Mark extensible enums in winit-core as #[non_exhaustive]

Event, input source, error, data transfer, native key, IME request,
scroll delta and fullscreen enums can now gain variants without a
breaking change. Backends and examples get wildcard arms where they
matched exhaustively; unknown IME requests are reported as
ImeRequestError::NotSupported and unknown fullscreen modes behave
like Fullscreen::Borderless(None).

winit-appkit/src/cursor.rs

@@ -34,7 +34,7 @@ impl CustomCursor {
     pub(crate) fn new(cursor: CustomCursorSource) -> Result<CustomCursor, RequestError> {
         let cursor = match cursor {
             CustomCursorSource::Image(cursor_image) => cursor_image,
-            CustomCursorSource::Animation { .. } | CustomCursorSource::Url { .. } => {
+            _ => {
                 return Err(NotSupportedError::new("unsupported cursor kind").into());
             },
         };

winit-appkit/src/dnd.rs

@@ -453,6 +453,7 @@ impl PasteboardWriterState {
             SendData::Uris(_) => None,
             SendData::String(string) => Some(NSString::from_str(&string).into()),
             SendData::Bytes(binary) => Some(NSData::from_vec(binary).into()),
+            _ => None,
         }
     }
 }

winit-appkit/src/event_loop.rs

@@ -248,6 +248,7 @@ impl RootActiveEventLoop for ActiveEventLoop {
                                     .chain(Vec::new().into_iter().map(ns_url_from_str)),
                             ),
                             SendData::Bytes(_) => None,
+                            _ => None,
                         }
                     })
                     .into_iter()

winit-appkit/src/window_delegate.rs

@@ -227,6 +227,7 @@ define_class!(
                 // in fullscreen, so we must've reached here by `set_fullscreen`
                 // as it updates the state
                 Some(Fullscreen::Borderless(_)) => (),
+                Some(_) => (),
                 // Otherwise, we must've reached fullscreen by the user clicking
                 // on the green fullscreen button. Update state!
                 None => {
@@ -667,6 +668,7 @@ fn new_window(
                 monitor.ns_screen(mtm).or_else(|| NSScreen::mainScreen(mtm))
             },
             Some(Fullscreen::Borderless(None)) => NSScreen::mainScreen(mtm),
+            Some(_) => NSScreen::mainScreen(mtm),
             None => None,
         };
         let frame = match &screen {
@@ -1683,7 +1685,7 @@ impl WindowDelegate {
                     let monitor = monitor.cast_ref::<MonitorHandle>().unwrap();
                     monitor.ns_screen(mtm)
                 },
-                Fullscreen::Borderless(None) => {
+                _ => {
                     if let Some(monitor) = self.current_monitor_inner() {
                         monitor.ns_screen(mtm)
                     } else {
@@ -1928,6 +1930,7 @@ impl WindowDelegate {
                 self.view().disable_ime();
                 return Ok(());
             },
+            _ => return Err(ImeRequestError::NotSupported),
         };
 
         if let Some((spot, size)) = request_data.cursor_area {