parse error in #[feature(...)] discards all mentioned features.

[lcnr]

#![feature(const_generics, use std::mem)]

pub struct Bar<const N: usize>;

and

#![feature(const_generics, 'other_feature)]

pub struct Bar<const N: usize>;

Both emit both a parse error and

error[E0658]: const generics are unstable
 --> src/lib.rs:3:22
  |
3 | pub struct Bar<const N: usize>;
  |                      ^
  |
  = note: see issue #74878 <https://github.com/rust-lang/rust/issues/74878> for more information
  = help: add `#![feature(min_const_generics)]` to the crate attributes to enable

Ideally we should still accept the features parsed before that error, so it would be nice to only emit the parse error here.

Split out from #80077, thanks @leonardo-m for opening that issue.

[varkor]

Presumably this is the behaviour for attributes in general: if a single item is malformed, the entire attribute won't be parsed. I wonder whether resolving this in general might lead to unexpected behaviour for other attributes.