Add documentation to `std::os::unix::io` describing Rust's stance on
`/proc/self/mem`, treating it as an external entity which is outside
the scope of Rust's safety guarantees.

Co-authored-by: Sean Stangl <sean.stangl@gmail.com>

Co-authored-by: Sean Stangl <sean.stangl@gmail.com>

Co-authored-by: Josh Triplett <josh@joshtriplett.org>

Co-authored-by: Josh Triplett <josh@joshtriplett.org>

Co-authored-by: Mara Bos <m-ou.se@m-ou.se>

This adopts the wording suggested in
rust-lang#97837 (comment).

Addresses Issue 95665 by including `hir::Node::ForeignItem` as a valid
type to visit in `diagnostic_hir_wf_check`.

Fixes rust-lang#95665

…-ou-se

Implement `core::slice::IterMut::as_mut_slice` and `impl<T> AsMut<[T]> for IterMut<'_, T>`

As per [the zulip discussion](https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/.60std.3A.3Aslice.3A.3AIterMut.3A.3Aas_mut_slice.60), the `AsMut` impl has been commented out, with a comment near the `#[unstable(...)]` to uncomment it when `as_mut_slice` gets stabilized.

…=kennytm

Panic when advance_slices()'ing too far and update docs.

This updates advance_slices() to panic when advancing too far, like advance() already does. And updates the docs to say so.

See rust-lang#62726 (comment)

… r=m-ou-se

Add `{Arc, Rc}::downcast_unchecked`

Part of rust-lang#90850.

Fix the generator example for `pin!()`

The previous generator example is not actually self-referential, since the reference is created after the yield.

CC rust-lang#93178 (tracking issue)

Windows: `CommandExt::async_pipes`

Discussed in tokio-rs/tokio#4670 was the need for third party crates to be able to force `process::Command::spawn` to create pipes as async.

This implements the suggestion for a `async_pipes` method that gives third party crates that option.

# Example:

```rust
use std::process::{Command, Stdio};

Command::new("cmd")
    .async_pipes(true)
    .stdin(Stdio::piped())
    .stdout(Stdio::piped())
    .stderr(Stdio::piped())
    .spawn()
    .unwrap();
```

…ou-se

`Stdio::makes_pipe`

Wrappers around `std::process::Command` may want to be able to override pipe creation. However, [`std::process::Stdio`](https://doc.rust-lang.org/std/process/struct.Stdio.html) is opaque so there's no way to tell if `Command` was told to create new pipes or not.

This is in some ways a more generic (and cross-platform) alternative to rust-lang#97149. However, unlike that feature, this comes with the price of the user needing to actually create their own pipes rather than reusing the std one. So I think it stands (or not) on its own.

# Example

```rust
#![feature(stdio_makes_pipe)]
use std::process::Stdio;

let io = Stdio::piped();
assert_eq!(io.makes_pipe(), true);
```

…em, r=m-ou-se

Document Rust's stance on `/proc/self/mem`

Add documentation to `std::os::unix::io` describing Rust's stance on
`/proc/self/mem`, treating it as an external entity which is outside
the scope of Rust's safety guarantees.

…nkov

Include ForeignItem when visiting types for WF check

Addresses Issue 95665 by including `hir::Node::ForeignItem` as a valid
type to visit in `diagnostic_hir_wf_check`.

Fixes rust-lang#95665