Macro rooted 

macro_rules! rooted {
    (&in($cx:expr) $($t:tt)*) => { ... };
    (in($cx:expr) let $($var:ident)+ = $init:expr) => { ... };
    (in($cx:expr) let $($var:ident)+: $type:ty = $init:expr) => { ... };
    (in($cx:expr) let $($var:ident)+: $type:ty) => { ... };
}