The iced package requires the messages to implement the Debug, Clone, and Send traits, none of which are available for closures. I was able to implement somethi

The iced package requires the messages to implement the Debug, Clone, and Send traits, none of which are available for closures. I was able to implement something similar to your example[0] but it only supports plain function pointers for the callbacks. The compiler wasn't able to derive a sufficiently general Debug trait for the function pointer due to an issue with the lifetime of the argument, so I had to implement that myself as well. [Edit: Ignore this next paragraph; as steveklabnik pointed out, this change has already been implemented and "Message(|c| c.value += 1)" is accepted by the latest stable version.] Incidentally, as long as there are situations where only function pointers can be used and not closures it would be really nice to have some support for anonymous function pointers in Rust (with the fn type and not just the Fn trait) so that one could write e.g. "Message(fn |c| c.value += 1)" instead of "Message({ fn f(c: &mut Counter) { c.value += 1 } f })". Or just infer the fn type for "closures" which don't actually close over any variables without the need for an extra keyword. [0] https://github.com/nybble41/iced-counter2/blob/master/src/main.rs https://github.com/nybble41/iced-counter2/blob/master/src/ma...