Docs for `MaybeUninit::assume_init` should indicate that reading uninit POD isn't sound

[tgross35]

The question about whether or not it is sound to read uninit data if it's an integer or other POD comes up from time to time. I think it's worth pointing this out in the MaybeUninit docs since that's probably the entrypoint for such a problem to come up, and currently the docs only mention Vec https://doc.rust-lang.org/nightly/std/mem/union.MaybeUninit.html#method.assume_init.

For reference:

use std::mem::MaybeUninit;

fn main() {
    let x = MaybeUninit::<u8>::uninit();
    let _ = unsafe { x.assume_init() };
}

error: Undefined Behavior: reading memory at alloc214[0x0..0x1], but memory is uninitialized at [0x0..0x1], and this operation requires initialized memory
 --> src/main.rs:5:22
  |
5 |     let _ = unsafe { x.assume_init() };
  |                      ^^^^^^^^^^^^^^^ Undefined Behavior occurred here
  |
  = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
  = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information

Uninitialized memory occurred at alloc214[0x0..0x1], in this allocation:
alloc214 (stack variable, size: 1, align: 1) {
    __                                              │ ░
}

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

Iirc we don't have a way to make this kind of behavior sound either without going via assembly.

Cc @RalfJung

[Sekar-C-Mca]

@rustbot claim

[RalfJung]

The type-level docs at https://doc.rust-lang.org/nightly/std/mem/union.MaybeUninit.html already explicitly discuss this. I am not sure it's worth duplicating that, but the method docs could more explicitly ask the reader to look at the type-level docs.