What if we just add <[MaybeUninit<T>; N]>::assume_init directly?

[scottmcm]

I recall (though can't find where) libs-api being unsatisfied with the transpose approach for this, so I figured I'd toss out this as another alternative.

Now that we don't need lang item hacks to add inherent impl blocks for built-in types, we can just add

impl<T, const N: usize> [MaybeUninit<T>; N] {
    pub const unsafe fn assume_init(self) -> [T; N];
}

That seems pretty reasonable -- the odds of another meaning for assume_init on generic overlays that might overlap feel low to me, and another specific meaning of it for arrays of a different type would still be possible.

If this is worth doing, I can add analogous things to &[MaybeUninit<T>] and &mut [MaybeUninit<T>] too.

cc #96097
r? @m-ou-se

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[SUPERCILEX]

Can you post the rationale on rust-lang/libs-team#122? The original reason for transpose was to also let you index into the array which this PR doesn't solve in the general case. I wonder if we should commit to having slice methods and offer an array to slice conversion and then let you do index operations with the slice instead of the array.

[scottmcm]

I'm hoping that we can just stabilize inline_const (I've proposed it). At that point, [const { MaybeUninit::uninit() }; N] works, which makes the MaybeUninit::uninit().transpose() much less needed, and thus I started thinking whether we could get away without the other transpose either.

[bors]

☔ The latest upstream changes (presumably #107634) made this pull request unmergeable. Please resolve the merge conflicts.

[anden3]

Hello @scottmcm! Just wanna let you know that this PR has a merge conflict :)

[JohnCSimon]

@scottmcm

Ping from triage: I'm closing this due to inactivity, Please reopen when you are ready to continue with this.
Note: if you are going to continue please open the PR BEFORE you push to it, else you won't be able to reopen - this is a quirk of github.
Thanks for your contribution.

@rustbot label: +S-inactive