Code shared from the Rust Playground

playground.rs

pub struct MyIntersperse<S> {
    source: S,
    interspersed: bool,
}

struct MyIntersperseProject<'a, S> {
    source: Pin<&'a mut S>,
    interspersed: &'a mut bool,
}

impl<S: Stream<Item = String>> MyIntersperse<S> {
    pub fn new(source: S) -> MyIntersperse<S> {
        MyIntersperse {
            source: Box::pin(source),
            interspersed: false,
        }
    }
    
    fn project(self: Pin<&mut Self>) -> MyIntersperseProject<'_, S> {
        let this = unsafe { Pin::into_inner_unchecked(self) };
        MyIntersperseProject {
            source: unsafe { Pin::new_unchecked(&mut this.source) },
            interspersed: &mut this.interspersed,
        }
    }
}

impl<S: Stream<Item = String>> Stream for MyIntersperse<S> {
    type Item = String;

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        let this = self.project();
        
        if !this.interspersed {
            this.interspersed = true;
            Poll::Ready(Some(String::from("---")))
        } else {
            match this.source.poll_next(cx) {
                Poll::Pending => Poll::Pending,
                Poll::Ready(None) => Poll::Ready(None),
                Poll::Ready(Some(s)) => {
                    this.interspersed = false;
                    Poll::Ready(Some(s))
                }
            }
        }
    }
}