Macro rustc_feature::active::declare_features

macro_rules! declare_features {
    (__status_to_bool active) => { ... };
    (__status_to_bool incomplete) => { ... };
    ($(
        $(#[doc = $doc:tt])* ($status:ident, $feature:ident, $ver:expr, $issue:expr, $edition:expr),
    )+) => { ... };
}