Tracking issue for release notes of #128570: Stabilize `asm_const`

[rustbot]

This issue tracks the release notes text for #128570.

• Issue is nominated for the responsible team (and T-release nomination is removed).
• Proposed text is drafted by team responsible for underlying change.
• Issue is nominated for release team review of clarity for wider audience.
• Release team includes text in release notes/blog posts.

Release notes text:

The section title will be de-duplicated by the release team with other release notes issues.
Prefer to use the standard titles from previous releases.
More than one section can be included if needed.

# Language
- [Stabilize `const` operands in inline assembly](https://github.com/rust-lang/rust/pull/128570)

Release blog section (if any, leave blank if no section is expected):

# Constants as assembly immediates

The `const` assembly operand now provides a way to use integers as immediates
without first storing them in a register. As an example, we implement a syscall to
[`write`](https://man7.org/linux/man-pages/man2/write.2.html) by hand: 

```rust
const WRITE_SYSCALL: c_int = 0x01; // syscall 1 is `write`
const STDOUT_HANDLE: c_int = 0x01; // `stdout` has file handle 1
const MSG: &str = "Hello, world!\n";

let written: usize;

// Signature: `ssize_t write(int fd, const void buf[], size_t count)`
unsafe {
    core::arch::asm!(
        "mov rax, {SYSCALL} // rax holds the syscall number",
        "mov rdi, {OUTPUT}  // rdi is `fd` (first argument)",
        "mov rdx, {LEN}     // rdx is `count` (third argument)",
        "syscall            // invoke the syscall",
        "mov {written}, rax // save the return value",
        SYSCALL = const WRITE_SYSCALL,
        OUTPUT = const STDOUT_HANDLE,
        LEN = const MSG.len(),
        in("rsi") MSG.as_ptr(), // rsi is `buf *` (second argument)
        written = out(reg) written,
    );
}

assert_eq!(written, MSG.len());
```

Output:

```text
Hello, world!
```

[Playground link](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=0cf8e21335b38011b49156c6c65929bc).

In the above, a statement such as `LEN = const MSG.len()` populates the format
specifier `LEN` with an immediate that takes the value of `MSG.len()`. This can be seen
in the generated assembly (the value is `14`):

```asm
lea     rsi, [rip + .L__unnamed_3]
mov     rax, 1    # rax holds the syscall number
mov     rdi, 1    # rdi is `fd` (first argument)
mov     rdx, 14   # rdx is `count` (third argument)
syscall # invoke the syscall
mov     rax, rax  # save the return value
```

See [the reference](https://doc.rust-lang.org/reference/inline-assembly.html)
for more details.

[traviscross]

@folkertdev, @Amanieu, @tgross35: Any thoughts for this?

cc @rust-lang/lang

[tgross35]

What exactly is needed here? I don't think there is a need for anything special, unless this merits a small demo in the blog post.

[Mark-Simulacrum]

At minimum, I would expect that the release notes section is updated to provide a 1-sentence description of the feature in human language, not just referencing the feature gate name.

[tgross35]

Okay - so the issue needs to be edited to replace the "Stabilize xxx" link with a description? It isn't clear from the top post what is actually expected to happen.

I'm also not sure why it shows up under "# Compatibility Notes" rather than "# Language", unless that is also expected to be adjusted.

[Mark-Simulacrum]

Yes, the section title is arbitrary (I'll look at modifying that for future with something more obvious like EDIT ME), and should be adjusted by the team.

Do you have a suggestion on how to make the request clearer? I'm happy (and probably best to) take that offline / to zulip.

[tgross35]

Okay, that makes sense. Rustbot could probably add more or less what you said here to the top post, a la "Edit this section to replace the automatically generated link with a succinct description of what changed. If the change is notable enough for inclusion in the blog post, add a section there.".

(I like this idea btw)

[traviscross]

@tgross35: It'd be good to write up a blog post section for this. It's a notable user-facing feature. If there happens to be an RfL use case for this (as there is for asm_goto), it might be interesting to briefly mention that so as to advertise that ongoing collaboration a bit.

Here's a good example of such a section:

• Tracking issue for release notes of #128596: stabilize const_fn_floating_point_arithmetic #129567 (comment)