Macro discrete_feature 

macro_rules! discrete_feature {
    ($(#[$meta:meta])* $vis:vis enum $feature: ident<$feature_name: path, $value: ty>) => { ... };
}

This macro expands to define an enum which already implements Parse and DiscreteFeature, for a one-liner definition of a DiscreteFeature.

Example

use css_parse::*;
use bumpalo::Bump;
use csskit_derives::{ToCursors, ToSpan};
use derive_atom_set::AtomSet;

// Your language atoms:
#[derive(Debug, Default, Copy, Clone, AtomSet, PartialEq)]
pub enum MyLangAtoms {
  #[default]
  _None,
  TestFeature,
}
impl MyLangAtoms {
  pub const ATOMS: MyLangAtoms = MyLangAtoms::_None;
}

// Define the Discrete Feature.
discrete_feature! {
    /// A discrete media feature: `(test-feature: big)`, `(test-feature: small)`
    #[derive(ToCursors, ToSpan, Debug)]
    pub enum TestFeature<MyLangAtoms::TestFeature, T![Ident]>
}

// Test!
assert_parse!(MyLangAtoms::ATOMS, TestFeature, "(test-feature)", TestFeature::Bare(_open, _ident, _close));
assert_parse!(MyLangAtoms::ATOMS, TestFeature, "(test-feature:big)", TestFeature::WithValue(_open, _ident, _colon, _feature, _close));