Change Executors to take &self

[seanmonstar]

The Executor and TypedExecutor traits both take &mut self in their spawn method. While this does make sense logically, it has this downside: you can't share an executor with an Arc (for example, an Arc<dyn Executor>). You can put the executor inside a Mutex, however most executors already can handle synchronization internally, frequently with some lock-free queue.

It might be a good idea to switch the spawn function to only take &self.

An alternative option is to leave them as &mut self, and implement Executor for &T for those that already have internal synchronization.

[LucioFranco]

@seanmonstar is there benefit to having them take &self over just having a cheap clone internally?

[seanmonstar]

The difficulty I ran into was when using it as a trait object, because then you can't make use of a cheap clone.

[LucioFranco]

Ah that makes a lot of sense, I think that is a good idea then!

[carllerche]

@seanmonstar the trait obj argument is solid. I am 👍

[hawkw]

I think this issue is no longer relevant, as the Executor and TypedExecutor traits no longer exist.