Auto merge of #122045 - matthiaskrgr:rollup-5l3vpn7, r=matthiaskrgr

Rollup of 9 pull requests

Successful merges:

 - #121065 (Add basic i18n guidance for `Display`)
 - #121744 (Stop using Bubble in coherence and instead emulate it with an intercrate check)
 - #121829 (Dummy tweaks (attempt 2))
 - #121857 (Implement async closure signature deduction)
 - #121894 (const_eval_select: make it safe but be careful with what we expose on stable for now)
 - #122014 (Change some attributes to only_local.)
 - #122016 (will_wake tests fail on Miri and that is expected)
 - #122018 (only set noalias on Box with the global allocator)
 - #122028 (Remove some dead code)

r? `@ghost`
`@rustbot` modify labels: rollup

compiler/rustc_abi/src/lib.rs

@@ -1612,8 +1612,9 @@ pub enum PointerKind {
     SharedRef { frozen: bool },
     /// Mutable reference. `unpin` indicates the absence of any pinned data.
     MutableRef { unpin: bool },
-    /// Box. `unpin` indicates the absence of any pinned data.
+    /// Box. `unpin` indicates the absence of any pinned data. `global` indicates whether this box
-    Box { unpin: bool },
+    /// uses the global allocator or a custom one.
+    Box { unpin: bool, global: bool },
 }
 
 /// Note that this information is advisory only, and backends are free to ignore it.
@@ -1622,6 +1623,8 @@ pub enum PointerKind {
 pub struct PointeeInfo {
     pub size: Size,
     pub align: Align,
+    /// If this is `None`, then this is a raw pointer, so size and alignment are not guaranteed to
+    /// be reliable.
     pub safe: Option<PointerKind>,
 }
 

compiler/rustc_ast/src/mut_visit.rs

@@ -1604,7 +1604,10 @@ pub fn noop_visit_capture_by<T: MutVisitor>(capture_by: &mut CaptureBy, vis: &mu
     }
 }
 
-/// Some value for the AST node that is valid but possibly meaningless.
+/// Some value for the AST node that is valid but possibly meaningless. Similar
+/// to `Default` but not intended for wide use. The value will never be used
+/// meaningfully, it exists just to support unwinding in `visit_clobber` in the
+/// case where its closure panics.
 pub trait DummyAstNode {
     fn dummy() -> Self;
 }
@@ -1679,19 +1682,6 @@ impl DummyAstNode for Stmt {
     }
 }
 
-impl DummyAstNode for Block {
-    fn dummy() -> Self {
-        Block {
-            stmts: Default::default(),
-            id: DUMMY_NODE_ID,
-            rules: BlockCheckMode::Default,
-            span: Default::default(),
-            tokens: Default::default(),
-            could_be_bare_literal: Default::default(),
-        }
-    }
-}
-
 impl DummyAstNode for Crate {
     fn dummy() -> Self {
         Crate {

compiler/rustc_codegen_cranelift/example/mini_core.rs

@@ -525,8 +525,11 @@ pub struct Unique<T: ?Sized> {
 impl<T: ?Sized, U: ?Sized> CoerceUnsized<Unique<U>> for Unique<T> where T: Unsize<U> {}
 impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Unique<U>> for Unique<T> where T: Unsize<U> {}
 
+#[lang = "global_alloc_ty"]
+pub struct Global;
+
 #[lang = "owned_box"]
-pub struct Box<T: ?Sized, A = ()>(Unique<T>, A);
+pub struct Box<T: ?Sized, A = Global>(Unique<T>, A);
 
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
 
@@ -536,7 +539,7 @@ impl<T> Box<T> {
             let size = intrinsics::size_of::<T>();
             let ptr = libc::malloc(size);
             intrinsics::copy(&val as *const T as *const u8, ptr, size);
-            Box(Unique { pointer: NonNull(ptr as *const T), _marker: PhantomData }, ())
+            Box(Unique { pointer: NonNull(ptr as *const T), _marker: PhantomData }, Global)
         }
     }
 }

compiler/rustc_codegen_cranelift/src/unsize.rs

@@ -74,10 +74,6 @@ fn unsize_ptr<'tcx>(
         | (&ty::RawPtr(ty::TypeAndMut { ty: a, .. }), &ty::RawPtr(ty::TypeAndMut { ty: b, .. })) => {
             (src, unsized_info(fx, *a, *b, old_info))
         }
-        (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) if def_a.is_box() && def_b.is_box() => {
-            let (a, b) = (src_layout.ty.boxed_ty(), dst_layout.ty.boxed_ty());
-            (src, unsized_info(fx, a, b, old_info))
-        }
         (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {
             assert_eq!(def_a, def_b);
 

compiler/rustc_codegen_gcc/example/mini_core.rs

@@ -472,6 +472,7 @@ pub trait Allocator {
 
 impl Allocator for () {}
 
+#[lang = "global_alloc_ty"]
 pub struct Global;
 
 impl Allocator for Global {}

compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs

@@ -454,9 +454,13 @@ pub fn type_di_node<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, t: Ty<'tcx>) -> &'ll D
         ty::RawPtr(ty::TypeAndMut { ty: pointee_type, .. }) | ty::Ref(_, pointee_type, _) => {
             build_pointer_or_reference_di_node(cx, t, pointee_type, unique_type_id)
         }
-        // Box<T, A> may have a non-1-ZST allocator A. In that case, we
+        // Some `Box` are newtyped pointers, make debuginfo aware of that.
-        // cannot treat Box<T, A> as just an owned alias of `*mut T`.
+        // Only works if the allocator argument is a 1-ZST and hence irrelevant for layout
-        ty::Adt(def, args) if def.is_box() && cx.layout_of(args.type_at(1)).is_1zst() => {
+        // (or if there is no allocator argument).
+        ty::Adt(def, args)
+            if def.is_box()
+                && args.get(1).map_or(true, |arg| cx.layout_of(arg.expect_ty()).is_1zst()) =>
+        {
             build_pointer_or_reference_di_node(cx, t, t.boxed_ty(), unique_type_id)
         }
         ty::FnDef(..) | ty::FnPtr(_) => build_subroutine_type_di_node(cx, unique_type_id),

