Macro rustc_ap_rustc_arena::which_arena_for_type

macro_rules! which_arena_for_type {
    ([] [$arena : expr]) => { ... };
    ([few $(, $attrs : ident) *] [$arena : expr]) => { ... };
    ([$ignore : ident $(, $attrs : ident) *] $args : tt) => { ... };
}