How to specify generic parameter of trait in trait bound?

help
1

I have a use case like this:

struct S;

trait PasswordHasher<T> {}

impl<H, P> Iterator for S
where
    H: PasswordHasher<P>,
{
    type Item = ();

    fn next(&mut self) -> Option<Self::Item> {
        todo!()
    }
}

Which does not compile.
I worked around this, by storing P as PhantomData in S as a workaround, but I'd rather not do this, since imho PhantomData is just noise when reading the code.

I also imagined that maybe this would work

struct S;

trait PasswordHasher<T> {}

impl<H> Iterator for S
where
    for<P> H: PasswordHasher<P>,
{
    type Item = ();

    fn next(&mut self) -> Option<Self::Item> {
        todo!()
    }
}

But HRTBs sadly only work with lifetimes at the moment.

Is there any way to implement this without resorting to PhantomData?

2

It would help to describe what you're actually trying to do.

Notably, the reason that doesn't work is that it doesn't make sense -- which P? Why that one? What are you doing with it? What if something implements PasswordHasher for multiple different Ps? Without having a call or a method, we have no idea what you're even trying to accomplish.

Maybe you want the <T> on a method in a non-generic PasswordHasher trait. Maybe you want a PhantomData. Maybe you want an associated type on PasswordHasher instead of a generic. But you have to say what you're trying to so.

3

The problem is, that this will work with any P.

I want to get rid of the PhantomData in the struct.
I can move the generic P into the functions in verify() and hash() without problems, but then, I get issues when specifying P on the iterator implementation.

Edit:
On second thought, I guess that my problem then boils down to having multiple implementations of Iterator for the same type, which is not possible due to its API using a single associated type.
I guess I just leave it as it is.