Misleading error message with empty trait

[frewsxcv]

/tmp $ cat program.rs

trait MyTrait {}

fn main() {}

/tmp $ rustc program.rs

program.rs:1:1: 1:17 error: parameter `Self` is never used
program.rs:1 trait MyTrait {}
             ^~~~~~~~~~~~~~~~
program.rs:1:17: 1:17 help: consider removing `Self` or using a marker such as `core::marker::PhantomFn`
error: aborting due to previous error

My program does not make use of any Self parameter

[rprichard]

Looks like a duplicate of #22598.