Replace uses of MaybeUninit::uninit_array() with inline const blocks.

[kpreid]

[This PR originally contained the changes in #125995 too. See edit history for the original PR description.]

The documentation of MaybeUninit::uninit_array() says:

Note: in a future Rust version this method may become unnecessary when Rust allows inline const expressions. The example below could then use let mut buf = [const { MaybeUninit::<u8>::uninit() }; 32];.

The PR adding it also said: #65580 (comment)

if it’s stabilized soon enough maybe it’s not worth having a standard library method that will be replaceable with let buffer = [MaybeUninit::<T>::uninit(); $N];

That time has come to pass — inline const expressions are stable — so MaybeUninit::uninit_array() is now unnecessary. The only remaining question is whether it is an important enough convenience to keep it around.

I believe it is net good to remove this function, on the principle that it is better to compose two orthogonal features (MaybeUninit and array construction) than to have a specific function for the specific combination, now that that is possible.

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[ChayimFriedman2]

I still prefer MaybeUninit::uninit_array() to [const { MaybeUninit::uninit() }; N], it's shorter and more readable IMHO.

[workingjubilee]

The examples and the compiler code should probably still prefer the stable option.

Technically the compared-against example was [MaybeUninit::uninit(); N], which implies that the array initializer is a const context.

[Noratrieb]

this fell down into the "i hope to get to this eventually but will probably never lol" part of my notification queue and i forgot to look at it again, sorry.
you have my approval for the first two commits, but I'd like a libs-api opinion on the last one. you can split out the first two into a new PR and i'll approve that, or you can keep them all like this if you're okay with letting the first two wait longer (as it will probably take a bit).
r? libs-api

[kpreid]

Split out the first two parts into #125995. I'll re-summarize this PR once that's merged.

[bors]

☔ The latest upstream changes (presumably #126016) made this pull request unmergeable. Please resolve the merge conflicts.

[kpreid]

This PR now consists solely of removing MaybeUninit::uninit_array().

[dtolnay]

Thanks!

I am attempting to start a T-libs-api FCP for this in #96097 (comment).

https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/rfcbot.20asleep