This adds support for CMSG handling on macOS and fixes it on OpenBSD
and other BSDs.

When traversing the CMSG list, the previous code had an exception for
Android where the next element after the last pointer could point to
the first pointer instead of NULL.  This is actually not specific to
Android: the `libc::CMSG_NXTHDR` implementation for Linux and
emscripten have a special case to return NULL when the length of the
previous element is zero; most other implementations simply return the
previous element plus a zero offset in this case.

This MR additionally adds `SocketAncillary::is_empty` because clippy
is right that it should be added.

And, now that we do that, we can remove the explanatory note since the
error span should make it clear what the disambiguator is.

… r=Amanieu

alloc: Added `as_slice` method to `BinaryHeap` collection

I initially asked about whether it is useful addition on https://internals.rust-lang.org/t/should-i-add-as-slice-method-to-binaryheap/13816, and it seems there were no objections, so went ahead with this PR.

> There is [`BinaryHeap::into_vec`](https://doc.rust-lang.org/std/collections/struct.BinaryHeap.html#method.into_vec), but it consumes the value. I wonder if there is API design limitation that should be taken into account. Implementation-wise, the inner buffer is just a Vec, so it is trivial to expose as_slice from it.

Please, guide me through if I need to add tests or something else.

UPD: Tracking issue rust-lang#83659

escape_ascii take 2

The previous PR, rust-lang#73111 was closed for inactivity; since I've had trouble in the past reopening closed PRs, I'm just making a new one.

I'm still running the tests locally but figured I'd open the PR in the meantime. Will fix whatever errors show up so we don't have to wait again for this.

r? ``@m-ou-se``

unix: Fix feature(unix_socket_ancillary_data) on macos and other BSDs

This adds support for CMSG handling on macOS and fixes it on OpenBSD and possibly other BSDs.

When traversing the CMSG list, the previous code had an exception for Android where the next element after the last pointer could point to the first pointer instead of NULL.  This is actually not specific to Android: the `libc::CMSG_NXTHDR` implementation for Linux and emscripten have a special case to return NULL when the length of the previous element is zero; most other implementations simply return the previous element plus a zero offset in this case.

This MR makes the check non-optional which fixes CMSG handling and a possible endless loop on such systems; tested with file descriptor passing on OpenBSD, Linux, and macOS.

This MR additionally adds `SocketAncillary::is_empty` because clippy is right that it should be added.

This belongs to the `feature(unix_socket_ancillary_data)` tracking issue:  rust-lang#76915

r? `@joshtriplett`

… r=jyn514

Lint on unknown intra-doc link disambiguators

…egg-test, r=lcnr

Add a regression test for issue-82792

Closes rust-lang#82792

r? ``@lcnr``

…span, r=RalfJung

Remove a FIXME resolved by rust-lang#73578

r? ``@RalfJung``

…=jonas-schievink

⬆️ rust-analyzer