Force Application::Message to implement Clone

A `Message` should represent an application event (e.g. user
interactions, command results, subscription results...). Therefore, it
should always consist of pure, cloneable data.

native/src/program.rs

@@ -11,7 +11,7 @@ pub trait Program: Sized {
     type Renderer: Renderer;
 
     /// The type of __messages__ your [`Program`] will produce.
-    type Message: std::fmt::Debug + Send;
+    type Message: std::fmt::Debug + Clone + Send;
 
     /// The type of [`Clipboard`] your [`Program`] will use.
     type Clipboard: Clipboard;

src/application.rs

@@ -99,7 +99,7 @@ pub trait Application: Sized {
     type Executor: Executor;
 
     /// The type of __messages__ your [`Application`] will produce.
-    type Message: std::fmt::Debug + Send;
+    type Message: std::fmt::Debug + Clone + Send;
 
     /// The data needed to initialize your [`Application`].
     type Flags;

src/sandbox.rs

@@ -88,7 +88,7 @@ use crate::{
 /// ```
 pub trait Sandbox {
     /// The type of __messages__ your [`Sandbox`] will produce.
-    type Message: std::fmt::Debug + Send;
+    type Message: std::fmt::Debug + Clone + Send;
 
     /// Initializes the [`Sandbox`].
     ///

winit/src/conversion.rs

@@ -223,7 +223,10 @@ pub fn menu<Message>(menu: &Menu<Message>) -> winit::window::Menu {
 pub fn menu_message<Message>(
     _menu: &Menu<Message>,
     id: isize,
-) -> Option<Message> {
+) -> Option<Message>
+where
+    Message: Clone,
+{
     println!("Menu entry activated: {}", id);
 
     None