Stabilize alloc_layout_extra

[CAD97]

Stabilizes #55724. Specifically, stabilizes the functions

// core::alloc
impl Layout {
    pub fn dangling(&self) -> ptr::NonNull<u8>;
    pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutError>;
    pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutError>;
    pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutError>;
}

This PR contains one change beyond stabilization: it changes Layout::repeat to not add trailing padding (i.e. be equivalent to repeated extend calls). This is a functionality change, and should be of special interest during FCP.

If libs-api prefers including adding trailing padding in this API, I can revert that part of this PR. Layout code is notorious for being perf-sensitive to the smallest changes, so probably wants a perf run or at least rollup=never while it includes code changes.

---

Arguments for trailing padding in Layout::repeat:

• Matches Layout::array in producing layout suitable for an array field, just with a dynamic Layout rather than a static T layout
• The layout without trailing padding can be easily acquired via multiple calls to Layout::extend
• layout.repeat(n)?.0.pad_to_align() subtracts the trailing padding just to add it back on again and the optimizer potentially won't see though that

Arguments against trailing padding in Layout::repeat

• Matches Layout::extend in not adding trailing padding, just extending the layout as necessary
• Makes layout.repeat(1) a no-op rather than padding to alignment
• Adding the trailing padding is easily done via Layout::pad_to_align
• Getting the layout without trailing padding requires an imperative loop of Layout::extend

[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? @kennytm

(rust-highfive has picked a reviewer for you, use r? to override)

[CAD97]

r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs +needs-fcp

[CAD97]

Reverted the Layout::padding_needed_for part of the stabilization, per @scottmcm's comments

[CAD97]

(triaging my open PRs)

(Trying) Rotating the assignee (it's been a month)

r? rust-lang/libs-api

Requesting I-libs-api-nominated T-libs-api nomination, for 1) FCP, 2) decision on whether Layout::repeat should add trailing padding (to match array layout) or not (to match field layout).

[bors]

☔ The latest upstream changes (presumably #95295) made this pull request unmergeable. Please resolve the merge conflicts.

[est31]

👋 Hello, I'm writing this comment in this stabilization PR to notify you, the authors of this PR, that #100591 has been merged, which implemented a change in how features are stabilized.

Your PR has been filed before the change, so will likely require modifications in order to comply with the new rules. I recommend you to:

1. rebase the PR onto latest master, so that uses of the placeholder are possible.
2. replace the version numbers in the PR with the placeholder CURRENT_RUSTC_VERSION. For language changes, this means the version numbers in accepted.rs (example: 4caedba). For library changes, this means the since fields (example e576a9b).

That's it! The CURRENT_RUSTC_VERSION placeholder will, as part of the release process, be replaced with the version number that the PR merged for. It can be used anywhere in rust-lang/rust, not just accepted.rs and the since fields.

If you have any questions, feel free to drop by the zulip stream, or ping me directly in this PR's thread. Thanks! 👋

[CAD97]

I'm going to close this PR in favor of opening a new one that just stabilizes the truly nonproblematic dangling and _packed.