• The ability to create async tasks that can be safely given access to borrowed data, similar to crossbeam or rayon scopes
  • There are potentially multiple routes with which this can be accomplished

Design notes

Today's Future trait lacks one fundamental capability compared to synchronous code: there is no (known?) way to "block" your caller and be sure that the caller will not continue executing until you agree. In synchronous code, you can use a closure and a destructor to achieve this, which is the technique used for things like rayon::scope and crossbeam's scoped threads. In async code, because the Future trait has a safe poll function, it is always possible to poll it part way and then mem::forget (or otherwise leak) the value; this means that one cannot have parallel threads executing and using those references.

Async functions are commonly written with borrowed references as arguments:

fn main() {
async fn do_something(db: &Db) { ... }

but important utilities like spawn and spawn_blocking require 'static tasks. Without "unfogettable" traits, the only way to circumvent this is with mechanisms like FuturesUnordered, which is then subject to footguns as described in Barbara battles buffered streams.

There are two main approaches under consideration to address this issue: