#[unstable] accepts an already-stable language feature #129814
Description
This should probably not be accepted:
#![feature(staged_api)]
#![stable(feature = "rust_test", since = "1.0.0")]
#[unstable(feature = "arbitrary_enum_discriminant", issue = "42")]
#[rustc_const_unstable(feature = "arbitrary_enum_discriminant", issue = "42")]
const fn my_fun() {}
fn main() {}arbitrary_enum_discriminant is already stable, so having an unstable function under this feature makes no sense.