An invalid input operand constraint was passed to the asm macro (third line).

Erroneous code example:

asm!("xor %eax, %eax"
     :
     : "=test"("a")
    );

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