Macro dispatch 

macro_rules! dispatch {
    (
        $(#[$attr:meta])*
        $vis:vis static $name:ident: Dispatch<$kernel_type:ty> = $candidates:expr;
    ) => { ... };
}

Declares a static Dispatch over a slice of Candidates, best first.

The slice may be a block whose tail expressions are #[cfg]-gated per architecture.