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -852,7 +852,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         }
 
         let instance = match intrinsic {
-            None | Some(ty::IntrinsicDef { name: sym::drop_in_place, .. }) => instance,
+            None => instance,
             Some(intrinsic) => {
                 let mut llargs = Vec::with_capacity(1);
                 let ret_dest = self.make_return_dest(

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -204,6 +204,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
 
     pub fn deref<Cx: LayoutTypeMethods<'tcx>>(self, cx: &Cx) -> PlaceRef<'tcx, V> {
         if self.layout.ty.is_box() {
+            // Derefer should have removed all Box derefs
             bug!("dereferencing {:?} in codegen", self.layout.ty);
         }
 

compiler/rustc_const_eval/src/interpret/place.rs

@@ -437,6 +437,7 @@ where
         trace!("deref to {} on {:?}", val.layout.ty, *val);
 
         if val.layout.ty.is_box() {
+            // Derefer should have removed all Box derefs
             bug!("dereferencing {}", val.layout.ty);
         }
 

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -359,14 +359,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             Ok(Some(match ty.kind() {
                 ty::Ref(_, ty, _) => *ty,
                 ty::RawPtr(mt) => mt.ty,
-                // We should only accept `Box` with the default allocator.
+                // We only accept `Box` with the default allocator.
-                // It's hard to test for that though so we accept every 1-ZST allocator.
+                _ if ty.is_box_global(*self.tcx) => ty.boxed_ty(),
-                ty::Adt(def, args)
-                    if def.is_box()
-                        && self.layout_of(args[1].expect_ty()).is_ok_and(|l| l.is_1zst()) =>
-                {
-                    args[0].expect_ty()
-                }
                 _ => return Ok(None),
             }))
         };

compiler/rustc_expand/src/base.rs

@@ -4,7 +4,6 @@ use crate::expand::{self, AstFragment, Invocation};
 use crate::module::DirOwnership;
 
 use rustc_ast::attr::MarkedAttrs;
-use rustc_ast::mut_visit::DummyAstNode;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Nonterminal};
 use rustc_ast::tokenstream::TokenStream;
@@ -582,6 +581,17 @@ impl DummyResult {
             tokens: None,
         })
     }
+
+    /// A plain dummy crate.
+    pub fn raw_crate() -> ast::Crate {
+        ast::Crate {
+            attrs: Default::default(),
+            items: Default::default(),
+            spans: Default::default(),
+            id: ast::DUMMY_NODE_ID,
+            is_placeholder: Default::default(),
+        }
+    }
 }
 
 impl MacResult for DummyResult {
@@ -650,7 +660,7 @@ impl MacResult for DummyResult {
     }
 
     fn make_crate(self: Box<DummyResult>) -> Option<ast::Crate> {
-        Some(DummyAstNode::dummy())
+        Some(DummyResult::raw_crate())
     }
 }
 

compiler/rustc_feature/src/builtin_attrs.rs

@@ -709,7 +709,9 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     // Internal attributes, Const related:
     // ==========================================================================
 
-    rustc_attr!(rustc_promotable, Normal, template!(Word), WarnFollowing, IMPL_DETAIL),
+    rustc_attr!(
+        rustc_promotable, Normal, template!(Word), WarnFollowing,
+        @only_local: true, IMPL_DETAIL),
     rustc_attr!(
         rustc_legacy_const_generics, Normal, template!(List: "N"), ErrorFollowing,
         INTERNAL_UNSTABLE
@@ -784,7 +786,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
          the given type by annotating all impl items with #[rustc_allow_incoherent_impl]."
     ),
     rustc_attr!(
-        rustc_box, AttributeType::Normal, template!(Word), ErrorFollowing,
+        rustc_box, AttributeType::Normal, template!(Word), ErrorFollowing, @only_local: true,
         "#[rustc_box] allows creating boxes \
         and it is only intended to be used in `alloc`."
     ),
@@ -806,11 +808,11 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     gated!(
         // Used in resolve:
         prelude_import, Normal, template!(Word), WarnFollowing,
-        "`#[prelude_import]` is for use by rustc only",
+        @only_local: true, "`#[prelude_import]` is for use by rustc only",
     ),
     gated!(
-        rustc_paren_sugar, Normal, template!(Word), WarnFollowing, unboxed_closures,
+        rustc_paren_sugar, Normal, template!(Word), WarnFollowing, @only_local: true,
-        "unboxed_closures are still evolving",
+        unboxed_closures, "unboxed_closures are still evolving",
     ),
     rustc_attr!(
         rustc_inherit_overflow_checks, Normal, template!(Word), WarnFollowing, @only_local: true,
@@ -826,27 +828,31 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
     rustc_attr!(
         rustc_test_marker, Normal, template!(NameValueStr: "name"), WarnFollowing,
-        "the `#[rustc_test_marker]` attribute is used internally to track tests",
+        @only_local: true, "the `#[rustc_test_marker]` attribute is used internally to track tests",
     ),
     rustc_attr!(
-        rustc_unsafe_specialization_marker, Normal, template!(Word), WarnFollowing,
+        rustc_unsafe_specialization_marker, Normal, template!(Word),
+        WarnFollowing, @only_local: true,
         "the `#[rustc_unsafe_specialization_marker]` attribute is used to check specializations"
     ),
     rustc_attr!(
-        rustc_specialization_trait, Normal, template!(Word), WarnFollowing,
+        rustc_specialization_trait, Normal, template!(Word),
+        WarnFollowing, @only_local: true,
         "the `#[rustc_specialization_trait]` attribute is used to check specializations"
     ),
     rustc_attr!(
-        rustc_main, Normal, template!(Word), WarnFollowing,
+        rustc_main, Normal, template!(Word), WarnFollowing, @only_local: true,
         "the `#[rustc_main]` attribute is used internally to specify test entry point function",
     ),
     rustc_attr!(
-        rustc_skip_array_during_method_dispatch, Normal, template!(Word), WarnFollowing,
+        rustc_skip_array_during_method_dispatch, Normal, template!(Word),
+        WarnFollowing, @only_local: true,
         "the `#[rustc_skip_array_during_method_dispatch]` attribute is used to exclude a trait \
         from method dispatch when the receiver is an array, for compatibility in editions < 2021."
     ),
     rustc_attr!(
-        rustc_must_implement_one_of, Normal, template!(List: "function1, function2, ..."), ErrorFollowing,
+        rustc_must_implement_one_of, Normal, template!(List: "function1, function2, ..."),
+        ErrorFollowing, @only_local: true,
         "the `#[rustc_must_implement_one_of]` attribute is used to change minimal complete \
         definition of a trait, it's currently in experimental form and should be changed before \
         being exposed outside of the std"
@@ -857,6 +863,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
     rustc_attr!(
         rustc_safe_intrinsic, Normal, template!(Word), WarnFollowing,
+        @only_local: true,
         "the `#[rustc_safe_intrinsic]` attribute is used internally to mark intrinsics as safe"
     ),
     rustc_attr!(
@@ -877,8 +884,14 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     // ==========================================================================
 
