Macro unsafe_impl_transmute_from_for_atomic

macro_rules! unsafe_impl_transmute_from_for_atomic {
    ($($($tyvar:ident)? => $atomic:ty [$prim:ty]),*) => { ... };
}

Implements TransmuteFrom for $atomic, $prim, and UnsafeCell<$prim>.

Safety

$atomic must have the same size and bit validity as $prim.