Trait fn returning impl Fn() cannot be used as a system when called from generic type

[benfrankel]

Bevy version

0.13.2

What you did

trait Foo {
    fn foo() -> impl Fn() {
        || {}
    }
}

enum ConcreteFoo {}

impl Foo for ConcreteFoo {}

fn frobnicate<GenericFoo: Foo>(app: &mut App) {
    app.add_systems(Update, <ConcreteFoo as Foo>::foo()); // Is a system
    app.add_systems(Update, <GenericFoo as Foo>::foo()); // Is not a system
}

What went wrong

The second foo() is not considered a system, so it doesn't compile.

Additional information

This looks like a Rust bug, but I have no idea what the source of the bug is (something to do with RPITIT?), and this affects my usage of Bevy, so I'm creating an issue here for now.

[benfrankel]

Workaround:

trait Foo {
    fn foo() -> BoxedSystem {
        Box::new(IntoSystem::into_system(|| {}))
    }
}

[benfrankel]

This bug applies to run conditions as well, but BoxedCondition is not a proper workaround because it doesn't impl Condition. You have to use .run_if_dyn instead of .run_if, but .run_if_dyn is a real pain to use because it doesn't return Self:

fn my_system() {}

fn always_box() -> BoxedCondition {
    Box::new(IntoSystem::into_system(|| true))
}

fn foo(app: &mut App) {
    app.add_systems(Update, my_system.run_if(always_box())); // Does not compile
    app.add_systems(Update, {
        let mut configs = my_system.into_configs();
        configs.run_if_dyn(always_box());
        configs
    }); // Does compile
}

[chescock]

I think you need fn foo() -> impl Fn() + Sync + Send + 'static {.

System requires those trait bounds, and for ConcreteFoo the compiler can see the actual type and validate them. But for GenericFoo it has to protect against something like

struct EvilFoo;
impl Foo for EvilFoo {
    fn foo() -> impl Fn() {
        let x = Cell::new(0);
        // This is not `Sync` and so cannot be a `System`.
        move || {
            x.set(0);
        }
    }
}

[benfrankel]

Interesting. That works. I guess this isn't a bug in Rust or Bevy, but it is surprising.