Why does `Iterator.find()` require a mutable `self` reference?

Question

From the documentation:

fn find<P>(&mut self, predicate: P) -> Option<Self::Item> 
where P: FnMut(&Self::Item) -> bool

I don't see why it needs a mutable ref to self. Can someone explain?

Answer 1

It needs to be able to mutate self because it is advancing the iterator. Each time you call next, the iterator is mutated:

fn next(&mut self) -> Option<Self::Item>;

Here is the implementation of find:

fn find<P>(&mut self, mut predicate: P) -> Option<Self::Item> where
    Self: Sized,
    P: FnMut(&Self::Item) -> bool,
{
    for x in self.by_ref() {
        if predicate(&x) { return Some(x) }
    }
    None
}