Macro parity_util_mem::malloc_size_of_is_0

macro_rules! malloc_size_of_is_0 {
    ($($ty:ty),+) => { ... };
    (any: $($ty:ident<$($gen:ident),+>),+) => { ... };
    ($($ty:ident<$($gen:ident),+>),+) => { ... };
}

Implement notion of 0 allocation size for some type(s).

if used for generics, by default it will require that generaic arguments should implement MallocSizeOf. This can be avoided with passing "any: " in front of type list.

use parity_util_mem::{malloc_size, malloc_size_of_is_0};

struct Data<P> {
	phantom: std::marker::PhantomData<P>,
}

malloc_size_of_is_0!(any: Data<P>);

// MallocSizeOf is NOT implemented for [u8; 333]
assert_eq!(malloc_size(&Data::<[u8; 333]> { phantom: std::marker::PhantomData }), 0);

and when no "any: "

use parity_util_mem::{malloc_size, malloc_size_of_is_0};

struct Data<T>(pub T);

// generic argument (`T`) must be `impl MallocSizeOf`
malloc_size_of_is_0!(Data<u8>);

assert_eq!(malloc_size(&Data(0u8)), 0);