Stable inline const using unstable intrinsics #149817

oli-obk · 2025-12-09T16:27:25Z

Having moved several intrinsics from sth that gets invoked directly in a const fn to sth invoked in a const block kind of broke our "indirectly usable intrinsic" logic. So now we're actually checking it again. This required changing some intrinsics to now be indirectly callable

r? @jdonszelmann

Remove the custom special case rejecting const stability on macros as it is now handled by the general check

rustbot · 2025-12-09T16:27:29Z

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

Some changes occurred to constck

cc @fee1-dead

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

⚠️ #[rustc_intrinsic_const_stable_indirect] controls whether intrinsics can be exposed to stable const
code; adding it needs t-lang approval.

cc @rust-lang/wg-const-eval

oli-obk · 2025-12-09T16:29:06Z

library/core/src/intrinsics/mod.rs

 #[rustc_intrinsic]
 #[miri::intrinsic_fallback_is_spec]
+#[rustc_intrinsic_const_stable_indirect]
 pub const unsafe fn const_allocate(_size: usize, _align: usize) -> *mut u8 {


this and make_global are needed in

rust/library/alloc/src/boxed/thin.rs

Line 334 in e2893f7

let alloc: *mut u8 = const_allocate(alloc_size, alloc_align);

This one really shouldn't be callable-from-stable, it is the very definition of not stable... same for const_make_global.

I would strongly prefer to add allow_const_unstable to new_unsize_zst -- and we should consider if it's really safe to use these highly experimental features there.

That said, Thin is entirely unstable, so we can also add allow_const_unstable with a comment saying that this isn't actually stable -- but then we should also add that as a blocker on the tracking issue as it seems easy to miss.

I would strongly prefer to add allow_const_unstable to new_unsize_zst
That does feel nicer, to guard it against any other uses. With a comment there why specifically this call needs it and is safe

RalfJung · 2025-12-09T17:25:37Z

Is this a fix for #132536? Or only a partial fix since it "just" applies to inline consts, not const items?

oli-obk · 2025-12-10T07:25:35Z

compiler/rustc_const_eval/src/check_consts/mod.rs

-                || self.tcx.sess.opts.unstable_opts.force_unstable_if_unmarked)
+        matches!(
+            self.const_kind,
+            Some(hir::ConstContext::ConstFn | hir::ConstContext::Const { inline: true })


Only a partial fix. I can remove the inline limitation here, but it has a lot of fallout that I haven't analyzed yet. I'll do that in a follow-up

RalfJung

Looking at the remaining intrinsics, they aren't actually used from const fn either, are they? We just expose them via regular functions? The fact that those use a constant internally seems more of an implementation detail -- and we haven't had the t-lang FCP that would be required to actually expose them in a const fn.

View changes since this review

RalfJung · 2025-12-10T07:58:23Z

library/core/src/intrinsics/mod.rs

 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_intrinsic]
+#[rustc_intrinsic_const_stable_indirect]
 pub const fn variant_count<T>() -> usize;


This isn't actually stable either, I think? std::mem::variant_count is still unstable.

RalfJung · 2025-12-10T07:58:48Z

library/core/src/intrinsics/mod.rs

 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_intrinsic]
+#[rustc_intrinsic_const_stable_indirect]
 pub const fn type_name<T: ?Sized>() -> &'static str;


This isn't const-stable, it can only be invoked via a regular function.

RalfJung · 2025-12-10T07:59:23Z

library/core/src/intrinsics/mod.rs

 #[unstable(feature = "core_intrinsics", issue = "none")]
 #[rustc_intrinsic]
+#[rustc_intrinsic_const_stable_indirect]
 pub const fn type_id<T: ?Sized + 'static>() -> crate::any::TypeId;


And neither is this const-stable.

