Stabilize `funnel_shifts` (including `const`) by tgross35 · Pull Request #161015 · rust-lang/rust

tgross35 · 2026-08-13T01:55:25Z

funnel_shl and funnel_shr have been around for close to a year, the unchecked versions for a number of months. These are reasonably small and uncontroversial, and it can be tricky to get similar performance with a fallback; stabilize them here.

Newly stable API:

impl {u8, u16, u32, u64, u128, usize} {
    pub const fn funnel_shl(self, right: Self, shift: u32) -> Self;
    pub const fn funnel_shr(self, right: Self, shift: u32) -> Self;

    pub const unsafe fn unchecked_funnel_shl(self, right: Self, shift: u32) -> Self;
    pub const unsafe fn unchecked_funnel_shr(self, right: Self, shift: u32) -> Self;
}

The tracking issue also mentions a wrapping_ version but it has not been implemented.

Closes: #145686 (tracking issue, wrapping versions will need a new issue)

This involves an intrinsic stabilization so needs FCP from both @rust-lang/lang and @rust-lang/libs-api.

@rustbot label +I-lang-nominated +I-libs-api-nominated

Cc others who have been involved: @RalfJung, @folkertdev, @sayantn

r? libs

rustbot · 2026-08-13T01:55:28Z

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

⚠️ #[rustc_allow_const_fn_unstable] needs careful audit to avoid accidentally exposing unstable
implementation details on stable.

cc @rust-lang/wg-const-eval

⚠️ #[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

miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

cc @rust-lang/miri

stdarch is developed in its own repository. If possible, consider making this change to rust-lang/stdarch instead.

cc @Amanieu, @folkertdev, @sayantn

tgross35 · 2026-08-13T02:00:21Z

+#[rustc_intrinsic_const_stable_indirect]
 #[rustc_intrinsic]
 #[rustc_nounwind]
-#[rustc_const_unstable(feature = "funnel_shifts", issue = "145686")]
-#[unstable(feature = "funnel_shifts", issue = "145686")]
 #[track_caller]
+#[rustc_allow_const_fn_unstable(const_trait_impl, core_intrinsics_fallbacks)]
 #[miri::intrinsic_fallback_is_spec]
 pub const unsafe fn unchecked_funnel_shl<T: [const] fallback::FunnelShift>(


Note that const_trait_impl is only needed for the fallback. We could implement a version without it if needed.

The fallback traits are gated behind core_intrinsics_fallbacks, I'm not sure whether there's a cleaner way to do this.

View changes since the review

jdahlstrom · 2026-08-13T14:27:53Z

This should be "right half", right?

You're right; #161065

traviscross · 2026-08-13T14:53:49Z

Thanks @tgross35.

@rfcbot fcp merge lang,libs

rust-rfcbot · 2026-08-13T14:53:52Z

@traviscross has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

scottmcm · 2026-08-13T18:45:05Z

Zero lang concerns; this is a value operation with fallback MIR showing it can be implemented with existing rust.

@rfcbot reviewed

scottmcm · 2026-08-13T18:47:45Z

A concern for the libs(-api) part: Is this actually guaranteed to panic for out-of-range shifts?

It feels inconsistent with other things. I'd expect that funnel_shift would wrap in release and panic in debug, and that there'd be a strict_funnel_shift that would always panic for out-of-range shifts.

And it's a bit weird to not have the wrapping version of this. It'd be trivial to implement -- just mask the shift passed to unchecked -- so why not just add it before stabilizing the set?

tgross35 · 2026-08-13T19:20:33Z

Wrapping by default was the original proposal but was decided against, see rust-lang/libs-team#642 (comment).

The wrapping_ version was accepted as part of that ACP, but it never got added. I don't think we'd want to stabilize it right away but also don't want it to block these any longer since they're ready and useful.

pthariensflame · 2026-08-14T17:07:15Z

We’ll implement the wrapping versions, independently of this PR.

EDIT: Done in #161119.

…r=nia-e Stabilize `funnel_shifts` (including `const`) `funnel_shl` and `funnel_shr` have been around for close to a year, the unchecked versions for a number of months. These are reasonably small and uncontroversial, and it can be tricky to get similar performance with a fallback; stabilize them here. Newly stable API: ```rust impl {u8, u16, u32, u64, u128, usize} { pub const fn funnel_shl(self, right: Self, shift: u32) -> Self; pub const fn funnel_shr(self, right: Self, shift: u32) -> Self; pub const unsafe fn unchecked_funnel_shl(self, right: Self, shift: u32) -> Self; pub const unsafe fn unchecked_funnel_shr(self, right: Self, shift: u32) -> Self; } ``` The tracking issue also mentions a `wrapping_` version but it has not been implemented. Closes: rust-lang#145686 (tracking issue, wrapping versions will need a new issue)

