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),) +) => { ... };
}