Rollup merge of #65580 - SimonSapin:maybeuninit-array, r=Amanieu

Add `MaybeUninit` methods `uninit_array`, `slice_get_ref`, `slice_get_mut`

Eventually these will hopefully become the idiomatic way to work with partially-initialized stack buffers.

All methods are unstable. Note that `uninit_array` takes a type-level `const usize` parameter, so it is blocked (at least in its current form) on const generics.

Example:

```rust
use std::mem::MaybeUninit;

let input = b"Foo";
let f = u8::to_ascii_uppercase;

let mut buffer: [MaybeUninit<u8>; 32] = MaybeUninit::uninit_array();
let vec;
let output = if let Some(buffer) = buffer.get_mut(..input.len()) {
    buffer.iter_mut().zip(input).for_each(|(a, b)| { a.write(f(b)); });
    unsafe { MaybeUninit::slice_get_ref(buffer) }
} else {
    vec = input.iter().map(f).collect::<Vec<u8>>();
    &vec
};

assert_eq!(output, b"FOO");
```

0 files changed, 0 insertions, 0 deletions