SystemParam as associated type bounds for use in a system.

[bilowik]

I've simplified my code down to the bare minimum to describe the problem. The goal is to remove a lot of boilerplate I have. I'm not sure what needs to be done in order for App::add_systems to take Trait::system with the given Trait::Assoc SystemParam (if even possible). Alternatively, maybe this looks like something a different pattern can handle?

use bevy::{
    prelude::*,
    ecs::system::SystemParam,
};

pub trait Trait {
    type Assoc: SystemParam + Send + Sync + 'static;

    fn system(query: Self::Assoc) {
        
    }
}

#[derive(Default)]
pub struct MyPlugin<T: Trait + Send + Sync>(std::marker::PhantomData<T>);

impl<T: Trait + Send + Sync + 'static> Plugin for MyPlugin<T> {
    fn build(&self, app: &mut App) {
        app.add_systems(Update, T::system);
    }
}


fn main() {}

error[E0277]: the trait bound `fn(<T as Trait>::Assoc) {<T as Trait>::system}: IntoSystem<(), (), _>` is not satisfied
   --> src/main.rs:19:33
    |
19  |         app.add_systems(Update, T::system);
    |             -----------         ^^^^^^^^^ the trait `IntoSystem<(), (), _>` is not implemented for fn item `fn(<T as Trait>::Assoc) {<T as Trait>::system}`, which is required by `fn(<T as Trait>::Assoc) {<T as Trait>::system}: IntoSystemConfigs<_>`
    |             |
    |             required by a bound introduced by this call
    |
    = help: the following other types implement trait `IntoSystemConfigs<Marker>`:
              <std::boxed::Box<(dyn bevy::prelude::System<In = (), Out = ()> + 'static)> as IntoSystemConfigs<()>>
              <NodeConfigs<std::boxed::Box<(dyn bevy::prelude::System<In = (), Out = ()> + 'static)>> as IntoSystemConfigs<()>>
              <(S0,) as IntoSystemConfigs<(SystemConfigTupleMarker, P0)>>
              <(S0, S1) as IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1)>>
              <(S0, S1, S2) as IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2)>>
              <(S0, S1, S2, S3) as IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2, P3)>>
              <(S0, S1, S2, S3, S4) as IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2, P3, P4)>>
              <(S0, S1, S2, S3, S4, S5) as IntoSystemConfigs<(SystemConfigTupleMarker, P0, P1, P2, P3, P4, P5)>>
            and 14 others
    = note: required for `fn(<T as Trait>::Assoc) {<T as Trait>::system}` to implement `IntoSystemConfigs<_>`
note: required by a bound in `bevy::prelude::App::add_systems`
   --> /home/aflores/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bevy_app-0.12.1/src/app.rs:409:23
    |
406 |     pub fn add_systems<M>(
    |            ----------- required by a bound in this associated function
...
409 |         systems: impl IntoSystemConfigs<M>,
    |                       ^^^^^^^^^^^^^^^^^^^^ required by this bound in `App::add_systems`

For more information about this error, try `rustc --explain E0277`.

[hymm]

You should use SystemParamItem for the system parameters type instead of SystemParam. I have a branch with an improved example here https://github.com/hymm/bevy/blob/3ab4f50cbf3082057df10b1d72accd55e83ae8e3/examples/ecs/generic_system.rs#L146

[GhostMinerPlus]

    use std::marker::PhantomData;

    use bevy::{
        app::{App, Plugin, Update},
        ecs::system::{SystemParam, SystemParamItem},
    };

    struct SomePlugin<S>(PhantomData<S>);

    unsafe impl<S> Send for SomePlugin<S> {}

    unsafe impl<S> Sync for SomePlugin<S> {}

    trait SomeTrait {}

    impl<S: SystemParam + 'static> Plugin for SomePlugin<S>
    where
        for<'w, 's> S::Item<'w, 's>: SomeTrait,
    {
        fn build(&self, app: &mut App) {
            app.add_systems(Update, some_sys::<S>);
        }
    }

    fn some_sys<S: SystemParam>(s: SystemParamItem<S>)
    where
        for<'w, 's> S::Item<'w, 's>: SomeTrait,
    {
    }

This seems not work. I do this, cause I don't want some_sys::<S> added duplicately