…uwer Rollup of 15 pull requests Successful merges: - #158936 (Add `std::fs::{Home|Media}Dirs`) - #129036 (Additional NonZero conversions) - #158997 (Avoid recording unnameable `extern crate` aliases in diagnostic metadata) - #161015 (Stabilize `funnel_shifts` (including `const`)) - #161712 (Stabilize `Result::into_{ok,err}`) - #162493 (Add support for -Zsanitizer-cfi-minimal-runtime) - #162655 (next solver: prefer to select impl candidates over global where-clause candidates) - #162862 (Fix intra doc link resolution when a doc comment is composed of both inner and outer doc comment) - #163200 (make `RustaceansAreAwesome` satisfy trait bounds) - #163331 (Move `Arc` and `Rc` into `rcs` mod) - #163427 (implement #![feature(gca_adts)]) - #163428 (do not complain about unstable target features on nightly) - #163444 (Add `stable_rustc` helper in `run-make-support`) - #163447 (Allow using different index types when reading and writing to tables) - #163450 (Force the correct type variable to never for method resolution on an adjusted never type)

rust-bors · 2026-09-29T04:18:30Z

⌛ Testing commit f7548dd with merge e666839...

Workflow: https://github.com/rust-lang/rust/actions/runs/36521046367

Stabilize `funnel_shifts` (including `const`) `funnel_shl` and `funnel_shr` have been around for close to a year, the unchecked versions for a number of months. These are reasonably small and uncontroversial, and it can be tricky to get similar performance with a fallback; stabilize them here. Newly stable API: ```rust impl {u8, u16, u32, u64, u128, usize} { pub const fn funnel_shl(self, right: Self, shift: u32) -> Self; pub const fn funnel_shr(self, right: Self, shift: u32) -> Self; pub const unsafe fn unchecked_funnel_shl(self, right: Self, shift: u32) -> Self; pub const unsafe fn unchecked_funnel_shr(self, right: Self, shift: u32) -> Self; } ``` The tracking issue also mentions a `wrapping_` version but it has not been implemented. Closes: #145686 (tracking issue, wrapping versions will need a new issue)

…r=nia-e Stabilize `funnel_shifts` (including `const`) `funnel_shl` and `funnel_shr` have been around for close to a year, the unchecked versions for a number of months. These are reasonably small and uncontroversial, and it can be tricky to get similar performance with a fallback; stabilize them here. Newly stable API: ```rust impl {u8, u16, u32, u64, u128, usize} { pub const fn funnel_shl(self, right: Self, shift: u32) -> Self; pub const fn funnel_shr(self, right: Self, shift: u32) -> Self; pub const unsafe fn unchecked_funnel_shl(self, right: Self, shift: u32) -> Self; pub const unsafe fn unchecked_funnel_shr(self, right: Self, shift: u32) -> Self; } ``` The tracking issue also mentions a `wrapping_` version but it has not been implemented. Closes: rust-lang#145686 (tracking issue, wrapping versions will need a new issue)

tgross35 · 2026-09-29T04:21:48Z

@bors retry yield to rollup

rust-bors · 2026-09-29T04:21:52Z

❗ You can only retry pull requests that are approved and have a previously failed auto build.

Hint: There is currently a pending auto build on this PR. To cancel it, run @bors cancel.

tgross35 · 2026-09-29T04:22:39Z

@bors yield ...?

rust-bors · 2026-09-29T04:22:42Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/36521046367

The next pull request likely to be tested is #161015.

rust-bors · 2026-09-29T04:22:50Z

⌛ Testing commit f7548dd with merge 5aeecdc...

Workflow: https://github.com/rust-lang/rust/actions/runs/36521366415

Stabilize `funnel_shifts` (including `const`) `funnel_shl` and `funnel_shr` have been around for close to a year, the unchecked versions for a number of months. These are reasonably small and uncontroversial, and it can be tricky to get similar performance with a fallback; stabilize them here. Newly stable API: ```rust impl {u8, u16, u32, u64, u128, usize} { pub const fn funnel_shl(self, right: Self, shift: u32) -> Self; pub const fn funnel_shr(self, right: Self, shift: u32) -> Self; pub const unsafe fn unchecked_funnel_shl(self, right: Self, shift: u32) -> Self; pub const unsafe fn unchecked_funnel_shr(self, right: Self, shift: u32) -> Self; } ``` The tracking issue also mentions a `wrapping_` version but it has not been implemented. Closes: #145686 (tracking issue, wrapping versions will need a new issue)

jhpratt · 2026-09-29T04:23:14Z

@bors yield to rollup

rust-bors · 2026-09-29T04:23:17Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/36521366415

The next pull request likely to be tested is #163465.

