Tracking issue for `#![feature(maybe_uninit_slice)]`

[Centril]

This is a tracking issue for the RFC "Deprecate uninitialized in favor of a new MaybeUninit type" (rust-lang/rfcs#1892).

Most of this has been stabilized, this issue now only tracks the below unstable methods.

Public API

impl<T> [MaybeUninit<T>] {
    pub unsafe fn assume_init_drop(&mut self);
    pub const unsafe fn assume_init_ref(&self) -> &[T];
    pub const unsafe fn assume_init_mut(&mut self) -> &mut [T];

    pub const fn slice_as_ptr(this: &[MaybeUninit<T>]) -> *const T;
    pub const fn slice_as_mut_ptr(this: &mut [MaybeUninit<T>]) -> *mut T;
}

Steps / History

• Implementation
• Make slice methods inherent: Add inherent versions of MaybeUninit methods for slices #129259
• Ensure documentation has examples
• Final comment period (FCP)
• Stabilization PR

Unresolved Questions

• Should slice_as_ptr/slice_as_mut_ptr be methods (with some Self parameter) instead of functions?
• slice_as_ptr/slice_as_mut_ptr get rid of the slice length, so .add(n) needs to be used on the result instead of a safer [n]. See accepted ACP Remove MaybeUninit::slice_as_(mut_)ptr and optionally add *const/*mut MaybeUninit<T> -> *const/*mut T type safe conversions libs-team#245 and its incomplete implementation Nuke slice_as{,_mut}_ptr methods of MaybeUninit #103133

[RalfJung]

Note: #63291 contains some APIs for creating uninitialized slices, but still does not have a good story for working with them, AFAIK.

Cc @SimonSapin

[asomers]

I think another missing function is a way to create a slice of uninitialized data from a Vec. Like this:

impl<'a, T> Vec<T> {
    pub fn as_uninit(&'a mut self) -> &'a mut [MaybeUninit<T>] {...}
}

Which would be used like this:

let v = Vec::<u8>::with_capacity(100);
let mut sl = v.as_uninit();
if 0 != some_ffi_func(sl.first_ptr_mut(), sl.len()) {
    return Err<()>;
} else {
    v.set_len(sl.len());
    return Ok(v);
}

There's no need for Vec::uninit() or Vec::assume_init(), because Vec::with_capacity and Vec::set_len already do pretty much the same thing. And for simple cases like this one Vec::as_mut_ptr() would suffice without the formality of creating a slice of MaybeUninits. However, the slice has the advantage that it knows its own length and it knows the lifetime of its data. The latter is especially useful when working with FFI functions that initialize the data asynchronously, such as aio_read.

[RalfJung]

Cc @Shnatsel see above -- seems related to rust-lang/rust-clippy#4483

[SimonSapin]

@asomers I agree that something like this would be good to have. However your proposal seems ambiguous: does the returned slice include the entire allocation (indices 0..capacity) or only the range that is not known to be initialized (indices len..capacity)?

Would an API that returns two disjoint (&mut [T], &mut [MaybeUninit<T>]) slices for indices 0..len and len..capacity also be useful?

[SimonSapin]

Back to the APIs tracked in this issue, could they be methods of [MaybeUninit<T>] that take &self instead of associated functions of MaybeUninit<T>? (Though it’s not obvious what they should be called in that case.)

We have precedent for a lang item for impl [u8] {…}

[asomers]

Good questions @SimonSapin. I think that the returned slice should include the entire allocation (after all, elements that are already initialized can still be represented as MaybeUninit. Also, I realized that the method I proposed isn't even good enough for my particular use case. I need a way to create an owned, maybe-uninitialized slice in a known memory location, and then transform that into an initialized vec. This is important for Tokio, whose asynchronous nature can thwart the borrow-checker.

impl<T> Vec<MaybeUninit<T>> {
    /// Transmute this `Vec` of possibly uninitialized data into a normal `Vec`, assuming that the
    /// first `len` elements are properly initialized.
    pub unsafe fn assume_init(self, len: usize) -> Vec<T> {...}
}
/// I'm not 100% sure it's necessary, but define this for symmetry's sake:
impl<T> Vec<T> {
    /// Transmute this `Vec` into a `Vec` of possibly uninitialized data whose length is equal to
    /// the original `Vec`'s capacity.
    pub fn into_uninit(self) -> Vec<MaybeUninit<T>> {...}
}

And it would be used like this:

let v_uninit = Vec::<MaybeUninit<u8>>::with_capacity(1024);
let result = some_ffi_func(v_uninit.as_mut_ptr(), v_uninit.capacity());
match result {
    Ok(len) => Ok(v_uninit.assume_init(len)),
    Err(e) => Err(e)
}

Also, regarding the original methods, should the return type be Option<NonNull<T>>? That way they could return None for 0-length slices.