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.

nia-e · 2026-09-28T16:24:46Z

@bors r+

rust-bors · 2026-09-28T16:24:50Z

📌 Commit f7548dd has been approved by nia-e

It is now in the queue for this repository.

…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)

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

jdahlstrom reviewed Aug 13, 2026

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traviscross added P-lang-drag-1 Lang team prioritization drag level 1. https://rust-lang.zulipchat.com/#narrow/channel/410516-t-lang T-lang Relevant to the language team labels Aug 13, 2026

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 28, 2026

JonathanBrouwer mentioned this pull request Sep 28, 2026

Rollup of 12 pull requests #163456

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JonathanBrouwer mentioned this pull request Sep 28, 2026

Rollup of 15 pull requests #163460

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tgross35 mentioned this pull request Sep 29, 2026

Rollup of 9 pull requests #163465

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tgross35 mentioned this pull request Sep 29, 2026

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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rustbot added this to the 1.101.0 milestone Sep 29, 2026

tgross35 deleted the stabilize-funnel-shifts branch September 29, 2026 07:22

github-actions Bot mentioned this pull request Oct 3, 2026

Toolchain upgrade to nightly-2026-10-11 flux-rs/flux#1811

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