Document all unsafety used in libcore and libstd

[oli-obk]

The repository still contains many instances of

// ignore-tidy-undocumented-unsafe

We should eliminate all of them by documenting the unsafe used in those files.

Documenting unsafe blocks works by adding a // SAFETY: comment infront of them explaining why the unsafe block is ok. If there is an explanation about the unsafety elsewhere in the file, you can also leave a // SAFETY: comment explaining that the overarching logic is explained elsewhere (and mention where, too!)

cc @Centril

not sure what labels to add to this issue.

See https://github.com/rust-lang/rust/pull/63793/files for examples and the introduction of the unsafety tidy check

[foeb]

Hi! I'm interested in working on this.

[foeb]

@rustbot claim

[LeSeulArtichaut]

@foeb I took the liberty of opening #71063. I hope you weren't working on those 3 files 😅

[hbina]

Hello,

Is anyone working on

rust/src/libcore/slice/sort.rs

Lines 35 to 55 in 2dc5b60

	unsafe {
	// If the first two elements are out-of-order...
	if len >= 2 && is_less(v.get_unchecked(1), v.get_unchecked(0)) {
	// Read the first element into a stack-allocated variable. If a following comparison
	// operation panics, `hole` will get dropped and automatically write the element back
	// into the slice.
	let mut tmp = mem::ManuallyDrop::new(ptr::read(v.get_unchecked(0)));
	let mut hole = CopyOnDrop { src: &mut *tmp, dest: v.get_unchecked_mut(1) };
	ptr::copy_nonoverlapping(v.get_unchecked(1), v.get_unchecked_mut(0), 1);
	for i in 2..len {
	if !is_less(v.get_unchecked(i), &*tmp) {
	break;
	}
	// Move `i`-th element one place to the left, thus shifting the hole to the right.
	ptr::copy_nonoverlapping(v.get_unchecked(i), v.get_unchecked_mut(i - 1), 1);
	hole.dest = v.get_unchecked_mut(i);
	}
	// `hole` gets dropped and thus copies `tmp` into the remaining hole in `v`.
	}

This can be shown to be safe because len > 2 => len - 2 > 0, right?

[RalfJung]

I did maybe_uninit.rs as part of #76217, not realizing that this was still an ongoing effort.^^ I checked the box above.

[poliorcetics]

With #87127, only some unsafe blocks in src/libcore/slice/sort.rs are left. I looked them over and I don't think I understand the algorithm well enough to make a SAFETY comment for them though

[mdsn]

With #88412 merged I believe libstd is now free of // ignore-tidy-undocumented-unsafe which means this issue might be done.