Right now the only arguments to Future::poll are self and the task scheduler context. Any borrows of mutable state have to be captured by self, which means only one future with access to the state can be inflight at a time. If `&mut State` was an argument on resume, then the actor could schedule many futures simultaneously, which each getting temporary access to the state whenever it makes progress.

Actix does exactly that, but it needs a bespoke ActorFuture trait with extra poll arguments. But until https://lang-team.rust-lang.org/design_notes/general_corouti... gets worked through, there is no way for actix to support async/await.