implementing-async-read.md

Implementing AsyncRead

AsyncRead is being used here as a "stand-in" for some widely used trait that appears in the standard library. The details of the trait are not important.

Self is send

In this scenario, the Self type being used to implement is sendable, but the actual future that is created is not.

struct MySocketBuddy {
    x: u32
}

impl AsyncRead for MySocketBuddy {
    async fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        let ssh_key = Rc::new(vec![....]);
        do_some_stuff(ssh_key.clone());
        something_else(self.x).await;
    }
    // ERROR: `ssh_key` is live over an await;
    //        Self implements Send
    //        therefore resulting future must implement Send
}

Self is not send

In this scenario, the Self type being used to implement is not sendable.

struct MySocketBuddy {
    x: u32,
    ssh_key: Rc<Vec<u8>>,
}

impl AsyncRead for MySocketBuddy {
    async fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        do_some_stuff(self.ssh_key.clone());
        something_else(self.x).await;
    }
    // OK
}