How `std::iter:: Map::next()` works?

Hi! I am confused about the working mechanism of std::iter::Map::next(), that is, Map in std::iter - Rust.

The source(Map in std::iter - Rust) indicates that will be called, where self is of type std::iter::Map. As per source code, it seems like an infinite recursion, which doesn't make sense. Could anyone give some clues?

std::iter::Map is a so-called iterated adapter, meaning it stores another iterator and implements the Iterator trait using an adapted version of that iterator.

Map, specifically, is defined like this:

pub struct Map<I, F> {
    // Used for `SplitWhitespace` and `SplitAsciiWhitespace` `as_str` methods
    pub(crate) iter: I,
    f: F,

where iter is an iterator and f the function doing the mapping.

The .next() call is defined like this:

fn next(&mut self) -> Option<B> { self.f)

This calls the .next() method on the iterator stored in self.iter, and maps it via the Option::map method. It would recurse indefinitely if it said instead of, but these iterators are distinct from each other and the former owns the latter.


Why? It doesn't call, it calls The field self.iter is a distinct value (and has a distinct type) from self.