Make the following methods of `Duration` unstable const under `duration_const_2`:
 - `from_secs_f64`
 - `from_secs_f32`
 - `mul_f64`
 - `mul_f32`
 - `div_f64`
 - `div_f32`

This results in all methods of `Duration` being (unstable) const.

Also adds tests for these methods in a const context, moved the test to `library` as part of rust-lang#76268.

Possible because of rust-lang#72449, which made the relevant `f32` and `f64` methods const.

Tracking issue: rust-lang#72440

Without this, it was not inlined in SyncOnceCell::into_inner(), causing
unecessary checks and dead code.

This note is not relevant to other operating systems.

…est in Miri

The atomic equivalent of Cell::from_mut.

See rust-lang#76200.

…m-mut, r=KodrAus

Add Atomic*::from_mut.

The atomic equivalent of [`Cell::from_mut`](https://doc.rust-lang.org/stable/std/cell/struct.Cell.html#method.from_mut).

…orse

Make all methods of `Duration` unstably const

Make the following methods of `Duration` unstable const under `duration_const_2`:
 - `from_secs_f64`
 - `from_secs_f32`
 - `mul_f64`
 - `mul_f32`
 - `div_f64`
 - `div_f32`

This results in all methods of `Duration` being (unstable) const.

Moved the tests to `library` as part of rust-lang#76268.

Possible because of rust-lang#72449, which made the relevant `f32` and `f64` methods const.

Tracking issue: rust-lang#72440

r? @ecstatic-morse

Simplify iter zip struct doc

…ll-drop, r=matklad

Simplify SyncOnceCell's `take` and `drop`.

Prevents copies by using `assume_init_read` and `assume_init_drop`.

Some cleanup changes and commenting

r? @nikomatsakis
Cc @eddyb

Add mailmap entry

…s-note, r=Mark-Simulacrum

Remove Windows details from Unix and VmWorks symlink() docstrings

This note is not relevant to other operating systems.

…lacrum

Fix liballoc test suite for Miri

Mostly, fix the regression introduced by rust-lang#75207 that caused slices (i.e., references) to be created to invalid memory or memory that has aliasing pointers that we want to keep valid. @dylni  this changes the type of `check_range` to only require the length, not the full reference to the slice, which indeed is all the information this function requires.

Also reduce the size of a test introduced in rust-lang#70793 to make it not take 3 minutes in Miri.

This makes https://github.com/RalfJung/miri-test-libstd work again.

Simplify iter chain struct doc

Fix CI LLVM to work on NixOS out of the box

r? @Mark-Simulacrum

Tested locally, seems to work!