Tracking Issue for `asm_cfg`: `#[cfg(...)]` within `asm!`

[folkertdev]

The feature gate for the issue is #![feature(asm_cfg)].

This feature was discussed in #140279. It allows configuring templates and operands in the assembly macros, for example:

asm!( // or global_asm! or naked_asm!
    "nop",
    #[cfg(target_feature = "sse2")]
    "nop",
    // ...
    #[cfg(target_feature = "sse2")]
    a = const 123, // only used on sse2
);

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A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
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Steps

• Implementation add asm_cfg: #[cfg(...)] within asm! #140367
• Adjust documentation (see instructions on rustc-dev-guide)
• Style updates for any new syntax (nightly-style-procedure)
  • Style team decision on new formatting
  • Formatting for new syntax has been added to the Style Guide
  • (non-blocking) Formatting has been implemented in rustfmt
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

Implementation history

• add asm_cfg: #[cfg(...)] within asm! #140367

@rustbot label +T-lang +F-asm +A-inline-assembly