Set conditional default backend

[fxsalazar]

No description provided.

[eladyn]

Thank you! This is something I wanted to do for quite some time now.

While this solution is already much better than what has been there before, maybe we could go for an even "smarter" default backend selection logic. I'm thinking of something like this:

const fn default_backend() -> Backend {
  #[cfg(all(feature = "portaudio_backend", target_os="macos")]
  return Backend::Portaudio;
  #[cfg(feature = "alsa_backend")]
  return Bacend::Alsa;
  #[cfg(feature = "portaudio_backend")]
  return Backend::Portaudio;

  todo!("other backends");
  
  #[cfg(not(any(feature = "pulseaudio_backend", feature = "portaudio_backend", ...)))]
  compile_error!("Please enable at least one backend!");
}

This would prevent things like this:

The application panicked (crashed).
Message:  Unknown backend: alsa.
Location: src/setup.rs:169

Although this code is only a rough sketch and quite verbose / ugly.

If you don't want to look into this further, that would be fine as well, and we can just merge this version! :)

[fxsalazar]

Yes, sure, I'll take a look to this approach.

[fxsalazar]

I decided to go with the BACKEND_VALUES which is already aggregating the features values. 🤔 does it make sense?

[eladyn]

Oh, right. That seems like a good idea! Apart from a small comment, LGTM!

[eladyn]

Thank you!

[slondr]

cool!