`feature` is missing from the built-in attibutes list

[Jules-Bertholet]

The following compiles on stable Rust:

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

While individual features are all unstable, the feature attribute itself is part of the stable Rust language, and should be documented in https://github.com/ferrocene/specification/blob/main/src/attributes.rst.

[PLeVasseur]

@traviscross -- which A- label is most appropriate here?

I searched through open and closed issues in the Reference mentioning feature, but didn't find a good match.

Given where they sit in the Reference, maybe A-attributes?

[traviscross]

That label is appropriate. Note the section you linked for the Reference is for a separate feature. We don't really document this attribute on the Reference side either, other than it appearing in our attribute index, so I've similarly filed:

• Document #![feature(..)] attribute? reference#2076