     rustc_attr!(TEST, rustc_effective_visibility, Normal, template!(Word), WarnFollowing),
-    rustc_attr!(TEST, rustc_outlives, Normal, template!(Word), WarnFollowing),
+    rustc_attr!(
-    rustc_attr!(TEST, rustc_capture_analysis, Normal, template!(Word), WarnFollowing),
+        TEST, rustc_outlives, Normal, template!(Word),
+        WarnFollowing, @only_local: true
+    ),
+    rustc_attr!(
+        TEST, rustc_capture_analysis, Normal, template!(Word),
+        WarnFollowing, @only_local: true
+    ),
     rustc_attr!(TEST, rustc_insignificant_dtor, Normal, template!(Word), WarnFollowing),
     rustc_attr!(TEST, rustc_strict_coherence, Normal, template!(Word), WarnFollowing),
     rustc_attr!(TEST, rustc_variance, Normal, template!(Word), WarnFollowing),

compiler/rustc_hir/src/lang_items.rs

@@ -267,6 +267,8 @@ language_item_table! {
     EhCatchTypeinfo,         sym::eh_catch_typeinfo,   eh_catch_typeinfo,          Target::Static,         GenericRequirement::None;
 
     OwnedBox,                sym::owned_box,           owned_box,                  Target::Struct,         GenericRequirement::Minimum(1);
+    GlobalAlloc,             sym::global_alloc_ty,     global_alloc_ty,            Target::Struct,         GenericRequirement::None;
+
     // Experimental language item for Miri
     PtrUnique,               sym::ptr_unique,          ptr_unique,                 Target::Struct,         GenericRequirement::Exact(1);
 

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -132,7 +132,8 @@ pub fn intrinsic_operation_unsafety(tcx: TyCtxt<'_>, intrinsic_id: LocalDefId) -
         | sym::fsub_algebraic
         | sym::fmul_algebraic
         | sym::fdiv_algebraic
-        | sym::frem_algebraic => hir::Unsafety::Normal,
+        | sym::frem_algebraic
+        | sym::const_eval_select => hir::Unsafety::Normal,
         _ => hir::Unsafety::Unsafe,
     };
 
@@ -247,7 +248,6 @@ pub fn check_intrinsic_type(
                 ],
                 Ty::new_unit(tcx),
             ),
-            sym::drop_in_place => (1, 0, vec![Ty::new_mut_ptr(tcx, param(0))], Ty::new_unit(tcx)),
             sym::needs_drop => (1, 0, vec![], tcx.types.bool),
 
             sym::type_name => (1, 0, vec![], Ty::new_static_str(tcx)),

compiler/rustc_hir_typeck/src/closure.rs

@@ -56,18 +56,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // It's always helpful for inference if we know the kind of
         // closure sooner rather than later, so first examine the expected
         // type, and see if can glean a closure kind from there.
-        let (expected_sig, expected_kind) = match closure.kind {
+        let (expected_sig, expected_kind) = match expected.to_option(self) {
-            hir::ClosureKind::Closure => match expected.to_option(self) {
+            Some(ty) => self.deduce_closure_signature(
-                Some(ty) => {
+                self.try_structurally_resolve_type(expr_span, ty),
-                    self.deduce_closure_signature(self.try_structurally_resolve_type(expr_span, ty))
+                closure.kind,
-                }
+            ),
-                None => (None, None),
+            None => (None, None),
-            },
-            // We don't want to deduce a signature from `Fn` bounds for coroutines
-            // or coroutine-closures, because the former does not implement `Fn`
-            // ever, and the latter's signature doesn't correspond to the coroutine
-            // type that it returns.
-            hir::ClosureKind::Coroutine(_) | hir::ClosureKind::CoroutineClosure(_) => (None, None),
         };
 
