Add peek and peek_from to UnixStream and UnixDatagram

[rijenkii]

This is my first PR, so I'm sure I've done some things wrong.

This PR:

• adds peek function to UnixStream;
• adds peek and peek_from to UnixDatagram;
• moves UnixDatagram::recv_from implementation to a private function recv_from_flags, as peek_from uses the same code, just with different flags.

I've taken the documentation from TcpStream and UdpStream, so it may or may not make sense (I'm bad with english words).
Also, I'm not sure what I should write in the unstable attribute, so I've made up the name and set the issue to "none".

Closes #68565.

[rust-highfive]

r? @joshtriplett

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[the8472]

See also #69864

[bors]

☔ The latest upstream changes (presumably #73265) made this pull request unmergeable. Please resolve the merge conflicts.

[joshtriplett]

This seems reasonable to me. It needs to be rebased (rather than back-merging from the latest rust-lang/rust), but otherwise, I think it'd be reasonable to merge this.

[rijenkii]

Done, but what about the issue = "none"? And should I add #[feature(unix_socket_peek)] anywhere else?

[bors]

☔ The latest upstream changes (presumably #75979) made this pull request unmergeable. Please resolve the merge conflicts.

[daniel5151]

Just adding my vote of interest to see this get merged!

I've had to roll my own extension trait for UnixStream that adds peek, and while it works fine, it would be nice if I could remove that bit of unsafe code from my project.

[joshtriplett]

This looks great to me. Could someone on @rust-lang/libs review and merge, please?

[Mark-Simulacrum]

It looks like this needs a rebase, but I'll r? @KodrAus for libs team unstable API approval. Implementation looks good to me too (r=me or Josh on that).

[KodrAus]

PR is 80% tests and docs? Looks good to me!

It looks like we've got a merge conflict to work through so once that's sorted I'll push it through.

[rijenkii]

I'll fix the conflicts in the next couple of days, a little bit busy rn

[rijenkii]

Done, I think
Rebases are hard

[Dylan-DPC-zz]

bors r=KodrAus

[Dylan-DPC-zz]

gah again ..

@bors r=KodrAus

[bors]

📌 Commit 64b8fd7 has been approved by KodrAus

[bors]

⌛ Testing commit 64b8fd7 with merge 9911160...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: KodrAus
Pushing 9911160 to master...

[dtolnay]

Please open a tracking issue for the new unstable methods and send a PR updating these issue = "..." attributes with the tracking issue number.

[rijenkii]

Tracking issue opened, PR sent