Code shared from the Rust Playground

playground.rs

// https://stackoverflow.com/questions/50519147/double-mutable-borrow-error-in-a-loop-happens-even-with-nll-on
// https://github.com/rust-lang/rust/issues/54663
// This will complie with polonius: rustc +nightly -Zpolonius --edition=2018 /tmp/p.rs
struct Foo(u8);

impl Foo {
    fn even<'i>(&'i mut self) -> &'i u8 {
        loop {
            match self.next() {
                Some(even) => return even,
                None => continue,
            }
        }
    }

    fn next<'i>(&'i mut self) -> Option<&'i u8> {
        self.0 += 1;
        if self.0 % 2 == 0 {
            Some(&self.0)
        } else {
            None
        }
    }
}

fn main() {
    let mut foo = Foo(1);
    println!("{:?}", foo.next());
}