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/slider.rs b/native/src/widget/slider.rs
index c49053f1..3255dd04 100644
--- a/native/src/widget/slider.rs
+++ b/native/src/widget/slider.rs
@@ -26,6 +26,7 @@ use std::{hash::Hash, ops::RangeInclusive};
 /// # use iced_native::{slider, renderer::Null};
 /// #
 /// # pub type Slider<'a, T, Message> = iced_native::Slider<'a, T, Message, Null>;
+/// #[derive(Clone)]
 /// pub enum Message {
 ///     SliderChanged(f32),
 /// }
@@ -53,6 +54,7 @@ pub struct Slider<'a, T, Message, Renderer: self::Renderer> {
 impl<'a, T, Message, Renderer> Slider<'a, T, Message, Renderer>
 where
     T: Copy + From<u8> + std::cmp::PartialOrd,
+    Message: Clone,
     Renderer: self::Renderer,
 {
     /// Creates a new [`Slider`].
@@ -168,8 +170,8 @@ impl<'a, T, Message, Renderer> Widget<Message, Renderer>
     for Slider<'a, T, Message, Renderer>
 where
     T: Copy + Into<f64> + num_traits::FromPrimitive,
-    Renderer: self::Renderer,
     Message: Clone,
+    Renderer: self::Renderer,
 {
     fn width(&self) -> Length {
         self.width
@@ -322,8 +324,8 @@ impl<'a, T, Message, Renderer> From<Slider<'a, T, Message, Renderer>>
     for Element<'a, Message, Renderer>
 where
     T: 'a + Copy + Into<f64> + num_traits::FromPrimitive,
-    Renderer: 'a + self::Renderer,
     Message: 'a + Clone,
+    Renderer: 'a + self::Renderer,
 {
     fn from(
         slider: Slider<'a, T, Message, Renderer>,