But the whole idea is that once a closure implements an interface its internal s...

steveklabnik · on Oct 25, 2018

Well, first of all, it's not a closure, it's a struct, though closures are syntax sugar for structs.

Second, what you said is true, but also not really relevant; mutability is part of the API in Rust, and so that is part of the interface.

To make it more concrete, https://doc.rust-lang.org/stable/std/iter/trait.Iterator.htm..., the protocol, takes &mut self. So it must be mutable. It's following the principle you're talking about.