Why does AsyncBufReadExt::fill_buf require Self: Unpin?

[Kixunil]

I'm writing an async function that takes a generic AsyncBufReader and repeatedly calls fill_buf, processes the bytes and then calls consume. I stumbled upon surprising Unpin requirement. Why is it needed? It looks to me it should be possible to call poll_fill_buf without unpinning. Is this some unfortunate API mistake or is there actual reason I'm missing?

Is it impossible to avoid Unpin so I should just require it in my generic function?

[taiki-e]

For soundness; see #2612 for more.

Using std::pin::pin, tokio::pin, Box::pin, etc. in your code may help you avoid adding Unpin requirements to your function.

[Darksonn]

It's because it takes &mut self rather than Pin<&mut Self>. You can still use it with !Unpin types by using Self = Pin<&mut T>.

[Kixunil]

It's because it takes &mut self rather than Pin<&mut Self>. You can still use it with !Unpin types by using Self = Pin<&mut T>.

This is what I was missing, thanks!