Rollup of 9 pull requests Successful merges: - #161015 (Stabilize `funnel_shifts` (including `const`)) - #161712 (Stabilize `Result::into_{ok,err}`) - #162493 (Add support for -Zsanitizer-cfi-minimal-runtime) - #163427 (implement #![feature(gca_adts)]) - #163390 (add `automatically_derived` attribute documentation) - #163428 (do not complain about unstable target features on nightly) - #163444 (Add `stable_rustc` helper in `run-make-support`) - #163447 (Allow using different index types when reading and writing to tables) - #163459 (Stabilize vec_try_remove)

Rollup of 9 pull requests Successful merges: - rust-lang/rust#161015 (Stabilize `funnel_shifts` (including `const`)) - rust-lang/rust#161712 (Stabilize `Result::into_{ok,err}`) - rust-lang/rust#162493 (Add support for -Zsanitizer-cfi-minimal-runtime) - rust-lang/rust#163427 (implement #![feature(gca_adts)]) - rust-lang/rust#163390 (add `automatically_derived` attribute documentation) - rust-lang/rust#163428 (do not complain about unstable target features on nightly) - rust-lang/rust#163444 (Add `stable_rustc` helper in `run-make-support`) - rust-lang/rust#163447 (Allow using different index types when reading and writing to tables) - rust-lang/rust#163459 (Stabilize vec_try_remove)

RalfJung · 2026-10-27T06:55:43Z

-#[rustc_const_unstable(feature = "funnel_shifts", issue = "145686")]
-#[unstable(feature = "funnel_shifts", issue = "145686")]
 #[track_caller]
+#[rustc_allow_const_fn_unstable(const_trait_impl, core_intrinsics_fallbacks)]


I just realized that we should not have landed this. This allows core_intrinsics_fallbacks to use arbitrary unstable const things that will then be exposed on stable.

rustc_allow_const_fn_unstable should only be used for lang features, never for library features.

There's still plenty of time to change/revert before it lands on stable on December 24.

What's supposed to happen with core_intrinsics_fallbacks? It looks like this is the first case to use one of those traits.

For non-trait fallbacks, what's supposed to happen is that all functions that can be called from stable are tagged #[rustc_const_stable_indirect]. That way we enforce recursive const stability through them.

#[rustc_const_stable_indirect] does not yet support traits. That will have to be fixed. It's not trivial because with traits we cannot in general know which function actually gets called. So we'll have to tag the trait method itself and then have to ensure that all impls are recursively const-checked.

Were things working correctly before? Rotate has been const stable and it was using #[rustc_allow_const_fn_unstable(const_trait_impl, funnel_shifts)] prior to this PR. Maybe we could get away with something similar by moving the traits to core_intrinsics_funnel_shift_fallback to at least limit potential issues.

(Note: the branch would be 12 November.)

If anything we could revert the const stabilization. But I don't think this PR caused the issue, so reverting it wouldn't fix it anyway—we still need rustc_allow_const_fn_unstable(some_library_feature) to support the long-existing const stability of rotate_*, unless we effectively revert bc809be and reimplement in the compiler again.

But I don't think that's worth it if there's a reasonable policy to be reached here.

Yeah I don't think we need to revert the stabilization, worst-case we add a native implementation.

Zulip discussion: #t-compiler/const-eval > Const stability for intrinsic fallback bodies?

#165114 resolves this concern.

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Aug 13, 2026

rustbot assigned nia-e Aug 13, 2026

rustbot added I-lang-nominated Nominated for discussion during a lang team meeting. I-libs-api-nominated [DEPRECATED; DO NOT USE] labels Aug 13, 2026

tgross35 added the needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. label Aug 13, 2026

tgross35 commented Aug 13, 2026

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tgross35 mentioned this pull request Aug 13, 2026

Tracking Issue for Integer Funnel Shifts #145686

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nia-e added I-libs-nominated Nominated for discussion during a libs team meeting. and removed I-libs-api-nominated [DEPRECATED; DO NOT USE] labels Aug 13, 2026

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tgross35 force-pushed the stabilize-funnel-shifts branch from cf9c57e to 11cd98d Compare August 13, 2026 08:46

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tgross35 force-pushed the stabilize-funnel-shifts branch from 11cd98d to c21507f Compare August 13, 2026 08:51

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Rollup of 12 pull requests #163456

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Rollup of 15 pull requests #163460

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Rollup of 9 pull requests #163465

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Rollup of 9 pull requests #163467

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rust-bors Bot merged commit 475b967 into rust-lang:main Sep 29, 2026
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Toolchain upgrade to nightly-2026-10-11 flux-rs/flux#1811

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clarfonthey mentioned this pull request Oct 27, 2026

stabilize unchecked_disjoint_bitor #165009

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