Tracking Issue for `slice_first_last_chunk` feature (`slice::{split_,}{first,last}_chunk{,_mut}`) #111774

clarfonthey · 2023-05-19T22:16:36Z

Feature gate: #![feature(slice_first_last_chunk)]

This is a tracking issue for the slice::{split_,}{first,last}_chunk{,_mut} API.

Public API

2024-09-06: Most of this is stable now, except for the const-ness of the mut methods.

impl [T] {
    pub const fn first_chunk<const N: usize>(&self) -> Option<&[T; N]>;
    pub const fn first_chunk_mut<const N: usize>(&mut self) -> Option<&mut [T; N]>;
    pub const fn last_chunk<const N: usize>(&self) -> Option<&[T; N]>;
    pub const fn last_chunk_mut<const N: usize>(&mut self) -> Option<&mut [T; N]>;
    pub const fn split_first_chunk<const N: usize>(&self) -> Option<(&[T; N], &[T])>;
    pub const fn split_first_chunk_mut<const N: usize>(&mut self) -> Option<(&mut [T; N], &mut [T])>;
    pub const fn split_last_chunk<const N: usize>(&self) -> Option<(&[T; N], &[T])>;
    pub const fn split_last_chunk_mut<const N: usize>(&mut self) -> Option<(&mut [T; N], &mut [T])>;
}

Note that this mirrors the existing, already-stabilised API:

impl [T] {
    pub const fn first(&self) -> Option<&T>;
    pub const fn first_mut(&mut self) -> Option<&mut T>;
    pub const fn last(&self) -> Option<&T>;
    pub const fn last_mut(&mut self) -> Option<&mut T>;
    pub const fn split_first(&self) -> Option<(&T, &[T])>;
    pub const fn split_first_mut(&mut self) -> Option<(&mut T, &mut [T])>;
    pub const fn split_last(&self) -> Option<(&T, &[T])>;
    pub const fn split_last_mut(&mut self) -> Option<(&mut T, &mut [T])>;
}

Steps / History

API change proposal (ACP)¹: ACP: slice::{split_,}{first,last}_chunk libs-team#69
Implementation: Add slice::{split_,}{first,last}_chunk{,_mut} #95198
Final comment period (FCP)²
Wait until Tracking Issue for feature(const_slice_split_at_mut) #101804 is stable
Stabilization PR

Unresolved Questions

Is this the best order for the tuple in split_last_chunk? Consistency with split_last would have it be -> Option<(&[T;N], &[T])> as was ACPed, but it might be confusing to have the two slice-like things in the opposite order from every other API I can think of that returns two subslices of the original. In Add slice::{split_,}{first,last}_chunk{,_mut} #95198 (comment), @scottmcm expected -> Option<(&[T;N], &[T])> for split_first_chunk, but for it to be -> Option<(&[T], &[T;N])> in split_last_chunk so the tuple is "in order", in some sense.
...

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faern · 2023-05-26T09:46:04Z

I have been yearning for better byte parsing with methods like these for a long time. So I jumped on trying out split_first_chunk immediately. Was hoping it would greatly simplify parsing out fields of known lengths from buffers of serialized data. And it did! It did simplify a case of: buffer.get(header_start..header_end) + <[u8; HEADER_LEN]>::try_from(header).unwrap() into just buffer.split_first_chunk::<HEADER_LEN>()? and I love this! Way less code, risk of indexing mistakes, and no unwraps/panics involved at all ✨

However, the lack of non-const-generic version of the same thing did bite me right after. It's really sad that slice::split_at is panicking and not returning Option<(&[T], &[T])>. (This is being proposed in #90410)

My experiment with this new API: mullvad/udp-over-tcp@8bdff7a#diff-0ff1562d6dac69ac5760658a8efce5e0368695f966f0c807bb5f1e023d1365b6R130-R131

clarfonthey · 2023-05-27T02:54:55Z

I'm a bit confused what you mean about there not being a const version of this API-- split_at and co. are already const-unstable, and this method is also const-unstable. There is a bit going on with the various get methods since they would rely on const traits (which are currently being removed to pave way for keyword generics), but at least those methods should work fine for you.

