Macro rustc_data_structures::declare_box_region_type

macro_rules! declare_box_region_type {
    (impl $v:vis
     $name: ident,
     $yield_type:ty,
     for($($lifetimes:tt)*),
     ($($args:ty),*) -> ($reti:ty, $retc:ty)
    ) => { ... };
    ($v:vis $name: ident, for($($lifetimes:tt)*), ($($args:ty),*) -> ($reti:ty, $retc:ty)) => { ... };
}