Please provide an easy way to delegate Executor

[joshtriplett]

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Description

I've found it useful to have a wrapper around a Pool, to distinguish between specific kinds of database connections (e.g. a user DB vs a global DB). I'd find my wrappers even more convenient to use if they implemented Executor. However, delegating Executor correctly requires implementing many methods with complex signatures (including a hidden method).

Prefered solution

I'd love to have some convenient way to delegate Executor to a field. I can see two straightforward ways to do this:

• A derive(Executor) macro, which has an #[executor] attribute identifying the field to delegate to.
• Something like an IntoExecutor trait, to be accepted by methods like execute instead of Executor, which has a single method that returns the trait. This would be simpler in some ways, but requires changing the signature of those methods. That should be a compatible change and shouldn't break any existing code.

Is this a breaking change? Why or why not?

The derive(Executor) approach is not breaking.

The IntoExecutor approach requires an API change but that API change should be compatible with existing code, and is unlikely to be breaking.