Add exit code to ControlFlow::Exit (#2100)

* Add exit code to control flow and impl on linux

* Fix examples to have an exit code

* Fix doc examples to use an exit code

* Improve documentation wording on the exit code

* Add exit code example

* Add exit code on windows

* Change i32 as exit code to u8

This avoids nasty surprises with negative numbers on some unix-alikes
due to two's complement.

* Fix android usages of ControlFlow::Exit

* Fix ios usages of ControlFlow::Exit

* Fix web usages of ControlFlow::Exit

* Add macos exit code

* Add changelog note

* Document exit code on display server disconnection

* Revert "Change i32 as exit code to u8"

This reverts commit f88fba0253b45de6a2ac0c3cbcf01f50503c9396.

* Change Exit to ExitWithCode and make an Exit const

* Revert "Add exit code example"

This reverts commit fbd3d03de9c2d7516c7a63da489c99f498b710df.

* Revert "Fix doc examples to use an exit code"

This reverts commit daabcdf9ef9e16acad715c094ae442529e39fcbc.

* Revert "Fix examples to have an exit code"

This reverts commit 0df486896b8d106acf65ba83c45cc88d60d228e1.

* Fix unix-alike to use ExitWithCode instead of Exit

* Fix windows to use ExitWithCode rather than Exit

* Silence warning about non-uppercase Exit const

* Refactor exit code handling

* Fix macos Exit usage and recover original semantic

* Fix ios to use ExitWithCode instead of Exit

* Update documentation to reflect ExitWithCode

* Fix web to use ExitWithCode when needed, not Exit

* Fix android to use ExitWithCode, not Exit

* Apply documenation nits

* Apply even more documentation nits

* Move change in CHANGELOG.md under "Unreleased"

* Try to use OS error code as exit code on wayland

src/platform_impl/windows/event_loop.rs

@@ -188,11 +188,11 @@ impl<T: 'static> EventLoop<T> {
     where
         F: 'static + FnMut(Event<'_, T>, &RootELW<T>, &mut ControlFlow),
     {
-        self.run_return(event_handler);
-        ::std::process::exit(0);
+        let exit_code = self.run_return(event_handler);
+        ::std::process::exit(exit_code);
     }
 
-    pub fn run_return<F>(&mut self, mut event_handler: F)
+    pub fn run_return<F>(&mut self, mut event_handler: F) -> i32
     where
         F: FnMut(Event<'_, T>, &RootELW<T>, &mut ControlFlow),
     {
@@ -209,13 +209,13 @@ impl<T: 'static> EventLoop<T> {
 
         let runner = &self.window_target.p.runner_shared;
 
-        unsafe {
+        let exit_code = unsafe {
             let mut msg = mem::zeroed();
 
             runner.poll();
             'main: loop {
                 if 0 == winuser::GetMessageW(&mut msg, ptr::null_mut(), 0, 0) {
-                    break 'main;
+                    break 'main 0;
                 }
                 winuser::TranslateMessage(&msg);
                 winuser::DispatchMessageW(&msg);
@@ -225,16 +225,20 @@ impl<T: 'static> EventLoop<T> {
                     panic::resume_unwind(payload);
                 }
 
-                if runner.control_flow() == ControlFlow::Exit && !runner.handling_events() {
-                    break 'main;
+                if let ControlFlow::ExitWithCode(code) = runner.control_flow() {
+                    if !runner.handling_events() {
+                        break 'main code;
+                    }
                 }
             }
-        }
+        };
 
         unsafe {
             runner.loop_destroyed();
         }
+
         runner.reset_runner();
+        exit_code
     }
 
     pub fn create_proxy(&self) -> EventLoopProxy<T> {
@@ -763,7 +767,7 @@ unsafe fn process_control_flow<T: 'static>(runner: &EventLoopRunner<T>) {
                 Box::into_raw(WaitUntilInstantBox::new(until)) as LPARAM,
             );
         }
-        ControlFlow::Exit => (),
+        ControlFlow::ExitWithCode(_) => (),
     }
 }