         let ClosureSignatures { bound_sig, mut liberated_sig } =
@@ -323,11 +317,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     fn deduce_closure_signature(
         &self,
         expected_ty: Ty<'tcx>,
+        closure_kind: hir::ClosureKind,
     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
         match *expected_ty.kind() {
             ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => self
                 .deduce_closure_signature_from_predicates(
                     expected_ty,
+                    closure_kind,
                     self.tcx
                         .explicit_item_bounds(def_id)
                         .iter_instantiated_copied(self.tcx, args)
@@ -336,7 +332,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             ty::Dynamic(object_type, ..) => {
                 let sig = object_type.projection_bounds().find_map(|pb| {
                     let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
-                    self.deduce_sig_from_projection(None, pb)
+                    self.deduce_sig_from_projection(None, closure_kind, pb)
                 });
                 let kind = object_type
                     .principal_def_id()
@@ -345,12 +341,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
             ty::Infer(ty::TyVar(vid)) => self.deduce_closure_signature_from_predicates(
                 Ty::new_var(self.tcx, self.root_var(vid)),
+                closure_kind,
                 self.obligations_for_self_ty(vid).map(|obl| (obl.predicate, obl.cause.span)),
             ),
-            ty::FnPtr(sig) => {
+            ty::FnPtr(sig) => match closure_kind {
-                let expected_sig = ExpectedSig { cause_span: None, sig };
+                hir::ClosureKind::Closure => {
-                (Some(expected_sig), Some(ty::ClosureKind::Fn))
+                    let expected_sig = ExpectedSig { cause_span: None, sig };
-            }
+                    (Some(expected_sig), Some(ty::ClosureKind::Fn))
+                }
+                hir::ClosureKind::Coroutine(_) | hir::ClosureKind::CoroutineClosure(_) => {
+                    (None, None)
+                }
+            },
             _ => (None, None),
         }
     }
@@ -358,6 +360,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     fn deduce_closure_signature_from_predicates(
         &self,
         expected_ty: Ty<'tcx>,
+        closure_kind: hir::ClosureKind,
         predicates: impl DoubleEndedIterator<Item = (ty::Predicate<'tcx>, Span)>,
     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
         let mut expected_sig = None;
@@ -386,6 +389,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     span,
                     self.deduce_sig_from_projection(
                         Some(span),
+                        closure_kind,
                         bound_predicate.rebind(proj_predicate),
                     ),
                 );
@@ -422,13 +426,22 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 ty::PredicateKind::Clause(ty::ClauseKind::Trait(data)) => Some(data.def_id()),
                 _ => None,
             };
-            if let Some(closure_kind) =
+
-                trait_def_id.and_then(|def_id| self.tcx.fn_trait_kind_from_def_id(def_id))
+            if let Some(trait_def_id) = trait_def_id {
-            {
+                let found_kind = match closure_kind {
-                expected_kind = Some(
+                    hir::ClosureKind::Closure => self.tcx.fn_trait_kind_from_def_id(trait_def_id),
-                    expected_kind
+                    hir::ClosureKind::CoroutineClosure(hir::CoroutineDesugaring::Async) => {
-                        .map_or_else(|| closure_kind, |current| cmp::min(current, closure_kind)),
+                        self.tcx.async_fn_trait_kind_from_def_id(trait_def_id)
-                );
+                    }
+                    _ => None,
+                };
+
+                if let Some(found_kind) = found_kind {
+                    expected_kind = Some(
+                        expected_kind
+                            .map_or_else(|| found_kind, |current| cmp::min(current, found_kind)),
+                    );
+                }
             }
         }
 
@@ -445,14 +458,21 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     fn deduce_sig_from_projection(
         &self,
         cause_span: Option<Span>,
+        closure_kind: hir::ClosureKind,
         projection: ty::PolyProjectionPredicate<'tcx>,
     ) -> Option<ExpectedSig<'tcx>> {
         let tcx = self.tcx;
 
         let trait_def_id = projection.trait_def_id(tcx);
-        // For now, we only do signature deduction based off of the `Fn` traits.
+
-        if !tcx.is_fn_trait(trait_def_id) {
+        // For now, we only do signature deduction based off of the `Fn` and `AsyncFn` traits,
-            return None;
+        // for closures and async closures, respectively.
+        match closure_kind {
+            hir::ClosureKind::Closure
+                if self.tcx.fn_trait_kind_from_def_id(trait_def_id).is_some() => {}
+            hir::ClosureKind::CoroutineClosure(hir::CoroutineDesugaring::Async)
+                if self.tcx.async_fn_trait_kind_from_def_id(trait_def_id).is_some() => {}
+            _ => return None,
         }
 
         let arg_param_ty = projection.skip_binder().projection_ty.args.type_at(1);

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -100,6 +100,15 @@ impl<'tcx> InferCtxt<'tcx> {
         let process = |a: Ty<'tcx>, b: Ty<'tcx>| match *a.kind() {
             ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) if def_id.is_local() => {
                 let def_id = def_id.expect_local();
+                if self.intercrate {
+                    // See comment on `insert_hidden_type` for why this is sufficient in coherence
+                    return Some(self.register_hidden_type(
+                        OpaqueTypeKey { def_id, args },
+                        cause.clone(),
+                        param_env,
+                        b,
+                    ));
+                }
                 match self.defining_use_anchor {
                     DefiningAnchor::Bind(_) => {
                         // Check that this is `impl Trait` type is
@@ -141,8 +150,10 @@ impl<'tcx> InferCtxt<'tcx> {
                         }
                     }
                     DefiningAnchor::Bubble => {}
-                    DefiningAnchor::Error => return None,
+                    DefiningAnchor::Error => {
-                };
+                        return None;
+                    }
+                }
                 if let ty::Alias(ty::Opaque, ty::AliasTy { def_id: b_def_id, .. }) = *b.kind() {
                     // We could accept this, but there are various ways to handle this situation, and we don't
                     // want to make a decision on it right now. Likely this case is so super rare anyway, that

compiler/rustc_middle/src/ty/layout.rs

@@ -969,6 +969,8 @@ where
         }
     }
 
+    /// Compute the information for the pointer stored at the given offset inside this type.
+    /// This will recurse into fields of ADTs to find the inner pointer.
     fn ty_and_layout_pointee_info_at(
         this: TyAndLayout<'tcx>,
         cx: &C,
@@ -1068,15 +1070,17 @@ where
                     }
                 }
 
