Properly implement Debug for Window and EventLoop types (#3297)

For EventLoop, EventLoopBuilder, EventLoopProxy and by requiring it as
a supertrait of Window and ActiveEventLoop.

It is especially useful for user to be able to know that Window is Debug.

src/platform_impl/web/event_loop/mod.rs

@@ -11,6 +11,7 @@ mod window_target;
 
 pub(crate) use window_target::ActiveEventLoop;
 
+#[derive(Debug)]
 pub struct EventLoop {
     elw: ActiveEventLoop,
 }

src/platform_impl/web/event_loop/proxy.rs

@@ -8,8 +8,10 @@ use crate::event_loop::EventLoopProxyProvider;
 use crate::platform_impl::web::event_loop::runner::WeakShared;
 use crate::platform_impl::web::r#async::{AtomicWaker, Wrapper};
 
+#[derive(Debug)]
 pub struct EventLoopProxy(Wrapper<WeakShared, Arc<State>, ()>);
 
+#[derive(Debug)]
 struct State {
     awoken: AtomicBool,
     waker: AtomicWaker,

src/platform_impl/web/event_loop/runner.rs

@@ -1,9 +1,9 @@
 use std::cell::{Cell, RefCell};
 use std::collections::{HashSet, VecDeque};
-use std::iter;
 use std::ops::Deref;
 use std::rc::{Rc, Weak};
 use std::sync::Arc;
+use std::{fmt, iter};
 
 use wasm_bindgen::prelude::Closure;
 use wasm_bindgen::JsCast;
@@ -26,6 +26,7 @@ use crate::platform_impl::platform::r#async::DispatchRunner;
 use crate::platform_impl::platform::window::Inner;
 use crate::window::WindowId;
 
+#[derive(Debug)]
 pub struct Shared(Rc<Execution>);
 
 impl Clone for Shared {
@@ -68,6 +69,12 @@ struct Execution {
     on_visibility_change: OnEventHandle<web_sys::Event>,
 }
 
+impl fmt::Debug for Execution {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Execution").finish_non_exhaustive()
+    }
+}
+
 enum RunnerEnum {
     /// The `EventLoop` is created but not being run.
     Pending,
@@ -96,6 +103,16 @@ struct Runner {
     event_loop: ActiveEventLoop,
 }
 
+impl fmt::Debug for Runner {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Runner")
+            .field("state", &self.state)
+            .field("app", &"<ApplicationHandler>")
+            .field("event_loop", &self.event_loop)
+            .finish()
+    }
+}
+
 impl Runner {
     pub fn new(app: Box<dyn ApplicationHandler>, event_loop: ActiveEventLoop) -> Self {
         Runner { state: State::Init, app, event_loop }

src/platform_impl/web/event_loop/window_target.rs

@@ -25,7 +25,7 @@ use crate::platform_impl::web::event_loop::proxy::EventLoopProxy;
 use crate::platform_impl::Window;
 use crate::window::{CustomCursor as RootCustomCursor, CustomCursorSource, Theme, WindowId};
 
-#[derive(Default)]
+#[derive(Default, Debug)]
 struct ModifiersShared(Rc<Cell<ModifiersState>>);
 
 impl ModifiersShared {
@@ -44,7 +44,7 @@ impl Clone for ModifiersShared {
     }
 }
 
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 pub struct ActiveEventLoop {
     pub(crate) runner: runner::Shared,
     modifiers: ModifiersShared,