[ER] Add warnings on Nightly for unused features

[leonardo-m]

This is a diagnostic enhancement request. This code compiles with no warnings on Nightly:

#![feature(min_const_generics)]
fn main() {}

But I'd like rustc to give a warning that the min_const_generics feature is unnecessary for this program. (Not even Clippy seems to give a warning). The same for all the other unused features.

[ehuss]

I believe this is a known issue, see #44232.

[leonardo-m]

OK, thank you, I suspected that.