faern · 2023-05-27T11:24:15Z

The added features under this tracking issue are awesome. I approve. What I mean is there is no method to split a slice where the index is not const generic and it does not panic (split_at but without panic is what I want. But that's what the issue I linked to is about)

safinaskar · 2023-06-01T13:03:56Z

Note previous similar proposal: #90091 . Both currently are implemented, so I think one of them should be removed from codebase

Xiretza · 2023-06-01T15:30:45Z

The previously stated advantage of this over the existing #90091 was "solv[ing] the Option-ness question" - I think #109049 should resolve that as well (while mostly keeping #90091 the same)?

tgross35 · 2023-11-01T06:33:42Z

Is this the best order for the tuple in split_last_chunk? Consistency with split_last would have it be -> Option<(&[T;N], &[T])> as was ACPed, but it might be confusing to have the two slice-like things in the opposite order from every other API I can think of that returns two subslices of the original. In #95198 (comment), @scottmcm expected -> Option<(&[T;N], &[T])> for split_first_chunk, but for it to be -> Option<(&[T], &[T;N])> in split_last_chunk so the tuple is "in order", in some sense.

I think this should be changed such that [a, b, c, d].split_last_chunk::<2>() returns ([a, b], [c, d]). This is not consistent specifically with split_{first,last} but it is consistent with str::rsplit_once and other buffer splitting methods (slice::split_at{_mut}, slice::align_to). I think if you could ever rsplit more than one item that it would make more sense to return their natural order too, rather than reversing.

Maybe we could justify that we want to be more consistent with rsplit_once and let [a, b, middle @ .., d, e] else ..., and call the pop-like split_{first,last} methods the special case?

Rollup merge of rust-lang#117561 - tgross35:split-array, r=scottmcm Stabilize `slice_first_last_chunk` This PR does a few different things based around stabilizing `slice_first_last_chunk`. They are split up so this PR can be by-commit reviewed, I can move parts to a separate PR if desired. This feature provides a very elegant API to extract arrays from either end of a slice, such as for parsing integers from binary data. ## Stabilize `slice_first_last_chunk` ACP: rust-lang/libs-team#69 Implementation: rust-lang#90091 Tracking issue: rust-lang#111774 This stabilizes the functionality from rust-lang#111774: ```rust impl [T] { pub const fn first_chunk<const N: usize>(&self) -> Option<&[T; N]>; pub fn first_chunk_mut<const N: usize>(&mut self) -> Option<&mut [T; N]>; pub const fn last_chunk<const N: usize>(&self) -> Option<&[T; N]>; pub fn last_chunk_mut<const N: usize>(&mut self) -> Option<&mut [T; N]>; pub const fn split_first_chunk<const N: usize>(&self) -> Option<(&[T; N], &[T])>; pub fn split_first_chunk_mut<const N: usize>(&mut self) -> Option<(&mut [T; N], &mut [T])>; pub const fn split_last_chunk<const N: usize>(&self) -> Option<(&[T], &[T; N])>; pub fn split_last_chunk_mut<const N: usize>(&mut self) -> Option<(&mut [T], &mut [T; N])>; } ``` Const stabilization is included for all non-mut methods, which are blocked on `const_mut_refs`. This change includes marking the trivial function `slice_split_at_unchecked` const-stable for internal use (but not fully stable). ## Remove `split_array` slice methods Tracking issue: rust-lang#90091 Implementation: rust-lang#83233 (review) This PR also removes the following unstable methods from the `split_array` feature, rust-lang#90091: ```rust impl<T> [T] { pub fn split_array_ref<const N: usize>(&self) -> (&[T; N], &[T]); pub fn split_array_mut<const N: usize>(&mut self) -> (&mut [T; N], &mut [T]); pub fn rsplit_array_ref<const N: usize>(&self) -> (&[T], &[T; N]); pub fn rsplit_array_mut<const N: usize>(&mut self) -> (&mut [T], &mut [T; N]); } ``` This is done because discussion at rust-lang#90091 and its implementation PR indicate a strong preference for nonpanicking APIs that return `Option`. The only difference between functions under the `split_array` and `slice_first_last_chunk` features is `Option` vs. panic, so remove the duplicates as part of this stabilization. This does not affect the array methods from `split_array`. We will want to revisit these once `generic_const_exprs` is further along. ## Reverse order of return tuple for `split_last_chunk{,_mut}` An unresolved question for rust-lang#111774 is whether to return `(preceding_slice, last_chunk)` (`(&[T], &[T; N])`) or the reverse (`(&[T; N], &[T])`), from `split_last_chunk` and `split_last_chunk_mut`. It is currently implemented as `(last_chunk, preceding_slice)` which matches `split_last -> (&T, &[T])`. The first commit changes these to `(&[T], &[T; N])` for these reasons: - More consistent with other splitting methods that return multiple values: `str::rsplit_once`, `slice::split_at{,_mut}`, `slice::align_to` all return tuples with the items in order - More intuitive (arguably opinion, but it is consistent with other language elements like pattern matching `let [a, b, rest @ ..] ...` - If we ever added a varidic way to obtain multiple chunks, it would likely return something in order: `.split_many_last::<(2, 4)>() -> (&[T], &[T; 2], &[T; 4])` - It is the ordering used in the `rsplit_array` methods I think the inconsistency with `split_last` could be acceptable in this case, since for `split_last` the scalar `&T` doesn't have any internal order to maintain with the other items. ## Unresolved questions Do we want to reserve the same names on `[u8; N]` to avoid inference confusion? rust-lang#117561 (comment) --- `slice_first_last_chunk` has only been around since early 2023, but `split_array` has been around since 2021. `@rustbot` label -T-libs +T-libs-api -T-libs +needs-fcp cc `@rust-lang/wg-const-eval,` `@scottmcm` who raised this topic, `@clarfonthey` implementer of `slice_first_last_chunk` `@jethrogb` implementer of `split_array` Zulip discussion: https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Stabilizing.20array-from-slice.20*something*.3F Fixes: rust-lang#111774

