update comment at MaybeUninit::uninit_array

[RalfJung]

#49147 is closed; this now instead needs inline const expressions (#76001).

[rust-highfive]

r? @dtolnay

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[tspiteri]

Isn't it already possible to write let mut buf = [{ const U: MaybeUninit<u8> = MaybeUninit::uninit(); U}; 32]; thus making the method already unnecessary? Or is that too cumbersome to be considered usable?

[RalfJung]

Yeah this is possible if you have the concrete type; it won't when generic types are involved.

Not sure what people prefer here, for now fixing the docs seemed easier than arguing for the method to be removed. ;)

[tspiteri]

Ah yes, I missed the generic case.

Fixing the docs now is a good idea, I just encountered it earlier this week! But it was for a concrete type. ;)

[dtolnay]

FYI @tspiteri that appears not to actually uninit.

#![feature(maybe_uninit_uninit_array)]

use std::mem::MaybeUninit;

pub fn f() -> [MaybeUninit<u8>; 32] {
    [{ const U: MaybeUninit<u8> = MaybeUninit::uninit(); U}; 32]
}

pub fn g() -> [MaybeUninit<u8>; 32] {
    MaybeUninit::uninit_array()
}

example::f:
        movq    %rdi, %rax
        xorps   %xmm0, %xmm0
        movups  %xmm0, 16(%rdi)
        movups  %xmm0, (%rdi)
        retq

example::g:
        movq    %rdi, %rax
        retq

Godbolt: https://rust.godbolt.org/z/8nnvPK1PM
A lot like #83657

[RalfJung]

FYI @tspiteri that appears not to actually uninit.

Ah, good point, currently uninitialized consts are not actually uninitialized it seems...

[bluss]

@RalfJung What about @rodrimati1992's generic code here - for [MaybeUninit<T>; N], which compiles on Rust 1.51 / part of arrayvec 0.7? https://github.com/bluss/arrayvec/blob/7b290b7aa52b0c66b9056cfda577363d005f39b8/src/utils.rs

It seems to contradict what was said about generic types here (?)

[RalfJung]

@bluss associated const are the only generic consts we currently have. So yes that is a way to work around this limitation.

This will also run into #83657 though.

[bluss]

Yep, that's where it was noticed 🙂

[dtolnay]

@bors r+ rollup

[bors]

📌 Commit 5cfc98f has been approved by dtolnay