Suggest associated const for incorrect use of `let` in traits

[Rageking8]

Given the following code: link

trait Trait {
    let x: i32;
}

fn main() {

}

The current output is:

   Compiling playground v0.0.1 (/playground)
error: non-item in item list
 --> src/main.rs:2:5
  |
1 | trait Trait {
  |             - item list starts here
2 |     let x: i32;
  |     ^^^ non-item starts here
3 | }
  | - item list ends here

error: could not compile `playground` due to previous error

Ideally the output should suggest changing the let to a const for associated const. Thanks.

[chenyukang]

@rustbot claim