// ./plugins/foo/src/lib.rs
use host::*;
use host::Plugin;

#[derive(Debug, Default)]
pub struct Foo;

impl Plugin for Foo {
    fn name(&self) -> &'static str {
        "foo"
    }
}
declare_plugin!(Foo, Foo::default);

// ./host/src/main.rs
use libloading::{Library, Symbol};
mod plugins;
use plugins::*;

type Constructor = unsafe fn() -> *mut Plugin;
fn main() {
    let lib = Library::new("foo").unwrap();
    unsafe {
        let func: Symbol<Constructor> = lib.get(b"_plugin_create").unwrap();
        let raw = func();
        let plugin = Box::from_raw(raw);
        println!("7 + 42 = {}", plugin.name());
    }
}


// ./host/src/lib.rs
mod plugins;
pub use plugins::*;

// ./host/src/plugins.rs
#[macro_export]
macro_rules! declare_plugin {
    ($plugin_type:ty, $constructor:path) => {
        #[no_mangle]
        pub extern "C" fn _plugin_create() -> *mut $crate::Plugin {
            // make sure the constructor is the correct type.
            let constructor: fn() -> $plugin_type = $constructor;

            let object = constructor();
            let boxed: Box<$crate::Plugin> = Box::new(object);
            Box::into_raw(boxed)
        }
    };
}

pub trait Plugin {
    fn name(&self) -> &'static str;
}