A clobber was surrounded by braces in the asm macro.

Erroneous code example:

asm!("mov $$0x200, %eax"
     :
     :
     : "{eax}"
    );

Considering that this would be a long explanation, we instead recommend you to take a look at the unstable book: https://doc.rust-lang.org/unstable-book/language-features/asm.html