Missing help when trying to enable stabilized library feature on stable

[jruderman]

Given the following code:

#![feature(discriminant_value)]
fn main() {
    let y = Some(1);
    println!("{:?}", std::mem::discriminant(&y));
}

The current output is:

error[E0554]: `#![feature]` may not be used on the stable release channel
 --> src/main.rs:1:1
  |
1 | #![feature(discriminant_value)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Ideally the output should look like:

error: ...
help: the feature `discriminant_value` has been stable since 1.21.0 and no longer requires an attribute to enable

(or it could be downgraded from error to warning)

#83722 fixed this for #![feature(min_const_generics)] but not for #![feature(discriminant_value)], somehow

[jyn514]

Hmm, this might be because it's a library feature and not a language feature?

[jyn514]

Mentoring instructions: include library features in this check:

rust/compiler/rustc_ast_passes/src/feature_gate.rs

Line 723 in 497ee32

let stable_since = lang_features

[chansuke]

@rustbot claim

[vishadGoyal]

Hi! I started working on this but didn't know I had to claim it first. It's my first time contributing to open source.
Anyway, so far I realized we need to combine these two Vectors instead of just using the declared_lang_features vector
the vectors

use of features

The remaining challenge is to combine two vectors with different data types.
Hope this helps @chansuke Best of luck!

[chansuke]

@vishadGoyal I appreciate your great work. You should work on this :)

[vishadGoyal]

@rustbot claim