-                // FIXME(eddyb) This should be for `ptr::Unique<T>`, not `Box<T>`.
+                // Fixup info for the first field of a `Box`. Recursive traversal will have found
+                // the raw pointer, so size and align are set to the boxed type, but `pointee.safe`
+                // will still be `None`.
                 if let Some(ref mut pointee) = result {
-                    if let ty::Adt(def, _) = this.ty.kind() {
+                    if offset.bytes() == 0 && this.ty.is_box() {
-                        if def.is_box() && offset.bytes() == 0 {
+                        debug_assert!(pointee.safe.is_none());
-                            let optimize = tcx.sess.opts.optimize != OptLevel::No;
+                        let optimize = tcx.sess.opts.optimize != OptLevel::No;
-                            pointee.safe = Some(PointerKind::Box {
+                        pointee.safe = Some(PointerKind::Box {
-                                unpin: optimize && this.ty.boxed_ty().is_unpin(tcx, cx.param_env()),
+                            unpin: optimize && this.ty.boxed_ty().is_unpin(tcx, cx.param_env()),
-                            });
+                            global: this.ty.is_box_global(tcx),
-                        }
+                        });
                     }
                 }
 

compiler/rustc_middle/src/ty/sty.rs

@@ -1999,6 +1999,27 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
+    /// Tests whether this is a Box using the global allocator.
+    #[inline]
+    pub fn is_box_global(self, tcx: TyCtxt<'tcx>) -> bool {
+        match self.kind() {
+            Adt(def, args) if def.is_box() => {
+                let Some(alloc) = args.get(1) else {
+                    // Single-argument Box is always global. (for "minicore" tests)
+                    return true;
+                };
+                if let Some(alloc_adt) = alloc.expect_ty().ty_adt_def() {
+                    let global_alloc = tcx.require_lang_item(LangItem::GlobalAlloc, None);
+                    alloc_adt.did() == global_alloc
+                } else {
+                    // Allocator is not an ADT...
+                    false
+                }
+            }
+            _ => false,
+        }
+    }
+
     /// Panics if called on any type other than `Box<T>`.
     pub fn boxed_ty(self) -> Ty<'tcx> {
         match self.kind() {

compiler/rustc_span/src/symbol.rs

@@ -896,6 +896,7 @@ symbols! {
         generic_const_items,
         generic_param_attrs,
         get_context,
+        global_alloc_ty,
         global_allocator,
         global_asm,
         globs,

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -26,7 +26,6 @@ use rustc_infer::traits::{util, FulfillmentErrorCode, TraitEngine, TraitEngineEx
 use rustc_middle::traits::query::NoSolution;
 use rustc_middle::traits::solve::{CandidateSource, Certainty, Goal};
 use rustc_middle::traits::specialization_graph::OverlapMode;
-use rustc_middle::traits::DefiningAnchor;
 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
 use rustc_middle::ty::visit::{TypeVisitable, TypeVisitableExt};
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeSuperVisitable, TypeVisitor};
@@ -207,7 +206,6 @@ fn overlap<'tcx>(
 
     let infcx = tcx
         .infer_ctxt()
-        .with_opaque_type_inference(DefiningAnchor::Bubble)
         .skip_leak_check(skip_leak_check.is_yes())
         .intercrate(true)
         .with_next_trait_solver(tcx.next_trait_solver_in_coherence())

compiler/rustc_ty_utils/src/abi.rs

@@ -452,7 +452,7 @@ fn adjust_for_rust_scalar<'tcx>(
             let no_alias = match kind {
                 PointerKind::SharedRef { frozen } => frozen,
                 PointerKind::MutableRef { unpin } => unpin && noalias_mut_ref,
-                PointerKind::Box { unpin } => unpin && noalias_for_box,
+                PointerKind::Box { unpin, global } => unpin && global && noalias_for_box,
             };
             // We can never add `noalias` in return position; that LLVM attribute has some very surprising semantics
             // (see <https://github.com/rust-lang/unsafe-code-guidelines/issues/385#issuecomment-1368055745>).

library/alloc/src/alloc.rs

@@ -50,6 +50,8 @@ extern "Rust" {
 #[unstable(feature = "allocator_api", issue = "32838")]
 #[derive(Copy, Clone, Default, Debug)]
 #[cfg(not(test))]
+// the compiler needs to know when a Box uses the global allocator vs a custom one
+#[cfg_attr(not(bootstrap), lang = "global_alloc_ty")]
 pub struct Global;
 
 #[cfg(test)]
@@ -385,6 +387,7 @@ pub const fn handle_alloc_error(layout: Layout) -> ! {
     }
 
     #[cfg(not(feature = "panic_immediate_abort"))]
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     unsafe {
         core::intrinsics::const_eval_select((layout,), ct_error, rt_error)
     }

library/alloc/src/boxed.rs

@@ -2062,6 +2062,9 @@ impl<Args: Tuple, F: AsyncFn<Args> + ?Sized, A: Allocator> AsyncFn<Args> for Box
 #[unstable(feature = "coerce_unsized", issue = "18598")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
 
+// It is quite crucial that we only allow the `Global` allocator here.
+// Handling arbitrary custom allocators (which can affect the `Box` layout heavily!)
+// would need a lot of codegen and interpreter adjustments.
 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Box<U>> for Box<T, Global> {}
 

library/alloc/tests/task.rs

@@ -4,6 +4,7 @@ use alloc::task::{LocalWake, Wake};
 use core::task::{LocalWaker, Waker};
 
