document cfg conditions on inline assembly templates and operands

[folkertdev]

Reference PR for:

• Stabilize asm_cfg rust#147736

[petrochenkov]

Again, it may be a mostly theoretical concern, but attributes are not supported here.
asm! rather have an attribute-like syntax in its DSL - rust-lang/rust#147736 (comment).

The "only cfg and cfg_attr" restriction is unique to all attribute positions and is a direct consequence of the above.

[folkertdev]

This list is about allowed positions, and with asm_cfg attributes are parsed on template strings and operands, so saying that they are a valid position for attributes feels right.

But yeah they aren't really supported like in the other places listed here.

[ehuss]

This is an interesting question because for example in things like #1890 we document that diagnostics are allowed in all attribute positions. I'm not sure how to resolve that.

[folkertdev]

Are there any diagnostics that are meaningful on a (macro call that expands to a) template string? We could parse and then silently discard them, but actually making the diagnostic attribute do something would be tough I think.

[folkertdev]

#[rustfmt::skip] could be meaningful, but assembly is not formatted at all currently, so in practice it won't do anything today.

[ehuss]

I don't know. You could have something silly like asm!(#[allow(named_asm_labels)] "warned: nop");, but I don't think that is something worth doing.

I'm not suggesting that it is something that should be changed. I'm just trying to think of how we can cleanly fit it in to the reference text.

[ehuss]

Can you add an example here showing cfg being used?

[folkertdev]

I added

core::arch::naked_asm!(
    #[cfg(not(panic = "abort"))]
    ".cfi_startproc",
    // ...
    "ret",
    #[cfg(not(panic = "abort"))]
    ".cfi_endproc",
);

Loosely based on https://github.com/nbdd0121/unwinding/blob/3bc28cec6c30eade0e1caa094b49db78101b19ef/src/unwinder/arch/aarch64.rs#L66C24-L66C40.

It's tough to come up with simple full examples, so hopefully this conveys the idea.