feat: implement Body::size_hint for MapFrame

[zino23]

Fix seanmonstar/reqwest#2497.

[seanmonstar]

A problem is that map_frame() can change the frames, perhaps adding or removing from the data, so the size hint is no longer accurate.

[zino23]

A problem is that map_frame() can change the frames, perhaps adding or removing from the data, so the size hint is no longer accurate.

Yes indeed. One rational for this implementation, though, is the construction of a MapFrame does not change size of the body immediately. The transformation of the frame occurs when the body is poll_frame-ed.

impl<B, F, B2> Body for MapFrame<B, F>
where
    B: Body,
    F: FnMut(Frame<B::Data>) -> Frame<B2>,
    B2: Buf,
{
    type Data = B2;
    type Error = B::Error;

    fn poll_frame(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Result<Frame<Self::Data>, Self::Error>>> {
        let this = self.project();
        match this.inner.poll_frame(cx) {
            Poll::Pending => Poll::Pending,
            Poll::Ready(None) => Poll::Ready(None),
            Poll::Ready(Some(Ok(frame))) => Poll::Ready(Some(Ok((this.f)(frame)))), // <- Transformation of the frame.
            Poll::Ready(Some(Err(err))) => Poll::Ready(Some(Err(err))),
        }
    }
}

So whether to relay the size hint of a MapFrame to the inner body depends on how we perceive the size hint. If it means the size hint before transformation, then this implementation is ok. If it means the real size hint after transformation, then there is no way to know it and we simply return SizeHint::default().

[seanmonstar]

Right, and there's no way for us to know what the closure is doing, so we have to toss the hint.

[zino23]

So I guess I'll close this PR?

Also you can close this as it is as expected: seanmonstar/reqwest#2497