the system() method is part of the IntoSystem trait that converts the function into a System object: https://github.com/bevyengine/bevy/blob/master/crates/bevy

the system() method is part of the IntoSystem trait that converts the function into a System object: https://github.com/bevyengine/bevy/blob/master/crates/bevy_ecs/src/system/into_system.rs#L193 https://github.com/bevyengine/bevy/blob/master/crates/bevy_e... There are macros below that implement that for Func types.