event: Unwrap user event in map_nonuser_event Err()

Currently separating an Event<T> into a user event T and Event<()> is
not ergonomic: when map_nonuser_event returns Err() the caller "knows"
this can only contain UserEvent(T), yet has to write additional
unpacking logic to extract the event, with an unnecessary
unreachable!().

Solve this by returning T directly in the Err case, instead of
Event<'a, T>.

CHANGELOG.md

@@ -1,6 +1,6 @@
 # Unreleased
 
-- On Android, calling `WindowEvent::Focused` now works properly instead of always returning false. 
+- On Android, calling `WindowEvent::Focused` now works properly instead of always returning false.
 - On Windows, fix alt-tab behaviour by removing borderless fullscreen "always on top" flag.
 - On Windows, fix bug preventing windows with transparency enabled from having fully-opaque regions.
 - **Breaking:** On Windows, include prefix byte in scancodes.
@@ -10,6 +10,7 @@
 - On Web, fix `WindowEvent::ReceivedCharacter` never being sent on key input.
 - On macOS, fix compilation when targeting aarch64
 - On X11, fix `Window::request_redraw` not waking the event loop.
+- **Breaking:** `map_nonuser_event` now returns the unwrapped user event `T` in `Err()`.
 
 # 0.23.0 (2020-10-02)
 

src/event.rs

@@ -143,10 +143,10 @@ impl<T: Clone> Clone for Event<'static, T> {
 }
 
 impl<'a, T> Event<'a, T> {
-    pub fn map_nonuser_event<U>(self) -> Result<Event<'a, U>, Event<'a, T>> {
+    pub fn map_nonuser_event<U>(self) -> Result<Event<'a, U>, T> {
         use self::Event::*;
         match self {
-            UserEvent(_) => Err(self),
+            UserEvent(user_event) => Err(user_event),
             WindowEvent { window_id, event } => Ok(WindowEvent { window_id, event }),
             DeviceEvent { device_id, event } => Ok(DeviceEvent { device_id, event }),
             NewEvents(cause) => Ok(NewEvents(cause)),