Enable API documentation for `std::os::wasi`.

This adds API documentation support for `std::os::wasi` modeled after
how `std::os::unix` works, so that WASI can be documented [here] along
with the other platforms.

[here]: https://doc.rust-lang.org/stable/std/os/index.html

Two changes of particular interest:

 - This changes the `AsRawFd` for `io::Stdin` for WASI to return
   `libc::STDIN_FILENO` instead of `sys::stdio::Stdin.as_raw_fd()` (and
   similar for `Stdout` and `Stderr`), which matches how the `unix`
   version works. `STDIN_FILENO` etc. may not always be explicitly
   reserved at the WASI level, but as long as we have Rust's `std` and
   `libc`, I think it's reasonable to guarantee that we'll always use
   `libc::STDIN_FILENO` for stdin.

 - This duplicates the `osstr2str` utility function, rather than
   trying to share it across all the configurations that need it.

r? `@alexcrichton`

Enable API documentation for `std::os::wasi`.

This adds API documentation support for `std::os::wasi` modeled after
how `std::os::unix` works, so that WASI can be documented [here] along
with the other platforms.

[here]: https://doc.rust-lang.org/stable/std/os/index.html

Two changes of particular interest:

 - This changes the `AsRawFd` for `io::Stdin` for WASI to return
   `libc::STDIN_FILENO` instead of `sys::stdio::Stdin.as_raw_fd()` (and
   similar for `Stdout` and `Stderr`), which matches how the `unix`
   version works. `STDIN_FILENO` etc. may not always be explicitly
   reserved at the WASI level, but as long as we have Rust's `std` and
   `libc`, I think it's reasonable to guarantee that we'll always use
   `libc::STDIN_FILENO` for stdin.

 - This duplicates the `osstr2str` utility function, rather than
   trying to share it across all the configurations that need it.

r? ``@alexcrichton``

Enable API documentation for `std::os::wasi`.

This adds API documentation support for `std::os::wasi` modeled after
how `std::os::unix` works, so that WASI can be documented [here] along
with the other platforms.

[here]: https://doc.rust-lang.org/stable/std/os/index.html

Two changes of particular interest:

 - This changes the `AsRawFd` for `io::Stdin` for WASI to return
   `libc::STDIN_FILENO` instead of `sys::stdio::Stdin.as_raw_fd()` (and
   similar for `Stdout` and `Stderr`), which matches how the `unix`
   version works. `STDIN_FILENO` etc. may not always be explicitly
   reserved at the WASI level, but as long as we have Rust's `std` and
   `libc`, I think it's reasonable to guarantee that we'll always use
   `libc::STDIN_FILENO` for stdin.

 - This duplicates the `osstr2str` utility function, rather than
   trying to share it across all the configurations that need it.

r? `@alexcrichton`

Enable API documentation for `std::os::wasi`.

This adds API documentation support for `std::os::wasi` modeled after
how `std::os::unix` works, so that WASI can be documented [here] along
with the other platforms.

[here]: https://doc.rust-lang.org/stable/std/os/index.html

Two changes of particular interest:

 - This changes the `AsRawFd` for `io::Stdin` for WASI to return
   `libc::STDIN_FILENO` instead of `sys::stdio::Stdin.as_raw_fd()` (and
   similar for `Stdout` and `Stderr`), which matches how the `unix`
   version works. `STDIN_FILENO` etc. may not always be explicitly
   reserved at the WASI level, but as long as we have Rust's `std` and
   `libc`, I think it's reasonable to guarantee that we'll always use
   `libc::STDIN_FILENO` for stdin.

 - This duplicates the `osstr2str` utility function, rather than
   trying to share it across all the configurations that need it.

r? ``@alexcrichton``

Rollup of 14 pull requests

Successful merges:

 - rust-lang#81794 (update tracking issue for `relaxed_struct_unsize`)
 - rust-lang#82057 (Replace const_cstr with cstr crate)
 - rust-lang#82370 (Improve anonymous lifetime note to indicate the target span)
 - rust-lang#82394 (:arrow_up: rust-analyzer)
 - rust-lang#82396 (Add Future trait for doc_spotlight feature doc)
 - rust-lang#82404 (Test hexagon-enum only when llvm target is present)
 - rust-lang#82419 (expand: Preserve order of inert attributes during expansion)
 - rust-lang#82420 (Enable API documentation for `std::os::wasi`.)
 - rust-lang#82421 (Add a `size()` function to WASI's `MetadataExt`.)
 - rust-lang#82442 (Skip emitting closure diagnostic when closure_kind_origins has no entry)
 - rust-lang#82473 (Use libc::accept4 on Android instead of raw syscall.)
 - rust-lang#82482 (Use small hash set in `mir_inliner_callees`)
 - rust-lang#82490 (Update cargo)
 - rust-lang#82494 (Substitute erased lifetimes on bad placeholder type)

Failed merges:

 - rust-lang#82448 (Combine HasAttrs and HasTokens into AstLike)

r? `@ghost`
`@rustbot` modify labels: rollup

As a followup to [this comment] in rust-lang#82420, this patch teaches rustdoc
how to display WASI.

[this comment]: rust-lang#82420 (comment)

Teach rustdoc how to display WASI.

As a followup to [this comment] in rust-lang#82420, this patch teaches rustdoc
how to display WASI.

[this comment]: rust-lang#82420 (comment)

r? `@alexcrichton`