Partially stabilize const_slice_from_raw_parts

[sugar700]

This doesn't stabilize methods working on mutable pointers.

This pull request continues from #94946.

Pinging @rust-lang/wg-const-eval this because I use rustc_allow_const_fn_unstable. I believe this is justifiable as it's already possible to use slice::from_raw_parts in stable by abusing transmute. The stable alternative to this would be to provide a stable const implementation of std::ptr::from_raw_parts (as it can already be implemented in stable).

use std::mem;

#[repr(C)]
struct Slice<T> {
    data: *const T,
    len: usize,
}

fn main() {
    let data: *const i32 = [1, 2, 3, 4].as_ptr();
    let len = 4;
    println!("{:?}", unsafe {
        mem::transmute::<Slice<i32>, &[i32]>(Slice { data, len })
    });
}

@rustbot modify labels: +T-libs-api

[rust-highfive]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs to request review from a libs-api team reviewer. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[rust-highfive]

r? @scottmcm

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[m-ou-se]

@rfcbot merge

cc @rust-lang/wg-const-eval

[rfcbot]

Team member @m-ou-se has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se
• @yaahc

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]

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

[rfcbot]

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.

[dtolnay]

Thanks!

[dtolnay]

@bors r+

[bors]

📌 Commit 08819803a89d82543261337786f19897f5761828 has been approved by dtolnay

It is now in the queue for this repository.

[sugar700]

Rebased.

[dtolnay]

@bors r+

[bors]

📌 Commit 96ef66a78dd52dd46ccfa3d58a171e9bf9e62be6 has been approved by dtolnay

It is now in the queue for this repository.

[RalfJung]

@bors r=dtolnay

[bors]

📌 Commit 0753fd1 has been approved by dtolnay

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0753fd1 with merge 95e7764...

[bors]

☀️ Test successful - checks-actions
Approved by: dtolnay
Pushing 95e7764 to master...

[rust-timer]

Finished benchmarking commit (95e7764): comparison url.

Instruction count

• Primary benchmarks: no relevant changes found
• Secondary benchmarks: 😿 relevant regressions found

	mean1	max	count2
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	1.4%	1.6%	4
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	N/A	N/A	0
All 😿🎉 (primary)	N/A	N/A	0

Max RSS (memory usage)

Results

• Primary benchmarks: no relevant changes found
• Secondary benchmarks: mixed results

	mean1	max	count2
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	3.1%	3.1%	1
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	-3.9%	-3.9%	1
All 😿🎉 (primary)	N/A	N/A	0

Cycles

Results

• Primary benchmarks: 🎉 relevant improvement found
• Secondary benchmarks: 😿 relevant regression found

	mean1	max	count2
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	2.9%	2.9%	1
Improvements 🎉 (primary)	-2.3%	-2.3%	1
Improvements 🎉 (secondary)	N/A	N/A	0
All 😿🎉 (primary)	-2.3%	-2.3%	1

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

Footnotes

1. the arithmetic mean of the percent change ↩ ↩² ↩³

2. number of relevant changes ↩ ↩² ↩³