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-win32/src/dnd.rs

@@ -247,6 +247,7 @@ impl TypedData for WinTypedData {
             },
             // Windows URI drag-and-drop can't be neatly expressed as a binary blob.
             SendData::Uris(_) => None,
+            _ => None,
         }
     }
 
@@ -924,6 +925,7 @@ unsafe fn send_data_to_stgmedium(data: SendData, hint: TypeHint) -> Option<STGME
             hglobal
         },
         SendData::Bytes(b) => alloc_hglobal_from(&b)?,
+        _ => return None,
     };
 
     let mut medium = unsafe { std::mem::zeroed::<STGMEDIUM>() };

winit-win32/src/event_loop.rs

@@ -431,7 +431,7 @@ impl RootActiveEventLoop for ActiveEventLoop {
     ) -> Result<CustomCursor, RequestError> {
         let cursor = match source {
             CustomCursorSource::Image(cursor) => cursor,
-            CustomCursorSource::Animation { .. } | CustomCursorSource::Url { .. } => {
+            _ => {
                 return Err(NotSupportedError::new("unsupported cursor kind").into());
             },
         };
@@ -1397,6 +1397,7 @@ unsafe fn public_window_callback_inner(
                                 window_pos.cy = old_monitor_rect.bottom - old_monitor_rect.top;
                             }
                         },
+                        _ => (),
                     }
                 }
             }

winit-win32/src/window.rs

@@ -967,7 +967,7 @@ impl CoreWindow for Window {
                         | Fullscreen::Borderless(Some(monitor)) => {
                             Some(Cow::Borrowed(monitor.cast_ref::<MonitorHandle>().unwrap()))
                         },
-                        Fullscreen::Borderless(None) => None,
+                        _ => None,
                     };
 
                     let monitor = monitor
@@ -1068,7 +1068,8 @@ impl CoreWindow for Window {
         match &request {
             ImeRequest::Enable(..) if cap.is_some() => return Err(ImeRequestError::AlreadyEnabled),
             ImeRequest::Update(_) if cap.is_none() => return Err(ImeRequestError::NotEnabled),
-            _ => (),
+            ImeRequest::Enable(..) | ImeRequest::Update(_) | ImeRequest::Disable => (),
+            _ => return Err(ImeRequestError::NotSupported),
         }
 
         let window = self.window;
@@ -1096,6 +1097,7 @@ impl CoreWindow for Window {
                     ImeContext::set_ime_allowed(window.hwnd(), false);
                     return;
                 },
+                _ => return,
             };
 
             if let Some((spot, size)) = request_data.cursor_area {