Closure should impl Debug

[Pauan]

Motivation

When implementing rustwasm/gloo#30, I created this type:

pub struct EventListener<'a> {
    target: EventTarget,
    kind: &'a str,
    callback: Option<Closure<FnMut(Event)>>,
}

I would really like to just slap a #[derive(Debug)] and call it a day, but Closure doesn't impl Debug, so I have to do this instead:

impl<'a> std::fmt::Debug for EventListener<'a> {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        f.debug_struct("EventListener")
            .field("target", &self.target)
            .field("kind", &self.kind)
            .field("callback", &"Closure { ... }")
            .finish()
    }
}

Proposed Solution

I propose that Closure should provide a Debug impl, which essentially just outputs Closure { ... } (or similar).

Alternatives

Maybe the Debug impl should expose more information? For example, it could call .toString() on the JS function.

[fitzgen]

+1 for sure. I've done this everything-except-the-closure debug impl many times as well.

Maybe the Debug impl should expose more information? For example, it could call .toString() on the JS function.

Don't think we should do this -- the function's source won't be anything that the user authored, just wasm-bindgen's internally defined JS source for closures.