Macro unpack_opt_ref 

macro_rules! unpack_opt_ref {
    ($args:ident => $arg1:ident) => { ... };
    ($args:ident => $arg1:ident,$arg2:ident) => { ... };
    ($args:ident => $arg1:ident,$arg2:ident,$arg3:ident) => { ... };
    ($args:ident => $arg1:ident,$arg2:ident,$arg3:ident,$arg4:ident) => { ... };
    ($args:ident => $arg1:ident,$arg2:ident,$arg3:ident,$arg4:ident,$arg5:ident) => { ... };
}

Try to efficiently unwrap a &Vec<T> into a &[T;n] for $arg1…$argn