Yes, in fact, you cannot even use async Rust without writing your own executor or bringing one in via a library. It is very, very much opt in. That was a hard constraint on the design.
However, I think what the parent is getting at is the feeling of the total package, not the technical details. If every library you want to use is async, you can't really "opt out" exactly, even if technically the feature is opt out.
However, I think what the parent is getting at is the feeling of the total package, not the technical details. If every library you want to use is async, you can't really "opt out" exactly, even if technically the feature is opt out.