Coroutines are one means of implementing async; the way they are implemented in Rust is more like building a state machine out of the async function. It can still be mapped to coroutines, and there are probably crates that use async and macros to make coroutines usable on stable, but the effort to have a stable language feature is still ongoing, with https://lang-team.rust-lang.org/design_notes/general_coroutines.html giving the overview.

I know painfully little of ActivityPub, but from a protocol design PoV, Lemmy altering its behavior when it sees Mastodon doesn’t sound desirable. I’d rather put metadata in the toots that allow better decisions, or even have two entities (linked by metadata) for channel-top-posts and channel-everything, if the latter is needed at all.

Ah – things do indeed start popping up once I follow it, and in mastodon terminology, the community is boosting every single comment. I’ll try that for some time to see if it’s practical, but it’s probably overkill.

Even subscribing to the communities is a somewhat theoretical thing – I can see https://lemmyrs.org/c/meta as @meta and follow that, but none of its posts are shown. (Or do only new posts show up once someone on my server is following it?)