jdonszelmann · 2025-12-10T08:04:29Z

compiler/rustc_ast_lowering/src/asm.rs

                    }
                    InlineAsmOperand::Const { anon_const } => hir::InlineAsmOperand::Const {
-                        anon_const: self.lower_const_block(anon_const),
+                        anon_const: self.lower_const_block(anon_const, &[]),


you can put at least some atributes on const operands. Iirc recently cfg was stabilized here? Though that one is of course expanded out early. Still wonder where those attributes go and whether they should be forwarded here 🤔

even if in this case it only matters for their stability attrs

jdonszelmann · 2025-12-10T08:05:58Z

compiler/rustc_attr_parsing/src/attributes/stability.rs

+        Allow(Target::AssocConst),
+        Allow(Target::Trait),
+        Allow(Target::Static),
+        Allow(Target::Expression), // FIXME: we really only want to allow inline consts


we could make up a new target for that 🤷‍♀️ . This would be its current only use but it's not that hard to do otherwise

jdonszelmann · 2025-12-10T08:08:30Z

compiler/rustc_expand/src/base.rs


        let stability = find_attr!(attrs, AttributeKind::Stability { stability, .. } => *stability);

        // FIXME(jdonszelmann): make it impossible to miss the or_else in the typesystem


I forgot what my own fixme here refers to tbh. I think it looks like its already handled? Could you remove it?

jdonszelmann · 2025-12-10T08:09:43Z

compiler/rustc_expand/src/base.rs

        let stability = find_attr!(attrs, AttributeKind::Stability { stability, .. } => *stability);

        // FIXME(jdonszelmann): make it impossible to miss the or_else in the typesystem
-        if let Some(sp) = find_attr!(attrs, AttributeKind::ConstStability { span, .. } => *span) {


why are we removing this check from macros? Don't quite follow how its related to the rest of your PR

jdonszelmann · 2025-12-10T08:12:18Z

library/core/src/intrinsics/mod.rs

 #[rustc_intrinsic]
 #[miri::intrinsic_fallback_is_spec]
+#[rustc_intrinsic_const_stable_indirect]
 pub const unsafe fn const_allocate(_size: usize, _align: usize) -> *mut u8 {


I would strongly prefer to add allow_const_unstable to new_unsize_zst
That does feel nicer, to guard it against any other uses. With a comment there why specifically this call needs it and is safe

jdonszelmann · 2025-12-10T08:13:18Z

tests/ui/attributes/const-stability-on-macro.rs


 #[rustc_const_stable(feature = "foo", since = "3.3.3")]
-//~^ ERROR macros cannot have const stability attributes
+//~^ ERROR attribute cannot be used on macro defs


yea again am not entirely sure why removing this error from const stability is part of this PR.

jdonszelmann · 2025-12-10T08:14:16Z

tests/ui/consts/min_const_fn/const-fn-lang-feature.rs


 fn main() {
-    const { my_fun2() };
+    #[rustc_const_unstable(feature = "abi_unadjusted", issue = "42")]


this test seems more what I expect the the goal of the PR to be. Maybe split the macro parts out and separately motivate it? Unless I'm missing how it's actually related

rustbot · 2025-12-10T08:14:23Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

oli-obk added 2 commits December 9, 2025 11:59

Remove accidental const stability marker on a struct

2c6db18

Restrict the set of things that const stability can be applied

d79f825

Remove the custom special case rejecting const stability on macros as it is now handled by the general check

rustbot assigned jdonszelmann Dec 9, 2025

oli-obk commented Dec 9, 2025

View reviewed changes

Require const blocks to only use const stable things

8666a33

oli-obk force-pushed the stable_inline_const_using_unstable_intrinsics branch from 5640606 to 8666a33 Compare December 9, 2025 16:34

oli-obk commented Dec 10, 2025

View reviewed changes

RalfJung reviewed Dec 10, 2025

View reviewed changes

jdonszelmann requested changes Dec 10, 2025

View reviewed changes

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Dec 10, 2025


		let stability = find_attr!(attrs, AttributeKind::Stability { stability, .. } => *stability);

		// FIXME(jdonszelmann): make it impossible to miss the or_else in the typesystem

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Stable inline const using unstable intrinsics #149817

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