Don't recommend ManuallyDrop to customize drop order
See
https://internals.rust-lang.org/t/need-for-controlling-drop-order-of-fields/12914/21
for the discussion.
TL;DR: ManuallyDrop is unsafe and footguny, but you can just ask the compiler to do all the work for you by re-ordering declarations.
Specifically, the original example from the docs is much better written as
```rust
struct Peach;
struct Banana;
struct Melon;
struct FruitBox {
melon: Melon,
// XXX: mind the relative drop order of the fields below
peach: Peach,
banana: Banana,
}
```
Remove internal and unstable MaybeUninit::UNINIT.
Looks like it is no longer necessary, as `uninit_array()` can be used instead in the few cases where it was needed.
(I wanted to just add `#[doc(hidden)]` to remove clutter from the documentation, but looks like it can just be removed entirely.)
Add MaybeUninit::assume_init_drop.
`ManuallyDrop`'s documentation tells the user to use `MaybeUninit` instead when handling uninitialized data. However, the main functionality of `ManuallyDrop` (`drop`) is not available directly on `MaybeUninit`. Adding it makes it easier to switch from one to the other.
I re-used the `maybe_uninit_extra` feature and tracking issue number (#63567), since it seems very related. (And to avoid creating too many features tracking issues for `MaybeUninit`.)
ManuallyDrop's documentation tells the user to use MaybeUninit instead
when handling uninitialized data. However, the main functionality of
ManuallyDrop (drop) was not available directly on MaybeUninit. Adding it
makes it easier to switch from one to the other.
rename get_{ref, mut} to assume_init_{ref,mut} in Maybeuninit
References #63568
Rework with comments addressed from #66174
Have replaced most of the occurrences I've found, hopefully didn't miss out anything
r? @RalfJung
(thanks @danielhenrymantilla for the initial work on this)
See also X-Link mem::{swap, take, replace}
Since it's easy to end up at one of these functions when you really wanted the other one, cross link them with descriptions of why you'd want to use them.
