Tracking Issue for maybe_uninit_array_assume_init

[joboet]

Feature gate: #![feature(maybe_uninit_array_assume_init)]

This is a tracking issue for MaybeUninit::array_assume_init.

Public API

// core::mem::maybe_uninit

impl<T> MaybeUninit<T> {
    pub unsafe fn array_assume_init<const N: usize>(array: [Self; N]) -> [T; N];
}

Steps / History

• Implementation: Add MaybeUninit method array_assume_init #80600
• Final commenting period (FCP)
• Stabilization PR

Unresolved Questions

• Should this be a const function?
• Should this be a method of array?
• Should this method be replaced by a conversion method from [MaybeUninit<T>; N] to MaybeUninit<[T; N]> (see comment).

[joboet]

@rustbot modify labels: +T-libs

[hellow554]

I think the question is, should this ever be stabilized? As you (?) wrote in the docs:

Note: in a future Rust version this method may become unnecessary when array literal syntax allows repeating const expressions. The example below could then use let mut buf = [MaybeUninit::::uninit(); 32];.

Maybe therefore this should stay an unstable option until (and later removed) until #49147 is stabilized?

[joboet]

This is the wrong issue, it does not cover MaybeUninit::uninit_array, you're looking for #65580, which seems not to have a tracking issue.

I do intend for this (MaybeUninit::array_assume_init) to become stable, even when mem::transmute supports const generics it is a handy abstraction.

[hellow554]

This is the wrong issue, it does not cover MaybeUninit::uninit_array, you're looking for #65580, which seems not to have a tracking issue.

I don't know why I picked that function and assumed this tracking issue was the correct one. Sorry for that!

[bstrie]

The documentation for this item suggests that it should be used with another unstable API, MaybeUninit::uninit_array. However that API is not contained under this feature flag, and furthermore I can't find a tracking issue for it. Should we use this issue to track that one as well?

[cole-miller]

Fwiw, this function is now used by hashbrown (nightly crate feature only, introduced in rust-lang/hashbrown#239). I'd love to see it stabilized soon! (Along with maybe_uninit_uninit_array.)

[the8472]

A possible alternative is to have a conversion method from [MaybeUninit<T>; N] to MaybeUninit<[T; N]> and then assume_init() the latter. Then a separate array_assume_init() function isn't needed.