Require `Clone` for `Message` early when needed

Prior to this change, the widgets that needed a `Clone` bound on `Message` to
implement the `Widget` trait could be created with a non-cloneable `Message`.

As a consequence, the compiler complained only when actually trying to use the
`Widget` trait. Normally, this happens when trying to `push` the widget in a
container or turn it into an `Element`.

Furthermore, the compiler error in this case does not mention `Message` nor the
`Clone` bound, but instead complains about a missing `From` implementation.
Thus, it can easily cause confusion!

This change introduces `Clone` bounds in the main implementation of the
widgets that need it to properly implement the `Widget` trait. As a
result, the compiler complains early when trying to create one of these widgets
with a non-cloneable `Message` and explicitly mentions that the `Message` needs
to implement `Clone`.

1 files changed, 4 insertions, 2 deletions

diff --git a/native/src/widget/button.rs b/native/src/widget/button.rs
index c932da2b..fd74563a 100644
--- a/native/src/widget/button.rs
+++ b/native/src/widget/button.rs
@@ -18,6 +18,7 @@ use std::hash::Hash;
 /// # type Button<'a, Message> =
 /// #     iced_native::Button<'a, Message, iced_native::renderer::Null>;
 /// #
+/// #[derive(Clone)]
 /// enum Message {
 ///     ButtonPressed,
 /// }
@@ -41,6 +42,7 @@ pub struct Button<'a, Message, Renderer: self::Renderer> {
 
 impl<'a, Message, Renderer> Button<'a, Message, Renderer>
 where
+    Message: Clone,
     Renderer: self::Renderer,
 {
     /// Creates a new [`Button`] with some local [`State`] and the given
@@ -142,8 +144,8 @@ impl State {
 impl<'a, Message, Renderer> Widget<Message, Renderer>
     for Button<'a, Message, Renderer>
 where
-    Renderer: self::Renderer,
     Message: Clone,
+    Renderer: self::Renderer,
 {
     fn width(&self) -> Length {
         self.width
@@ -272,8 +274,8 @@ pub trait Renderer: crate::Renderer + Sized {
 impl<'a, Message, Renderer> From<Button<'a, Message, Renderer>>
     for Element<'a, Message, Renderer>
 where
-    Renderer: 'a + self::Renderer,
     Message: 'a + Clone,
+    Renderer: 'a + self::Renderer,
 {
     fn from(
         button: Button<'a, Message, Renderer>,