Clarify the use of PluginRegistrar as a trait object

[seatonullberg]

Thank you for making this tutorial, it has been a huge help for me!

My only point of confusion has been with the use of PluginRegistrar as a trait object in random/src/lib.rs:

extern "C" fn register(registrar: &mut dyn PluginRegistrar) {
    registrar.register_function("random", Box::new(Random));
}

Then trait PluginRegistrar is implemented for struct PluginRegistrar in app/src/main.rs:

impl plugins_core::PluginRegistrar for PluginRegistrar {
    // --snip--
}

And that is the only implementation.

It seems to me like it is unnecessary to use a trait object here when the struct PluginRegistrar could be used directly.
This would require that the core module implements and exports both struct FunctionProxy and struct PluginRegistrar to be used in app. Am I missing something that prevents this or is it a matter of preference?

[StevenLu2004]

@Michael-F-Bryan Thank you so much for making this tutorial!

@seatonullberg I think it comes down to two things. First, having a trait PluginRegistrar allows for alternative implementations that depend on the app. Second, struct PluginRegistrar has a field lib: Rc<Library> that gets cloned to a FunctionProxy for every registered function, which means it depends on the libloading crate, and including the struct directly in plugins_core will add an unnecessary dependency of libloading to every plugin using plugins_core.