo<[i32]>` is not implemented for `dyn Foo`
    |
 note: required by a bound in `Cast::cast`
-  --> $DIR/issue-71659.rs:19:15
+  --> $DIR/issue-71659.rs:22:15
    |
 LL |     fn cast<T: ?Sized>(&self) -> &T
    |        ---- required by a bound in this associated function

tests/ui/unsized/issue-71659.next.stderr

@@ -0,0 +1,18 @@
+error[E0277]: the trait bound `dyn Foo: CastTo<[i32]>` is not satisfied
+  --> $DIR/issue-71659.rs:33:15
+   |
+LL |     let x = x.cast::<[i32]>();
+   |               ^^^^ the trait `CastTo<[i32]>` is not implemented for `dyn Foo`
+   |
+note: required by a bound in `Cast::cast`
+  --> $DIR/issue-71659.rs:22:15
+   |
+LL |     fn cast<T: ?Sized>(&self) -> &T
+   |        ---- required by a bound in this associated function
+LL |     where
+LL |         Self: CastTo<T>,
+   |               ^^^^^^^^^ required by this bound in `Cast::cast`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/unsized/issue-71659.rs

@@ -1,3 +1,6 @@
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
+
 #![feature(unsize)]
 
 use std::marker::Unsize;