async equivalent of std::io::LineWriter?

[anp]

AsyncBufReadExt::lines exists but there's no async+line-buffered generic writer available AFAICT.

In scope?

[taiki-e]

I would accept a PR to add this. (I've previously tried this a bit, but I remember finding it difficult.)

[FelipeLema]

hi, there

I was trying to address this by adapting std::io::LineWriter and its LineWriterShime sidekick, but I'm having trouble handling Pins and references.

Particularly, I'm not understanding how a WIP-futures_util::io::LineWriterShim can receive WIP-futures_util::io::LineWriter 's buffer writer.

If, instead, I try to merge LineWriterShim into LineWriter, then I have trouble with newly-added-BufWriter::write_to_buffer_unchecked()

Perhaps I should do a draft PR instead 🤔

[FelipeLema]

let me re-phrase the above

I'm not certain of how to "balance out" refs and pinning.

For doing behavioural stuff, all is present in std::io::LineWriter, but it's all doing mutable refs (specifically, writing directly to BufWriter's internal buffer). For doing async stuff, all is done with Pin::new, #[pin] and such.

Both pin-ey and ref-ey code don't play well with each other.

For example, how would I write a poll_write that may write to a BufWriter's internal buffer? Right now, rustc is complaining that I cannot declare flush_if_completed_line() to receive self as Pin<&mut Self or as &mut Self (not with the same error, of course)

[taiki-e]

For example, how would I write a poll_write that may write to a BufWriter's internal buffer? Right now, rustc is complaining that I cannot declare flush_if_completed_line() to receive self as Pin<&mut Self or as &mut Self (not with the same error, of course)

In #2477, you need to always use Pin<&mut Self> instead of &mut self.