Stabilize `slice_first_last_chunk` This PR does a few different things based around stabilizing `slice_first_last_chunk`. They are split up so this PR can be by-commit reviewed, I can move parts to a separate PR if desired. This feature provides a very elegant API to extract arrays from either end of a slice, such as for parsing integers from binary data. ## Stabilize `slice_first_last_chunk` ACP: rust-lang/libs-team#69 Implementation: rust-lang/rust#90091 Tracking issue: rust-lang/rust#111774 This stabilizes the functionality from rust-lang/rust#111774: ```rust impl [T] { pub const fn first_chunk<const N: usize>(&self) -> Option<&[T; N]>; pub fn first_chunk_mut<const N: usize>(&mut self) -> Option<&mut [T; N]>; pub const fn last_chunk<const N: usize>(&self) -> Option<&[T; N]>; pub fn last_chunk_mut<const N: usize>(&mut self) -> Option<&mut [T; N]>; pub const fn split_first_chunk<const N: usize>(&self) -> Option<(&[T; N], &[T])>; pub fn split_first_chunk_mut<const N: usize>(&mut self) -> Option<(&mut [T; N], &mut [T])>; pub const fn split_last_chunk<const N: usize>(&self) -> Option<(&[T], &[T; N])>; pub fn split_last_chunk_mut<const N: usize>(&mut self) -> Option<(&mut [T], &mut [T; N])>; } ``` Const stabilization is included for all non-mut methods, which are blocked on `const_mut_refs`. This change includes marking the trivial function `slice_split_at_unchecked` const-stable for internal use (but not fully stable). ## Remove `split_array` slice methods Tracking issue: rust-lang/rust#90091 Implementation: rust-lang/rust#83233 (review) This PR also removes the following unstable methods from the `split_array` feature, rust-lang/rust#90091: ```rust impl<T> [T] { pub fn split_array_ref<const N: usize>(&self) -> (&[T; N], &[T]); pub fn split_array_mut<const N: usize>(&mut self) -> (&mut [T; N], &mut [T]); pub fn rsplit_array_ref<const N: usize>(&self) -> (&[T], &[T; N]); pub fn rsplit_array_mut<const N: usize>(&mut self) -> (&mut [T], &mut [T; N]); } ``` This is done because discussion at #90091 and its implementation PR indicate a strong preference for nonpanicking APIs that return `Option`. The only difference between functions under the `split_array` and `slice_first_last_chunk` features is `Option` vs. panic, so remove the duplicates as part of this stabilization. This does not affect the array methods from `split_array`. We will want to revisit these once `generic_const_exprs` is further along. ## Reverse order of return tuple for `split_last_chunk{,_mut}` An unresolved question for #111774 is whether to return `(preceding_slice, last_chunk)` (`(&[T], &[T; N])`) or the reverse (`(&[T; N], &[T])`), from `split_last_chunk` and `split_last_chunk_mut`. It is currently implemented as `(last_chunk, preceding_slice)` which matches `split_last -> (&T, &[T])`. The first commit changes these to `(&[T], &[T; N])` for these reasons: - More consistent with other splitting methods that return multiple values: `str::rsplit_once`, `slice::split_at{,_mut}`, `slice::align_to` all return tuples with the items in order - More intuitive (arguably opinion, but it is consistent with other language elements like pattern matching `let [a, b, rest @ ..] ...` - If we ever added a varidic way to obtain multiple chunks, it would likely return something in order: `.split_many_last::<(2, 4)>() -> (&[T], &[T; 2], &[T; 4])` - It is the ordering used in the `rsplit_array` methods I think the inconsistency with `split_last` could be acceptable in this case, since for `split_last` the scalar `&T` doesn't have any internal order to maintain with the other items. ## Unresolved questions Do we want to reserve the same names on `[u8; N]` to avoid inference confusion? rust-lang/rust#117561 (comment) --- `slice_first_last_chunk` has only been around since early 2023, but `split_array` has been around since 2021. `@rustbot` label -T-libs +T-libs-api -T-libs +needs-fcp cc `@rust-lang/wg-const-eval,` `@scottmcm` who raised this topic, `@clarfonthey` implementer of `slice_first_last_chunk` `@jethrogb` implementer of `split_array` Zulip discussion: https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Stabilizing.20array-from-slice.20*something*.3F Fixes: #111774

