Make IoSlice and IoSliceMut methods unstably const

[thaliaarchi]

This enables constructing IoSlice and IoSliceMut in const contexts without users needing to break the platform abstraction by doing the cast themselves. This is useful for statics.

Since all current and plausible future platforms use simple slice-like structs, this should not impose a burden when adding a new platform with a different struct.

[rustbot]

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[thaliaarchi]

If this looks good, I'll open a tracking issue and link it.

[ibraheemdev]

r? libs-api

[thaliaarchi]

@BurntSushi Friendly ping. Would you mind taking a look at this?

[tgross35]

This seems fine; if we wind up with an obscure platform for which the vector type can't be constructed at comptime for some reason, we would probably use the unsupported fallback instead.

Nominated.

[tgross35]

For after discussion and updates,
r? libs

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[Amanieu]

FCP since this requires that any iovec-like structure is always const-constructible without calling any system API functions. This is already the case for unix and Windows.

@rfcbot merge

[rust-rfcbot]

Team member @Amanieu 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
• @the8472

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.

[thaliaarchi]

@rustbot ready
(I neglected to remove the tag after resolving review feedback)

[rust-rfcbot]

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

[rust-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.

[tgross35]

r? tgross35
@bors r+ rollup

[bors]

📌 Commit 3cf5b88 has been approved by tgross35

It is now in the queue for this repository.

[Zalathar]

Failed in rollup: #148120 (comment)

@bors r-