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.

This changes `self.ptr.as_mut()` with `get_mut_unchecked` which
does not use an intermediate reference.  Arc<T> already handled this
case properly.

This avoids overlapping a reference covering the data field,
which may be changed due in concurrent conditions. This fully
fixed the UB mainfested with `new_cyclic`.

Adds missing descriptions for the modules std::os::linux::fs and std::os::windows::io.
Also adds punctuation for consistency with other descriptions.

…, r=shepmaster

Add saturating methods for `Duration`

In some project, I needed a `saturating_add` method for `Duration`. I implemented it myself but i thought it would be a nice addition to the standard library as it matches closely with the integers types.

3 new methods have been introduced and are gated by the new `duration_saturating_ops` unstable feature:
* `Duration::saturating_add`
* `Duration::saturating_sub`
* `Duration::saturating_mul`

If have left the tracking issue to `none` for now as I want first to understand if those methods would be acceptable at all. If agreed, I'll update the PR with the tracking issue.

Further more, to match the behavior of integers types, I introduced 2 associated constants:
* `Duration::MIN`: this one is somehow a duplicate from `Duration::zero()` method, but at the time this method was added, `MIN` was rejected as it was considered a different semantic (see rust-lang#72790 (comment)).
* `Duration::MAX`

Both have been gated by the already existing unstable feature `duration_constants`, I can introduce a new unstable feature if needed or just re-use the `duration_saturating_ops`.

We might have to decide whether:
* `MIN` should be replaced by `ZERO`?
* associated constants over methods?

rustdoc: fix min_const_generics with ty::Param

fixes rust-lang#75913

r? @varkor cc @jyn514

…it-drop, r=RalfJung

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 (rust-lang#63567), since it seems very related. (And to avoid creating too many features tracking issues for `MaybeUninit`.)

Eliminate mut reference UB in Drop impl for Rc<T>

This changes `self.ptr.as_mut()` with `get_mut_unchecked` which
does not use an intermediate reference.  Arc<T> already handled this
case properly.

Fixes rust-lang#76509

Update `std::os` module documentation.

Adds missing descriptions for the modules `std::os::linux::fs` and `std::os::windows::io`.
Also adds punctuation for consistency with other descriptions.

…=lcnr

Finish off revisions for const generics UI tests.

This time it really does fix it. 😅 The only ones left are `min-and-full-same-time.rs`, which doesn't need it, and `array-impls/` which check the feature indirectly.

Fixes rust-lang#75279.

r? @lcnr

…ary-ops, r=jyn514

Add missing examples on binary core traits

r? @jyn514