Support rust's AsyncFn types for EventHandlers

[musjj]

Problem

Steps To Reproduce

use dioxus::prelude::*;

#[server]
async fn save_dog(image: String) -> Result<(), ServerFnError> {
    Ok(())
}

#[component]
fn DogView() -> Element {
    let src = use_signal(|| "https://images.dog.ceo/breeds/pitbull/dog-3981540_1280.jpg");
    let save = async move |_| {
        save_dog(src.cloned().to_owned()).await.unwrap();
    };

    rsx! {
        img { src }
        div {
            button { onclick: save, "save!" }
        }
    }
}

Error:

error: async closure does not implement `FnMut` because it captures state from its environment
   --> src/main.rs:104:16
    |
104 |     let save = async move |_| {
    |                ^^^^^^^^^^^^^^
...
113 |             button { onclick: save, "save!" }
    |                      ------------- required by a bound introduced by this call
    |
    = note: required for `Callback<dioxus::prelude::Event<MouseData>>` to implement `SuperFrom<{async closure@src/main.rs:104:16: 104:30}, MarkerWrapper<dioxus::dioxus_core::events::AsyncMarker>>`
    = note: required for `{async closure@src/main.rs:104:16: 104:30}` to implement `SuperInto<Callback<dioxus::prelude::Event<MouseData>>, MarkerWrapper<dioxus::dioxus_core::events::AsyncMarker>>`

But it works when you use an inner async block instead:

-    let save = async move |_| {
+    let save = move |_| async move {
         save_dog(src.cloned().to_owned()).await.unwrap();
     };

Expected behavior

Async closures has been stabilized, so it should be expected that it works with Dioxus.

Screenshots

Environment:

• Dioxus version: v0.6.0
• Rust version: v1.85.0-nightly (45d11e51b 2025-01-01)
• OS info: Linux
• App platform: web

Questionnaire

[ealmloff]

Async closures are still unstable and can only be used in the nightly edition of rust. Dioxus could accept async closures, but all of the async closure logic would need to be gated under the nightly version of rust

[ifsheldon]

Now that Rust 1.85 and Edition 2024 have been released with async closures stabilized. I think it's time to revisit this.