Clippy suggests rewriting hex literal as decimal to avoid "unnecessary" cast

[vilcans]

Clippy suggests I rewrite a hexadecimal value as decimal, but I wrote it in hex for a reason and want to keep it that way.

Minimal reproduction:

let a: f32 = 0xff as f32;

Result:

warning: casting integer literal to `f32` is unnecessary
   --> src/main.rs:113:22
    |
113 |         let a: f32 = 0xff as f32;
    |                      ^^^^^^^^^^^ help: try: `255_f32`
    |
    = note: `#[warn(clippy::unnecessary_cast)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast

$ cargo clippy -V
clippy 0.0.212 (4ee1206 2020-02-01)

[flip1995]

Duplicate of #5220

Fixed in #5257