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

Erroneous code example:

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

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

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