Draft time-travel debugging feature

program/Cargo.toml

@@ -10,9 +10,12 @@ categories.workspace = true
 keywords.workspace = true
 rust-version.workspace = true
 
+[lints]
+workspace = true
+
+[features]
+time-travel = []
+
 [dependencies]
 iced_graphics.workspace = true
 iced_runtime.workspace = true
-
-[lints]
-workspace = true

program/src/lib.rs

@@ -22,7 +22,7 @@ pub trait Program: Sized {
     type State;
 
     /// The message of the program.
-    type Message: Send + std::fmt::Debug + 'static;
+    type Message: Message + 'static;
 
     /// The theme of the program.
     type Theme: Default + theme::Base;
@@ -642,3 +642,17 @@ impl<P: Program> Instance<P> {
         self.program.scale_factor(&self.state, window)
     }
 }
+
+/// A trait alias for the [`Message`](Program::Message) of a [`Program`].
+#[cfg(feature = "time-travel")]
+pub trait Message: Send + std::fmt::Debug + Clone {}
+
+#[cfg(feature = "time-travel")]
+impl<T: Send + std::fmt::Debug + Clone> Message for T {}
+
+/// A trait alias for the [`Message`](Program::Message) of a [`Program`].
+#[cfg(not(feature = "time-travel"))]
+pub trait Message: Send + std::fmt::Debug {}
+
+#[cfg(not(feature = "time-travel"))]
+impl<T: Send + std::fmt::Debug> Message for T {}