What about the other way? There could be a trait that means “references have the same escape semantics as a stack frame”, perhaps called NotLeaky, and then a variant of tokio spawn could require NotLeaky, and return a non-‘static future. NotLeaky could be inferred in the same way as Send and Sync.

As a bonus, high-availability systems could require NotLeaky at the top of their event loop, precluding runtime memory leaks.

Edit: that wouldn’t work, since the future could be leaked by the caller… will read that reference.