RalfJung · 2024-09-06T21:27:43Z

This has been closed, but is still listed as the tracking issue for const_slice_first_last_chunk. But with const_mut_refs in FCP, I think we can go ahead and stabilize const_slice_first_last_chunk as well. :)

Cc @rust-lang/libs-api

dtolnay · 2024-09-06T21:38:39Z

@rfcbot fcp merge

const for these already-stable functions:

rfcbot · 2024-09-06T21:38:42Z

Team member @dtolnay 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!

See this document for info about what commands tagged team members can give me.

rfcbot · 2024-09-24T15:15:06Z

🔔 This is now entering its final comment period, as per the review above. 🔔

rfcbot · 2024-10-04T15:23:57Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

Rollup merge of rust-lang#130428 - ink-feather-org:stabilize-const-split-at-mut, r=RalfJung Stabilize `const_slice_split_at_mut` and `const_slice_first_last_chunk` Stabilizes rust-lang#101804 and the remainder of rust-lang#111774. FCP proposed in the tracking issue. Requires rust-lang#130403 (or it would need a rustc_allow_const_fn_unstable for it) Stabilized const API: ```rust // slice impl [T] { pub const fn split_at_mut(&mut self, mid: usize) -> (&mut [T], &mut [T]); pub const fn split_at_mut_unchecked(&mut self, mid: usize) -> (&mut [T], &mut [T]); pub const fn split_at_mut_checked(&mut self, mid: usize) -> Option<(&mut [T], &mut [T])>; pub const fn first_chunk_mut<const N: usize>(&mut self) -> Option<&mut [T; N]>; pub const fn last_chunk_mut<const N: usize>(&mut self) -> Option<&mut [T; N]>; pub const fn split_first_chunk_mut<const N: usize>(&mut self) -> Option<(&mut [T; N], &mut [T])>; pub const fn split_last_chunk_mut<const N: usize>(&mut self) -> Option<(&mut [T; N], &mut [T])>; } ``` Closes rust-lang#101804 Closes rust-lang#111774 cc `@RalfJung`

