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

Erroneous code example:

```compile_fail,E0663
llvm_asm!("xor %eax, %eax"
          :
          : "+test"("a")
         );
```

Considering that this would be a long explanation, we instead recommend you
take a look at the [`asm`] chapter of the Unstable book:

[asm]: https://doc.rust-lang.org/stable/unstable-book/library-features/asm.html