cast_sign_loss: value can't be negative if guarded by assert:

[matthiaskrgr]

similar to #2728 however, the value was not a constant in my original code, instead I protected against negative values with an assert().
clippy 0.0.212

#[cfg_attr(feature = "cargo-clippy", warn(cast_sign_loss))]

fn main() {
    let x = get_some_number();
    assert!(x > 0);
    let y = "hi".to_string().repeat(x as usize);
    println!("{}", y);
}

pub fn get_some_number() -> i32 {
	5
}

warning: casting i32 to usize may lose the sign of the value
 --> src/main.rs:6:37
  |
6 |     let y = "hi".to_string().repeat(x as usize);
  |                                     ^^^^^^^^^^
  |
note: lint level defined here
 --> src/main.rs:1:43
  |
1 | #[cfg_attr(feature = "cargo-clippy", warn(cast_sign_loss))]
  |                                           ^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#cast_sign_loss