clarfonthey added C-tracking-issue Category: An issue tracking the progress of sth. like the implementation of an RFC T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. labels May 19, 2023

clarfonthey mentioned this issue May 19, 2023

ACPs in tracking issue template for rust repository rust-lang/libs-team#225

Open

faern mentioned this issue May 26, 2023

split_at family should return an Result #90410

Closed

safinaskar mentioned this issue Jun 1, 2023

Tracking Issue for split_array #90091

Open

2 tasks

scottmcm mentioned this issue Sep 2, 2023

opt-level=z fails to remove bounds checks/panics even if every other opt-level succeeds #115463

Closed

photino mentioned this issue Sep 20, 2023

Run the framework in the stable Rust zino-rs/zino#51

Closed

scottmcm mentioned this issue Oct 25, 2023

Add slice::{split_,}{first,last}_chunk{,_mut} #95198

Merged

tgross35 mentioned this issue Nov 1, 2023

array::split_array*(): return arrays instead of slices #111829

Closed

tgross35 mentioned this issue Nov 4, 2023

Stabilize slice_first_last_chunk #117561

Merged

bors closed this as completed in 64461da Jan 20, 2024

hubofvalley mentioned this issue Jul 21, 2024

While compiling storage v0.1.0 got an error message 0glabs/0g-da-node#14

Open

RalfJung reopened this Sep 6, 2024

RalfJung added the I-libs-api-nominated Nominated for discussion during a libs-api team meeting. label Sep 6, 2024

rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Sep 6, 2024

dtolnay removed the I-libs-api-nominated Nominated for discussion during a libs-api team meeting. label Sep 6, 2024

RalfJung mentioned this issue Sep 19, 2024

Tracking issue for &mut T in const contexts (const_mut_refs) #57349

Closed

8 tasks

rfcbot added final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. and removed proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. labels Sep 24, 2024

This was referenced Sep 29, 2024

Stabilize const ptr::write* and mem::replace #130954

Merged

Stabilize const_slice_split_at_mut and const_slice_first_last_chunk #130428

Merged

rfcbot added finished-final-comment-period The final comment period is finished for this PR / Issue. to-announce Announce this issue on triage meeting and removed final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. labels Oct 4, 2024

rustbot mentioned this issue Oct 4, 2024

Tracking issue for release notes of #111774: Tracking Issue for slice_first_last_chunk feature (slice::{split_,}{first,last}_chunk{,_mut}) #131247

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bors closed this as completed in 386a0ba Oct 5, 2024

apiraino removed the to-announce Announce this issue on triage meeting label Oct 17, 2024

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Tracking Issue for `slice_first_last_chunk` feature (`slice::{split_,}{first,last}_chunk{,_mut}`) #111774

Tracking Issue for `slice_first_last_chunk` feature (`slice::{split_,}{first,last}_chunk{,_mut}`) #111774

clarfonthey commented May 19, 2023 •

edited by RalfJung

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Tracking Issue for slice_first_last_chunk feature (slice::{split_,}{first,last}_chunk{,_mut}) #111774

Tracking Issue for slice_first_last_chunk feature (slice::{split_,}{first,last}_chunk{,_mut}) #111774

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clarfonthey commented May 19, 2023 • edited by RalfJung Loading

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Tracking Issue for `slice_first_last_chunk` feature (`slice::{split_,}{first,last}_chunk{,_mut}`) #111774

Tracking Issue for `slice_first_last_chunk` feature (`slice::{split_,}{first,last}_chunk{,_mut}`) #111774

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