iter::fuse not behaving as expected

[JEphron]

The docs led me to believe that fuse would yield items until it saw a None, and then yield only Nones. Maybe I'm misunderstanding, but

let v = vec!["0", "a", "1", "2"];
let it: Vec<Option<i32>> = v.iter().map(|x| x.parse().ok()).fuse().inspect(|x| println!("inspected {:?}",x)).collect();
println!("it:{:?}", it);

outputs

inspected Some(0)
inspected None
inspected Some(1)
inspected Some(2)
it:[Some(0), None, Some(1), Some(2)]  

whereas I'd expect to see something like it:[Some(0), None, None, None]

rustc 1.28.0-nightly (cb20f68 2018-05-21)

[ordovicia]

The map() yields Some(Some(0)), Some(None), Some(Some(1)), Some(Some(2)), followed by None.
The iterator is not fused by Some(None).

fn main() {
    let mut iter = ["0", "a", "1", "2"].iter().map(|x| x.parse::<i32>().ok());

    for _ in 0..5 {
        println!("{:?}", iter.next());
    }
    // => Some(Some(0)), Some(None), Some(Some(1)), Some(Some(2)), None
}

The fuse() method is used for iterator that may return Some after it returns None, claiming that it has nothing to yield.
E.g. Incoming network buffers.

[hellow554]

Somewhat related to #50225 ?

[Manishearth]

Yeah, this is behaving as expected. fuse is about iterators that may "start" again after finishing, not about iterators over an option.

[pacman82]

This is how fused is supposed to behave. it.next() in this example is returning Option<Option<i32>>. With the outer Optionindicating if the iterator has reached its end, and the inner Option indicating if parsing has been successful. The inner Optionis part of the element type iterated over and it being None should not cause fuse to do anything.