Observers, systems and commands that use closures and panic require explicit return type after Rust 2024 edition

[Shatur]

Bevy version

v0.16.0-rc.3

[Optional] Relevant system information

Rust 1.86.0, 2024 edition.

What you did

fn test() {
    let mut app = App::new();
    app.add_observer(|_: Trigger<TestEvent>| {
        panic!();
    });
}

#[derive(Event)]
struct TestEvent;

What went wrong

The code no longer compiles, though it worked in Bevy 0.15.0 (which used the 2021 edition).

It also compiles when I explicitly specify the return type ():

fn test() {
    let mut app = App::new();
    app.add_observer(|_: Trigger<TestEvent>| -> () {
        panic!();
    });
}

#[derive(Event)]
struct TestEvent;

[alice-i-cecile]

As discussed on Discord with @NthTensor, this is due to Rust 2024, and is not our bug. It's arguably not a bug at all!

See https://doc.rust-lang.org/edition-guide/rust-2024/never-type-fallback.html for the context on this change, which recommends the same workaround of explicitly specifying the return type. The same code will compile just fine as functions rather than closures, because the return type of functions is never inferred (even if it's defaulted to ()).