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

Erroneous code example:

```compile_fail,E0662
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