Support for const string interpolation into inline assembly

[dingxiangfei2009]

Related to this comment.

Maybe related to #93332

This feature request targets the inline assembly macro asm! and globally scope assembly global_asm! to support direct string interpolation into the assembly template.

The semantic works very much like a format! in a narrower sense, that only constant string is supported. The proposed macro word is interpolate $expr where $expr is a const-evaluatable expression that yields a &'static str constant value.

An example of how it would work is as follows.

trait Helper {
    const SRC: &'static str;
}

fn make_it_work<H: Helper>(h: &H, x: i64) {
    asm!(
        "mov {0}, {1}",
        in(reg) x,
        interpolate H::SRC
    );
}

struct H;
impl Helper for H {
    const SRC: &'static str = "MAGIC";
}

fn invoke() {
    make_it_work(&H, 42);
}

The one and only instantiation of asm! macro, when completely expanded through codegen, might have yield the following assembly line.

    mov rcx, MAGIC

[dingxiangfei2009]

Requests from #132012 (comment) is fulfilled.

Bonus question I guess.

Do we want in(..) and out(..) constraints to accept const values as well? I imagine that it would be helpful to additionally mark registers clobbered, for instance.

[dingxiangfei2009]

@rustbot claim

[Lokathor]

I'm unclear why this is interpolate instead of sticking with const. The value is still a constant input to the asm, as far as I can tell.

[thomcc]

It might be surprising to users that const on a string doesn't somehow give the assemnbly a refrerence to that constant string, rather than interpolating it directly.