Add support for `async`?

[GuillaumeGomez]

In docs.rs, we have some big pages we currently split in pieces so the async is not too much slowed down. However, if askama was async (either through a feature or by default, although I'd prefer through a feature to not have too many dependencies by default) by adding new async_render and async_render_into methods, we could then put back together the pieces into a big one and let async handle it.

What do you think @Kijewski ?

[Kijewski]

I am not sure if I understand you correctly. With the feature enabled, the templates should implement something like TryStream<Ok = String, Error = askama::Error>? Or ↓?

trait Template {
    async fn async_render_into<D>(dest: &mut D) -> Result<(), askama::Error>
    where
        D: Sink<String, Error: Into<askama::Error>> + ?Sized;
}

I think both would be viable options, and could be turned into one-another with not too much work. But implementing either could mean quite some work. I think this might be a useful feature, though!

[syphar]

I feel like TryStream would be the more usable approach?

[GuillaumeGomez]

Implementing TryStream seems better indeed.

[syphar]

btw, in any case a small stream adapter might be needed if you work with a webserver, because they expect Bytes. But much less work than bridging between render_into(Write) and the expected TryStream

[syphar]

Another thought:
you might also go more low-level, and use AyncRead / AsyncWrite, with ReaderStream you could easily convert it.

Though, I'm not sure if web frameworks rather work with Stream?