Stabilize vec::Drain::as_slice

[CAD97]

and add AsRef<[T]> for Drain<'_, T>.

Tracking issue: #58957. Does not stabilize slice::IterMut::as_slice yet. cc @cuviper
This PR proposes stabilizing just the vec::Drain::as_slice part of that tracking issue.

My ultimate goal here: being able to use for<T, I: Iterator<Item=T> + AsRef<[T]>> I to refer to vec::IntoIter, vec::Drain, and eventually array::IntoIter, as an approximation of the set of by-value iterators that can be "previewed" as by-ref iterators. (Actually expressing that as a trait requires GAT.)

[rust-highfive]

r? @shepmaster

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[CAD97]

Marked as stable in 1.46 as with FCP this will definitely miss the 1.45 beta cutoff.

[cuviper]

Why not IterMut too? I don't think they're all that controversial, just nobody has gotten around to stabilization yet.

[CAD97]

Mainly because I have a use case for vec::Drain<T>: AsRef<[T]>, but not for slice::IterMut.

slice::IterMut might also want as_slice_mut(&mut self) -> &mut [T] and/or AsMut<[T]>, and though those can always be added later, I don't think it's necessary to tie stabilization of the two together. (They're already under separate feature gates anyway, after all.)

[dtolnay]

@rust-lang/libs: this stabilizes https://doc.rust-lang.org/std/vec/struct.Drain.html#method.as_slice and adds an equivalent AsRef impl.

impl<'a, T> vec::Drain<'a, T> {
    pub fn as_slice(&self) -> &[T];
}

impl<'a, T> AsRef<[T]> for vec::Drain<'a, T> {...}

Stabilizing these commits us to the constraint that Drain keeps the remaining elements to drain contiguous in memory i.e. Vec::drain does not eagerly shuffle the elements or move parts of the range to different places (though it may eagerly move the entire range all together). Not that we would want to do those things so that seems fine.

@rfcbot fcp merge

[rfcbot]

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

• @Amanieu
• @SimonSapin
• @dtolnay
• @sfackler
• @withoutboats

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.

The RFC will be merged soon.

[dtolnay]

@bors r+

[bors]

📌 Commit 738f848 has been approved by dtolnay

[RalfJung]

@bors rollup

[jplatte]

The milestone on this says 1.45, while the attribute says 1.46. Which one is correct?

[shepmaster]

• https://doc.rust-lang.org/1.45.0/std/vec/struct.Drain.html#method.as_slice
• https://doc.rust-lang.org/1.46.0/std/vec/struct.Drain.html#method.as_slice

[cuviper]

Milestones used to be manually applied, but then there was a beta branch before this PR merged.