How to write a Rust generator that returns an async_std stream?

Question

Let's say I have a trait IMyWriter that has a function flush_to_disk that gets an iterator of the pieces to write. Each piece that's written should yield a Result object.

use async_trait::async_trait;
use async_std::prelude::StreamExt;

#[async_trait]
trait IMyWriter<'a> {
   async fn flush_to_disk<'b, 'c: 'a + 'b, Iter, Results>(&mut self, pieces: Iter) -> Results 
    where
      Iter: IntoIterator<Item = u64>,
      Results: StreamExt<Item=Result<(), std::io::Error>>;
}

I'm having trouble with doing the mapping in the implementation:

use async_std::io::File;
use async_trait::async_trait;

struct MyWriter {
    file: Mutex<File>;
}

#[async_trait]
impl IMyWriter for MyWriter {
   async fn flush_to_disk<'b, 'c: 'a + 'b, Iter, Results>(&mut self, pieces: Iter) -> Results 
    where
      Iter: IntoIterator<Item = u64>,
      Results: StreamExt<Item=Result<(), std::io::Error>> {
      pieces.into_iter().map(|piece| async {
         let f = self.file.lock().await;
         // Do I/O on file
         Ok()
      })
   }
}

This obviously complains:

type parameter `Results`, found struct `std::iter::Map`

I can't for the life of me figure out how to return the results for the stream of asynchronous writes that happen.