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update comment at MaybeUninit::uninit_array #83568

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Mar 30, 2021
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update comment at MaybeUninit::uninit_array #83568

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6 changes: 3 additions & 3 deletions library/core/src/mem/maybe_uninit.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -319,9 +319,9 @@ impl<T> MaybeUninit<T> {
/// Create a new array of `MaybeUninit<T>` items, in an uninitialized state.
///
/// Note: in a future Rust version this method may become unnecessary
/// when array literal syntax allows
/// [repeating const expressions](https://github.com/rust-lang/rust/issues/49147).
/// The example below could then use `let mut buf = [MaybeUninit::<u8>::uninit(); 32];`.
/// when Rust allows
/// [inline const expressions](https://github.com/rust-lang/rust/issues/76001).
/// The example below could then use `let mut buf = [const { MaybeUninit::<u8>::uninit() }; 32];`.
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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?

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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. ;)

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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. ;)

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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

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FYI @tspiteri that appears not to actually uninit.

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

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@bluss bluss Mar 30, 2021 •
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@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 (?)

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@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.

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Yep, that's where it was noticed 🙂

///
/// # Examples
///
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