Guess the language by the error

[sassa_nf]

Ok, can we pass a function that consumes items instead of returning an Iterator?

error[E0666]: nested `impl Trait` is not allowed
   --> src/main.rs:103:63
    |
103 |    fn scan<T>(&mut self, z: T, mut f: impl FnMut(HeapElement, impl std::io::Read, T) -> T) -> std::io::Result<T> {
    |                                       ------------------------^^^^^^^^^^^^^^^^^^---------
    |                                       |                       |
    |                                       |                       nested `impl Trait` here
    |                                       outer `impl Trait`

Computer says "no".

Comments

[juan_gandhi]

They have a pretty weird type system. I don't know how to interpret all this.

[sassa_nf]

Well, I know how to proceed - &mut dyn std::io::Read instead of impl std::io::Read, but it all feels like fighting problems that shouldn't be problems.

The other funny thing is why f needs to be mut, when it is of type FnMut.

[juan_gandhi]

It's all pretty weird.