 #[test]
+#[cfg_attr(miri, should_panic)] // `will_wake` doesn't guarantee that this test will work, and indeed on Miri it fails
 fn test_waker_will_wake_clone() {
     struct NoopWaker;
 
@@ -19,6 +20,7 @@ fn test_waker_will_wake_clone() {
 }
 
 #[test]
+#[cfg_attr(miri, should_panic)] // `will_wake` doesn't guarantee that this test will work, and indeed on Miri it fails
 fn test_local_waker_will_wake_clone() {
     struct NoopWaker;
 

library/core/src/ffi/c_str.rs

@@ -428,10 +428,13 @@ impl CStr {
             unsafe { &*(bytes as *const [u8] as *const CStr) }
         }
 
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: The const and runtime versions have identical behavior
         // unless the safety contract of `from_bytes_with_nul_unchecked` is
         // violated, which is UB.
-        unsafe { intrinsics::const_eval_select((bytes,), const_impl, rt_impl) }
+        unsafe {
+            intrinsics::const_eval_select((bytes,), const_impl, rt_impl)
+        }
     }
 
     /// Returns the inner pointer to this C string.
@@ -719,6 +722,9 @@ const unsafe fn const_strlen(ptr: *const c_char) -> usize {
         unsafe { strlen(s) }
     }
 
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     // SAFETY: the two functions always provide equivalent functionality
-    unsafe { intrinsics::const_eval_select((ptr,), strlen_ct, strlen_rt) }
+    unsafe {
+        intrinsics::const_eval_select((ptr,), strlen_ct, strlen_rt)
+    }
 }

library/core/src/fmt/mod.rs

@@ -633,6 +633,23 @@ pub use macros::Debug;
 /// [tostring]: ../../std/string/trait.ToString.html
 /// [tostring_function]: ../../std/string/trait.ToString.html#tymethod.to_string
 ///
+/// # Internationalization
+///
+/// Because a type can only have one `Display` implementation, it is often preferable
+/// to only implement `Display` when there is a single most "obvious" way that
+/// values can be formatted as text. This could mean formatting according to the
+/// "invariant" culture and "undefined" locale, or it could mean that the type
+/// display is designed for a specific culture/locale, such as developer logs.
+///
+/// If not all values have a justifiably canonical textual format or if you want
+/// to support alternative formats not covered by the standard set of possible
+/// [formatting traits], the most flexible approach is display adapters: methods
+/// like [`str::escape_default`] or [`Path::display`] which create a wrapper
+/// implementing `Display` to output the specific display format.
+///
+/// [formatting traits]: ../../std/fmt/index.html#formatting-traits
+/// [`Path::display`]: ../../std/path/struct.Path.html#method.display
+///
 /// # Examples
 ///
 /// Implementing `Display` on a type:

library/core/src/intrinsics.rs

@@ -2514,6 +2514,8 @@ extern "rust-intrinsic" {
     /// intrinsic will be replaced with a call to `called_in_const`. It gets
     /// replaced with a call to `called_at_rt` otherwise.
     ///
+    /// This function is safe to call, but note the stability concerns below.
+    ///
     /// # Type Requirements
     ///
     /// The two functions must be both function items. They cannot be function
@@ -2523,45 +2525,47 @@ extern "rust-intrinsic" {
     /// the two functions, therefore, both functions must accept the same type of
     /// arguments. Both functions must return RET.
     ///
-    /// # Safety
+    /// # Stability concerns
     ///
-    /// The two functions must behave observably equivalent. Safe code in other
+    /// Rust has not yet decided that `const fn` are allowed to tell whether
-    /// crates may assume that calling a `const fn` at compile-time and at run-time
+    /// they run at compile-time or at runtime. Therefore, when using this
-    /// produces the same result. A function that produces a different result when
+    /// intrinsic anywhere that can be reached from stable, it is crucial that
-    /// evaluated at run-time, or has any other observable side-effects, is
+    /// the end-to-end behavior of the stable `const fn` is the same for both
-    /// *unsound*.
+    /// modes of execution. (Here, Undefined Behavior is considered "the same"
+    /// as any other behavior, so if the function exhibits UB at runtime then
+    /// it may do whatever it wants at compile-time.)
     ///
     /// Here is an example of how this could cause a problem:
     /// ```no_run
     /// #![feature(const_eval_select)]
     /// #![feature(core_intrinsics)]
     /// # #![allow(internal_features)]
-    /// use std::hint::unreachable_unchecked;
+    /// # #![cfg_attr(bootstrap, allow(unused))]
     /// use std::intrinsics::const_eval_select;
     ///
-    /// // Crate A
+    /// // Standard library
+    /// # #[cfg(not(bootstrap))]
     /// pub const fn inconsistent() -> i32 {
     ///     fn runtime() -> i32 { 1 }
     ///     const fn compiletime() -> i32 { 2 }
     ///
-    ///     unsafe {
+    //      // ⚠ This code violates the required equivalence of `compiletime`
-    //          // ⚠ This code violates the required equivalence of `compiletime`
+    ///     // and `runtime`.
-    ///         // and `runtime`.
+    ///     const_eval_select((), compiletime, runtime)
-    ///         const_eval_select((), compiletime, runtime)
-    ///     }
     /// }
+    /// # #[cfg(bootstrap)]
+    /// # pub const fn inconsistent() -> i32 { 0 }
     ///
-    /// // Crate B
+    /// // User Crate
     /// const X: i32 = inconsistent();
     /// let x = inconsistent();
-    /// if x != X { unsafe { unreachable_unchecked(); }}
+    /// assert_eq!(x, X);
     /// ```
     ///
-    /// This code causes Undefined Behavior when being run, since the
+    /// Currently such an assertion would always succeed; until Rust decides
-    /// `unreachable_unchecked` is actually being reached. The bug is in *crate A*,
+    /// otherwise, that principle should not be violated.
-    /// which violates the principle that a `const fn` must behave the same at
-    /// compile-time and at run-time. The unsafe code in crate B is fine.
     #[rustc_const_unstable(feature = "const_eval_select", issue = "none")]
+    #[cfg_attr(not(bootstrap), rustc_safe_intrinsic)]
     pub fn const_eval_select<ARG: Tuple, F, G, RET>(
         arg: ARG,
         called_in_const: F,

