Variable is both used and unused when assigning `asm!()` to it

[jonas-schievink]

#![feature(asm)]

fn main() {
    unsafe {
        let x: () = asm!("nop");
    }
}

warning: unused variable: `x`
 --> src/main.rs:5:13
  |
5 |         let x: () = asm!("nop");
  |             ^ help: if this is intentional, prefix it with an underscore: `_x`
  |
  = note: `#[warn(unused_variables)]` on by default

error[E0381]: use of possibly-uninitialized variable: `x`
 --> src/main.rs:5:13
  |
5 |         let x: () = asm!("nop");
  |             ^ use of possibly-uninitialized `x`