Macro define_cast 

macro_rules! define_cast {
    (unsafe { $vis:vis $name:ident $(<$tyvar:ident $(: ?$optbound:ident)?>)? = $src:ty => $dst:ty }) => { ... };
}

Defines a named Cast implementation.

Safety

The caller must ensure that, given src: *mut $src, src as *mut $dst is a size-preserving or size-shrinking cast.