library/core/src/num/f32.rs

@@ -1153,8 +1153,11 @@ impl f32 {
             // Stability concerns.
             unsafe { mem::transmute(x) }
         }
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: We use internal implementations that either always work or fail at compile time.
-        unsafe { intrinsics::const_eval_select((self,), ct_f32_to_u32, rt_f32_to_u32) }
+        unsafe {
+            intrinsics::const_eval_select((self,), ct_f32_to_u32, rt_f32_to_u32)
+        }
     }
 
     /// Raw transmutation from `u32`.
@@ -1245,8 +1248,11 @@ impl f32 {
             // Stability concerns.
             unsafe { mem::transmute(x) }
         }
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: We use internal implementations that either always work or fail at compile time.
-        unsafe { intrinsics::const_eval_select((v,), ct_u32_to_f32, rt_u32_to_f32) }
+        unsafe {
+            intrinsics::const_eval_select((v,), ct_u32_to_f32, rt_u32_to_f32)
+        }
     }
 
     /// Return the memory representation of this floating point number as a byte array in

library/core/src/num/f64.rs

@@ -1146,8 +1146,11 @@ impl f64 {
             // Stability concerns.
             unsafe { mem::transmute::<f64, u64>(rt) }
         }
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: We use internal implementations that either always work or fail at compile time.
-        unsafe { intrinsics::const_eval_select((self,), ct_f64_to_u64, rt_f64_to_u64) }
+        unsafe {
+            intrinsics::const_eval_select((self,), ct_f64_to_u64, rt_f64_to_u64)
+        }
     }
 
     /// Raw transmutation from `u64`.
@@ -1243,8 +1246,11 @@ impl f64 {
             // Stability concerns.
             unsafe { mem::transmute::<u64, f64>(rt) }
         }
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: We use internal implementations that either always work or fail at compile time.
-        unsafe { intrinsics::const_eval_select((v,), ct_u64_to_f64, rt_u64_to_f64) }
+        unsafe {
+            intrinsics::const_eval_select((v,), ct_u64_to_f64, rt_u64_to_f64)
+        }
     }
 
     /// Return the memory representation of this floating point number as a byte array in

library/core/src/panicking.rs

@@ -117,6 +117,7 @@ pub const fn panic_nounwind_fmt(fmt: fmt::Arguments<'_>, force_no_backtrace: boo
         panic_fmt(fmt);
     }
 
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     // SAFETY: const panic does not care about unwinding
     unsafe {
         super::intrinsics::const_eval_select((fmt, force_no_backtrace), comptime, runtime);

library/core/src/ptr/const_ptr.rs

@@ -48,8 +48,11 @@ impl<T: ?Sized> *const T {
             }
         }
 
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: The two versions are equivalent at runtime.
-        unsafe { const_eval_select((self as *const u8,), const_impl, runtime_impl) }
+        unsafe {
+            const_eval_select((self as *const u8,), const_impl, runtime_impl)
+        }
     }
 
     /// Casts to a pointer of another type.
@@ -806,6 +809,7 @@ impl<T: ?Sized> *const T {
     where
         T: Sized,
     {
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: The comparison has no side-effects, and the intrinsic
         // does this check internally in the CTFE implementation.
         unsafe {
@@ -1623,8 +1627,11 @@ impl<T: ?Sized> *const T {
             ptr.align_offset(align) == 0
         }
 
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: The two versions are equivalent at runtime.
-        unsafe { const_eval_select((self.cast::<()>(), align), const_impl, runtime_impl) }
+        unsafe {
+            const_eval_select((self.cast::<()>(), align), const_impl, runtime_impl)
+        }
     }
 }
 

library/core/src/ptr/mod.rs

@@ -995,6 +995,7 @@ pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
         };
     }
 
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     // SAFETY: the caller must guarantee that `x` and `y` are
     // valid for writes and properly aligned.
     unsafe {

library/core/src/ptr/mut_ptr.rs

@@ -48,8 +48,11 @@ impl<T: ?Sized> *mut T {
             }
         }
 
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: The two versions are equivalent at runtime.
-        unsafe { const_eval_select((self as *mut u8,), const_impl, runtime_impl) }
+        unsafe {
+            const_eval_select((self as *mut u8,), const_impl, runtime_impl)
+        }
     }
 
     /// Casts to a pointer of another type.
@@ -1896,8 +1899,11 @@ impl<T: ?Sized> *mut T {
             ptr.align_offset(align) == 0
         }
 
+        #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
         // SAFETY: The two versions are equivalent at runtime.
-        unsafe { const_eval_select((self.cast::<()>(), align), const_impl, runtime_impl) }
+        unsafe {
+            const_eval_select((self.cast::<()>(), align), const_impl, runtime_impl)
+        }
     }
 }
 

library/core/src/slice/index.rs

@@ -35,6 +35,7 @@ where
 #[track_caller]
 #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 const fn slice_start_index_len_fail(index: usize, len: usize) -> ! {
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     // SAFETY: we are just panicking here
     unsafe {
         const_eval_select(
@@ -63,6 +64,7 @@ const fn slice_start_index_len_fail_ct(_: usize, _: usize) -> ! {
 #[track_caller]
 #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 const fn slice_end_index_len_fail(index: usize, len: usize) -> ! {
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     // SAFETY: we are just panicking here
     unsafe {
         const_eval_select((index, len), slice_end_index_len_fail_ct, slice_end_index_len_fail_rt)
@@ -87,8 +89,11 @@ const fn slice_end_index_len_fail_ct(_: usize, _: usize) -> ! {
 #[track_caller]
 #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 const fn slice_index_order_fail(index: usize, end: usize) -> ! {
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     // SAFETY: we are just panicking here
-    unsafe { const_eval_select((index, end), slice_index_order_fail_ct, slice_index_order_fail_rt) }
+    unsafe {
+        const_eval_select((index, end), slice_index_order_fail_ct, slice_index_order_fail_rt)
+    }
 }
 
