rust-portaudio | PortAudio bindings and wrappers for Rust | Wrapper library

1. Why was Rust able to allow me to use SineGenerator from main(), but won't let me use Combine which takes the same objects (with, I assume, the same lifetimes)?

A 'static bound on a type parameter means that the type may not borrow anything that lives shorter than 'static. Your SineGenerator doesn't borrow anything, so it respects that bound. Combine, on the other hand, doesn't respect the bound because it contains borrowed pointers, and you instantiate it with a lifetime that is shorter than 'static by storing references to local variables in the Combine object.

1. Is there a better way to implement Combine that will allow me to do what I want to do here? I had to take references because Trait types don't have defined size at compile-time.

The typical solution is to use type parameters instead of trait objects.