Macro slice

macro_rules! slice {
    ($s:expr, .., $end:expr) => { ... };
    ($s:expr, $start:expr, ..) => { ... };
    ($s:expr, $start:expr, $end:expr) => { ... };
}

Slice s by some start and end bounds.

This is equivalent to doing s[start..end], but works in a const context.