 // FIXME const-hack

library/core/src/str/mod.rs

@@ -86,6 +86,7 @@ use iter::{MatchesInternal, SplitNInternal};
 #[rustc_allow_const_fn_unstable(const_eval_select)]
 #[cfg(not(feature = "panic_immediate_abort"))]
 const fn slice_error_fail(s: &str, begin: usize, end: usize) -> ! {
+    #[cfg_attr(not(bootstrap), allow(unused_unsafe))] // on bootstrap bump, remove unsafe block
     // SAFETY: panics for both branches
     unsafe {
         crate::intrinsics::const_eval_select(

tests/codegen/function-arguments.rs

@@ -2,6 +2,7 @@
 #![crate_type = "lib"]
 #![feature(dyn_star)]
 #![feature(generic_nonzero)]
+#![feature(allocator_api)]
 
 use std::mem::MaybeUninit;
 use std::num::NonZero;
@@ -182,6 +183,15 @@ pub fn _box(x: Box<i32>) -> Box<i32> {
   x
 }
 
+// With a custom allocator, it should *not* have `noalias`. (See
+// <https://github.com/rust-lang/miri/issues/3341> for why.) The second argument is the allocator,
+// which is a reference here that still carries `noalias` as usual.
+// CHECK: @_box_custom(ptr noundef nonnull align 4 %x.0, ptr noalias noundef nonnull readonly align 1 %x.1)
+#[no_mangle]
+pub fn _box_custom(x: Box<i32, &std::alloc::Global>) {
+  drop(x)
+}
+
 // CHECK: noundef nonnull align 4 ptr @notunpin_box(ptr noundef nonnull align 4 %x)
 #[no_mangle]
 pub fn notunpin_box(x: Box<NotUnpin>) -> Box<NotUnpin> {

tests/ui/abi/compatibility.rs

@@ -160,6 +160,7 @@ mod prelude {
         pub _marker: PhantomData<T>,
     }
 
+    #[lang = "global_alloc_ty"]
     pub struct Global;
 
     #[lang = "owned_box"]

tests/ui/async-await/async-closures/signature-deduction.rs

@@ -0,0 +1,10 @@
+//@ check-pass
+//@ edition: 2021
+
+#![feature(async_closure)]
+
+async fn foo(x: impl async Fn(&str) -> &str) {}
+
+fn main() {
+    foo(async |x| x);
+}

tests/ui/intrinsics/const-eval-select-backtrace.rs

@@ -12,8 +12,5 @@ fn uhoh() {
 const fn c() {}
 
 fn main() {
-    // safety: this is unsound and just used to test
+    std::intrinsics::const_eval_select((), c, uhoh);
-    unsafe {
-        std::intrinsics::const_eval_select((), c, uhoh);
-    }
 }

tests/ui/intrinsics/const-eval-select-backtrace.run.stderr

@@ -1,3 +1,3 @@
-thread 'main' panicked at $DIR/const-eval-select-backtrace.rs:17:9:
+thread 'main' panicked at $DIR/const-eval-select-backtrace.rs:15:5:
 Aaah!
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

tests/ui/intrinsics/const-eval-select-stability.rs

@@ -13,7 +13,7 @@ const fn nothing(){}
 
 #[stable(since = "1.0", feature = "hey")]
 #[rustc_const_stable(since = "1.0", feature = "const_hey")]
-pub const unsafe fn hey() {
+pub const fn hey() {
     const_eval_select((), nothing, log);
     //~^ ERROR `const_eval_select` is not yet stable as a const fn
 }

tests/ui/intrinsics/const-eval-select-x86_64.rs

@@ -22,9 +22,7 @@ fn eq_rt(x: [i32; 4], y: [i32; 4]) -> bool {
 }
 
 const fn eq(x: [i32; 4], y: [i32; 4]) -> bool {
-    unsafe {
+    const_eval_select((x, y), eq_ct, eq_rt)
-        const_eval_select((x, y), eq_ct, eq_rt)
-    }
 }
 
 fn main() {

tests/ui/intrinsics/const-eval-select.rs

@@ -13,9 +13,9 @@ fn no() -> bool {
     false
 }
 
-// not a sound use case; testing only
+// not allowed on stable; testing only
 const fn is_const_eval() -> bool {
-    unsafe { const_eval_select((), yes, no) }
+    const_eval_select((), yes, no)
 }
 
 fn main() {

tests/ui/rfcs/rfc-2632-const-trait-impl/effects/minicore.rs

@@ -511,6 +511,7 @@ const fn drop<T: ~const Destruct>(_: T) {}
 
 extern "rust-intrinsic" {
     #[rustc_const_stable(feature = "const_eval_select", since = "1.0.0")]
+    #[rustc_safe_intrinsic]
     fn const_eval_select<ARG: Tuple, F, G, RET>(
         arg: ARG,
         called_in_const: F,
@@ -525,5 +526,5 @@ fn test_const_eval_select() {
     const fn const_fn() {}
     fn rt_fn() {}
 
-    unsafe { const_eval_select((), const_fn, rt_fn); }
+    const_eval_select((), const_fn, rt_fn);
 }