`#[unstable]` accepts an already-stable language feature

[RalfJung]

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.

[CastilloDel]

@rustbot claim

[CastilloDel]

Should we also reject features that don't exist? Right now, this test would fail (because it would compile without problems)

//@ check-fail

#![feature(staged_api)]
#![stable(feature = "rust_test", since = "1.0.0")]

#[unstable(feature = "name_of_a_feature_that_does_not_exist", issue = "42")]
const fn my_fun() {}

fn main() {}

[RalfJung]

"unstable" attributes is how library features come into existence. So yes we should accept features that do not yet exist.