How to write where clause for closure with context

help
1

This is a follow up to this already answered question, except I don't think I can use the question here as my function here has context.

Consider the following example:

pub trait SillyTrait<T1> {
    fn silly(self);
}

pub fn silly2<T1, T2>(x : T1, y: T2) where T2 : SillyTrait<T1> {
    <T2 as SillyTrait<T1>>::silly(y);
}

pub fn silly3<T1>(f : impl Fn(T1, i64) -> i64, x : T1)
where typeof(f2) : SillyTrait<T1>
{
    let f2 = |y : i64| f(x, y);
    silly2(x, f2);
}

It's just typeof(f2) that I can't write here, but as silly3 seems valid (at least for some impls of SillyTrait) I should be able to write it I presume. But casting to an fn doesn't seem to work as f2 has context and writing a global function also won't work I don't think because I'll get all sorts of nasty race conditions if it's used at the same time (which Rust is supposed to check against) so I'm a bit lost about how to write this but I'm sure it's possible as it would be crazy for it not to be.

2

You can't do this yet, you can't reference types defined inside the function you are putting a where clause for.

You can manually desugar the closure and implement SillyTrait for that type. This will give you a type that you can name. You can see how to desugar closures in my blog on closures here:

3

Would it be possible to create an example that is a bit closer to your actual goal? Your current example has a generic trait SillyTrait<T1> where the type T1 is unused, then a function fn silly2<T1, T2>(x : T1, y: T2) where T2 : SillyTrait<T1> where the value x and type T1 are unused. And then a silly3 that does not make syntactic sense. It's hard to guess how you might achieve your goals when they are so well hidden.

4

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