asm!: `options` should work multiple times, to simplify macros

[joshtriplett]

To simplify the implementation of macros wrapping asm!, I think options should work if specified multiple times, with the semantic that no conflicting options may be provided. (We could also do "last option wins", but I think I'd slightly prefer "don't specify conflicting options" instead.)

This would, for instance, make it much easier to implement an asm_att! macro that passes all its arguments to asm! and adds options(att_syntax), without having to scan all the tokens it receives, look for an existing options, and append to it if found.

Sample code that I think should work (printing 9):

#![feature(asm)]

fn main() {
    let mut i: u32 = 5;
    unsafe {
        asm!(
            "add $4, %rax",
            inout("rax") i,
            options(att_syntax),
            options(nostack),
        );
    }
    println!("{}", i);
}

[Amanieu]

That seems like an extremely niche use case that would only really enable an asm_att! macro and nothing else. I don't expect such a macro to actually be used in practice since adding options(att_syntax) is already very easy.

[joshtriplett]

A large codebase that uses primarily AT&T syntax will want to spell it as briefly as possible; I expect to see such a macro in common use.

If you believe this would complicate the parser unnecessarily, then I would be open to a crate-based solution that shows how to implement the macro taking existing options into account.

[cesarb]

with the semantic that no conflicting options may be provided. (We could also do "last option wins", but I think I'd slightly prefer "don't specify conflicting options" instead.)

I think a simpler semantic would be "multiple options is identical to a single options with the options concatenated". That is, options(A, B), options(C, D) would have the exact same semantics as options(A, B, C, D).

This would be good not only for macros, but also for readability (some people might prefer multiple options, one on each line, instead of being forced to use a single options, when the list of options starts to get too long).

[joshtriplett]

@cesarb That sounds reasonable to me.

[Amanieu]

This is actually a very trivial change to the parsers, so it should be easy to add.

[camelid]

@rustbot claim

[camelid]

Oops, didn't realize it was claimed! Sorry

@rustbot release-assignment

I assume you were planning to do this, @Amanieu?