Inline `asm!` only checked when used

[phil-opp]

I tried this code:

#![feature(asm)]

fn main() {
    // errors if the following line is uncommented
    // foo()
}

pub fn foo() {
    unsafe {
        asm!("xchgfoobarbaz bx, bx");
    }
}

(playground)

I expected to see this happen: Compilation error because xchgfoobarbaz is not a valid assembly instruction.

Instead, this happened: The code compiles without errors or warnings. A compile error only occurs if the foo() call is uncommented in main.

Meta

Nightly version on playground: 2020-12-22 bb1fbbf

Originally reported by @tomaka in rust-osdev/x86_64#218.

[Aaron1011]

In general, I don't think it's possible to solve this. The 'invalid instruction mnemonic' error comes from LLVM - if the asm! call occurs in a generic function, then we have no way of generating LLVM IR if it's not called (and therefore monomorphized) at least once.

[asquared31415]

If I'm not mistaken, this is a duplicate of #36718 (though its title and original code are no longer entirely correct)

[Amanieu]

Closing as duplicate of #36718.