Macro local_key_if_separate_extern 

macro_rules! local_key_if_separate_extern {
    ([] $($K:tt)*) => { ... };
    ([(separate_provide_extern) $($rest:tt)*] $($K:tt)*) => { ... };
    ([$other:tt $($modifiers:tt)*] $($K:tt)*) => { ... };
}

If separate_provide_extern, then the key can be projected to its local key via <$K as AsLocalKey>::LocalKey.