Where to get num_bytes_read from, when using new AsyncRead::poll_read?

[Urhengulas]

Hello, it's me again 😸

If one was previously using the num_bytes_read of the return of AsyncRead::poll_read: Where can one get this information from now, since it isn't returned anymore?

Actually the docs still says it would be returned, but as far as I get the function signature it is not... (trait.AsyncRead.poll_read).

---

Corresponding code: https://github.com/snapview/tokio-tungstenite/blob/master/src/compat.rs#L142-L160

[Urhengulas]

Is it possible to just call ReadBud::remaining() -> usize before and after the call to AsyncRead::poll_read, onto the ReadBuf passed to it to get the num_bytes_read?

fn run(a: impl AsyncRead, b: &mut [u8], c: &mut Context) -> usize {
    let mut read_buf = ReadBuf::new(b);
    let before = read_buf.remaining();
    a.poll_read(c, &mut read_buf);
    let num_bytes_read = before - read_buf.remaining();
    num_bytes_read
}

Should this work or am I missing something?

[Darksonn]

Since you created the ReadBuf, you know that it starts with filled() empty. Thus, the number of bytes can be obtained with read_buf.filled().len() after calling poll_read. You may enjoy this snippet:

tokio/tokio-util/src/io/poll_read_buf.rs

Lines 59 to 63 in 8d17261

	let n = {
	let mut buf = ReadBuf::uninit(buf.bytes_mut());
	ready!(read.poll_read(cx, &mut buf)?);
	buf.filled().len()
	};

[Urhengulas]

But isn't the ReadBuf already filled, if it is created from a non-empty &mut [u8]?

[Darksonn]

No, when created from an &mut [u8] it will have filled() empty. The thing that isn't empty will be initialized().

[Urhengulas]

Ahh, I see. Thank you very much :D