An invalid syntax was passed to the second argument of an `asm` macro line.

Erroneous code example:

```compile_fail,E0661
let a;
asm!("nop" : "r"(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