Derive ZeroableInOption, PodInOption traits for transparent wrapper types

[reinerp]

For

#[repr(transparent)]
struct Id(NonZeroU32);

this type is indeed zeroable in Option, but implementing it requires the user to write the word unsafe. Would be helpful for bytemuck to automatically derive it, perhaps via the TransparentWrapper deriving.

[Lokathor]

I honestly don't think that this can be done, because the proc-macro just gets the input on a token level. This means that all the proc-macro sees is that there's an identifier it got as input which says "NonZeroU32", but the proc-macro is not able to ask the compiler "does this token mean the real ::core::num::NonZeroU32? or did they put a type alias on something to trick me?"

I'd love to be wrong! I can leave this issue open a while and link it in the dark-arts discord channel, maybe someone there will have some way to make it work.

[reinerp]

Here's a possible way. Given this:

#[derive(ZeroableInOption)]
#[repr(transparent)]
struct Foo(Bar);

the proc-macro should check that #[repr(transparent)] is present, and if so generate the following code:

unsafe impl ZeroableInOption for Foo where Bar: ZeroableInOption {}

Likewise for PodInOption. See playground.

[Lokathor]

I always forget that you can put a where clause on a concrete type.

Okay, that seems solid. If you do the PR (or find someone to do it for you) then I can have the proc-macro experts check it out and we can probably add this.

But i am not a proc-macro person, so I definitely won't be implementing this myself.