What problem does this solve or what need does it fill?

Currently scopes cannot be nested, which means tasks spawned in a scope cannot spawn more tasks.

What solution would you like?

Modify Scope definition (for wasm and non-wasm) to use interior mutability.
That way many spawn-able scopes can be passed deeper.

This in both current cases means wrapping the results vector with a thread-safe interior mutability type, like Mutex or RwLock.

What alternative(s) have you considered?

A alternative to scopes mentioned in #4287 could cover some of the nesting use cases.

Additional context

This solution was used in other crates such as crossbeam