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
);

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
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Steps

• Implement the RFC (cc @rust-lang/XXX -- can anyone write up mentoring
  instructions?)
• 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

• Should the parser be changed to parse specifically string literals and item macro invocations (rather than general expressions)?
• How should non-cfg attributes be handled?

Implementation history

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