ExtensionTest.rs

use std::any::Any;
use std::ffi::CStr;
use std::os::raw::c_void;

// ###########################
// macros to make things easy.
// ###########################

macro_rules! make_interface {
    ($uri:expr, $interface:ty, $instance:expr) => {
        fn extension_uri() -> &'static CStr {
            const URI: &[u8] = b"urn:dummy-state#interface\0";
            unsafe { CStr::from_bytes_with_nul_unchecked(URI) }
        }

        const INTERFACE: StateInterface = StateInterface {
            save: Self::extern_save,
        };
    };
}

macro_rules! export_interface {
    ($uri:expr, $($extension:ident),*) => {
        $(
        if <Self as $extension>::extension_uri() == $uri {
            return Some(&<Self as $extension>::INTERFACE);
        }
        )*
    }
}

// #####################
// Extension definitions
// #####################

#[repr(C)]
pub struct StateInterface {
    save: unsafe extern "C" fn(*mut c_void) -> u32,
}

/// The extension.
pub trait State: Sized {
    /// To be implemented!
    fn save(&mut self) -> u32;

    /// Raw adapter for `save`.
    unsafe extern "C" fn extern_save(plugin: *mut c_void) -> u32 {
        let plugin = (plugin as *mut Self).as_mut().unwrap();
        plugin.save()
    }

    make_interface!(
        b"urn:dummy-state#interface\0",
        StateInterface,
        StateInterface {
            save: Self::extern_save,
        }
    );
}

// ###########
// Application
// ###########

/// Our plugin.
pub struct Plugin {
    state: u32,
}

/// The implementation.
impl State for Plugin {
    fn save(&mut self) -> u32 {
        self.state
    }
}

impl Plugin {
    pub fn extension_data(uri: &CStr) -> Option<&'static dyn Any> {
        // These three lines could be generated.
        export_interface![uri, State];
        None
    }
}

/// Handled by the library.
pub unsafe extern "C" fn extension_data(uri: *const i8) -> *const c_void {
    let uri = CStr::from_ptr(uri);
    match Plugin::extension_data(uri) {
        Some(extension_data) => extension_data as *const _ as *const c_void,
        None => std::ptr::null(),
    }
}