Timer::reset can't soundly have an optimized implementation

[sfackler]

In 1.0.0-rc.1, the new Timer trait has a method to reset an existing Sleep to a new deadline. The default implementation simply creates a new Sleep and writes it over the old one, but presumably this method exists to it can be overridden with a better implementation when working with a timer like Tokio's that can have its deadline dynamically adjusted.

However, since the Sleep is provided as a dyn Sleep and Sleep has no downcasting APIs, there is no safe way to confirm that the Sleep instance is one created by this Timer implementation. That effectively means that the current default implementation is the only sound way of implementing the method.

[seanmonstar]

Sigh, yea, I forgot about this. And Rust still hasn't made this any easier; we can't just say trait Sleep: Any. So, let's see, I think these are the options:

• Copy the methods from Any into Sleep, which allows checking type_id and then calling downcast_mut. To make it safe, it probably needs to implement those methods in a blanket impl. Do we just impl<F: Future> Sleep for F {}?
• Maybe we can move the method to trait Sleep { fn reset(self: Pin<&mut Self>, timer: &dyn Timer) {}? That might just require having downcast methods on Timer instead...
• Ugh.

[sfackler]

Sleep: Any won't handle it, but you can manually add downcasting to the Sleep trait.