Reference Bundle

[cyypherus]

Hello,

These "no" cases are the functionality of the API that I would like to accomplish.

fn main() {
    let op = B { op: Box::new(|_| {}) };
    let mut u = U {};
    let mut v = V {};
    for _ in 0..10 {
        // (op.op)(&mut u); // yes
        
        // (op.op)(&mut v); // yes
        
        // (op.op)(&mut (&u, &v)); // no

        // struct UV<'a> { // no
        //     u: &'a mut U,
        //     v: &'a mut V
        // }
        // (op.op)(&mut UV {u: &mut u, v: &mut v});
    }
}

struct U;
struct V;

struct B<T> {
    op: Box<dyn Fn(&mut T)>
}

Rust Playground

My understanding is that in both the "no" cases the generic T in B is resolving to a single lifetime like B<T<'a>> & to the compiler it looks as if B will store T<'a>.

What are my options to move forward?
Is there a way to allow the users of the API to bundle & pass whatever mutable state they want to?

Thank you for your help in advance!

[quinedot]

@cyypherus

Rust Playground

My understanding is that in both the "no" cases the generic T in B is resolving to a single lifetime like B<T<'a>

Yes.

@cyypherus

& to the compiler it looks as if B will store T<'a>.

You can think of it that way if you like, but it's more like the single lifetime is part of the type that is assumed to have a destructor, so the lifetime has to be alive at the drop points.

That's mostly a pedantic correction for this particular case I suppose.

@cyypherus

What are my options to move forward?

Depends on the goal. Declaring op after u and v works for the playground.

There's no fully general solution for "closure higher-ranked over an arbitrary number of lifetimes". So ultimately, probably, it will be a task for users of the API to work around.