These methods are the pre-stabilized API for obtaining peer credentials
from an `AF_UNIX` socket, part of the `unix_socket_ancillary_data` feature.

Their current behavior is to get/set one of the `SO_PASSCRED` (Linux),
`LOCAL_CREDS_PERSISTENT` (FreeBSD), or `LOCAL_CREDS` (NetBSD) socket
options. On other targets the `{set_}passcred()` methods do not exist.

There are two problems with this approach:

1. Having public methods only exist for certain targets isn't permitted
   in a stable `std` API.

2. These options have generally similar purposes, but they are non-POSIX
   and their details can differ in subtle and surprising ways (such as
   whether they continue to be set after the next call to `recvmsg()`).

Splitting into OS-specific extension traits is the preferred solution to
both problems.

When reading through this, I got slightly hung up thinking the `let` it was referring to was the `let tmp` on line 25, which was confusing considering the comment states that the temporary is freed at the end of the block. I think adding this clarification could potentially help some beginners like myself without being overly verbose.

To the module where it is used, so it doesn't have to be `pub`.

This just moves the server-relevant parts of handles into `server.rs`.
It introduces a new higher-order macro `with_api_handle_types` to avoid
some duplication.

This fixes two `FIXME` comments, and makes things clearer, by not having
server code in `client.rs`.

Clearer wording

fix typo in documentation for std::fs::Permissions

Please check and re-check this PR carefully to see if I got this right.

But by my logic, if the `read_only` function returns `true`, I would not expect be able to write to the file (it being read only); so this text is meant to clarify that `read_only` being `false` doesn't mean *you* can actually write to the file, just that "in general" someone is able to.

…anieu,dtolnay

Convert `Unix{Datagram,Stream}::{set_}passcred()` to per-OS traits

These methods are the pre-stabilized API for obtaining peer credentials from an `AF_UNIX` socket, part of the `unix_socket_ancillary_data` feature.

Their current behavior is to get/set one of the `SO_PASSCRED` (Linux), `LOCAL_CREDS_PERSISTENT` (FreeBSD), or `LOCAL_CREDS` (NetBSD) socket options. On other targets the `{set_}passcred()` methods do not exist.

There are two problems with this approach:

1. Having public methods only exist for certain targets isn't permitted in a stable `std` API.

2. These options have generally similar purposes, but they are non-POSIX and their details can differ in subtle and surprising ways (such as whether they continue to be set after the next call to `recvmsg()`).

Splitting into OS-specific extension traits is the preferred solution to both problems.

Update E0716.md for clarity

When reading through this, I got slightly hung up thinking the `let` it was referring to was the `let tmp` on line 25, which was confusing considering the comment states that the temporary is freed at the end of the block. I think adding this clarification could potentially help some beginners like myself without being overly verbose.

Display short types for unimplemented trait

Shortens unimplemented trait diagnostics. Now shows:

```
error[E0277]: `Option<Option<Option<...>>>` doesn't implement `std::fmt::Display`
  --> $DIR/on_unimplemented_long_types.rs:4:17
   |
LL |   pub fn foo() -> impl std::fmt::Display {
   |                   ^^^^^^^^^^^^^^^^^^^^^^ `Option<Option<Option<...>>>` cannot be formatted with the default formatter
LL |
LL | /     Some(Some(Some(Some(Some(Some(Some(Some(Some(S...
LL | |         Some(Some(Some(Some(Some(Some(Some(Some(So...
LL | |             Some(Some(Some(Some(Some(Some(Some(Som...
LL | |                 Some(Some(Some(Some(Some(Some(Some...
...  |
LL | |         ))))))))))),
LL | |     )))))))))))
   | |_______________- return type was inferred to be `Option<Option<Option<...>>>` here
   |
   = help: the trait `std::fmt::Display` is not implemented for `Option<Option<Option<...>>>`
   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0277`.
```

I'm not 100% sure if this is desirable, or if we should just let the long types remain long. This is also kinda a short-term bandaid solution. The real long term solution is to properly migrate `rustc_trait_selection`'s error reporting to use translatable diagnostics and then properly handle type name printing.

Fixes rust-lang#121687.

…est31

Move `gather_comments`.

To the module where it is used, so it doesn't have to be `pub`.

r? ``@est31``

Move `HandleStore` into `server.rs`.

This just moves the server-relevant parts of handles into `server.rs`. It introduces a new higher-order macro `with_api_handle_types` to avoid some duplication.

This fixes two `FIXME` comments, and makes things clearer, by not having server code in `client.rs`.

r? ``@bjorn3``

…=cuviper

Remove hidden use of Global

Fixes rust-lang#121797

…les, r=workingjubilee

Use the guaranteed precision of a couple of float functions in docs

…r-errors

 Account for unmet T: !Copy in E0277 message

```
error[E0277]: the trait bound `T: !Copy` is not satisfied
  --> $DIR/simple.rs:10:16
   |
LL |     not_copy::<T>();
   |                ^ the trait bound `T: !Copy` is not satisfied
```
instead of the current

```
error[E0277]: the trait bound `T: !Copy` is not satisfied
  --> $DIR/simple.rs:10:16
   |
LL |     not_copy::<T>();
   |                ^ the trait `!Copy` is not implemented for `T`
```