有很多功能

fn add_people(mut commands: Commands) {
    commands.spawn((Person, Name("A".to_string())));
    commands.spawn((Person, Name("B".to_string())));
    commands.spawn((Person, Name("C".to_string())));
    commands.spawn((Person, Name("D".to_string())));
}

fn greet_people(query: Query<&Name, With<Person>>) {
    for name in query.iter() {
        println!("hello {}!", name.0);
    }
}
fn hello_world() {
    println!("hello world");
}

这些是我想调用的函数
pub fn add_startup_system（&mut self，system：impl IntoSystemDescriptor）
pub fn add_system（&mut self，system：impl IntoSystemDescriptor）
像这样

App::new()
        .add_plugins(DefaultPlugins)
        .add_startup_system(hello_world)
        .add_startup_system(add_people)
        .add_system(greet_people)
        .run();

以上函数类型为fn（Commands）fn（Query<&Name，With>）fn（），均满足目标IntoSystemDescriptor

IntoSystemDescriptor的定义是

pub trait IntoSystemDescriptor<Params> {
    fn into_descriptor(self) -> SystemDescriptor;
    /// Assigns a run criteria to the system. Can be a new descriptor or a label of a
    /// run criteria defined elsewhere.
    fn with_run_criteria<Marker>(
        self,
        run_criteria: impl IntoRunCriteria<Marker>,
    ) -> SystemDescriptor;

    /// Assigns a label to the system; there can be more than one, and it doesn't have to be unique.
    fn label(self, label: impl SystemLabel) -> SystemDescriptor;

    /// Specifies that the system should run before systems with the given label.
    fn before<Marker>(self, label: impl AsSystemLabel<Marker>) -> SystemDescriptor;

    /// Specifies that the system should run after systems with the given label.
    fn after<Marker>(self, label: impl AsSystemLabel<Marker>) -> SystemDescriptor;

    /// Marks this system as ambiguous with any system with the specified label.
    /// This means that execution order between these systems does not matter,
    /// which allows [some warnings](crate::schedule::ReportExecutionOrderAmbiguities) to be silenced.
    fn ambiguous_with<Marker>(self, label: impl AsSystemLabel<Marker>) -> SystemDescriptor;

    /// Specifies that this system should opt out of
    /// [execution order ambiguity detection](crate::schedule::ReportExecutionOrderAmbiguities).
    fn ignore_all_ambiguities(self) -> SystemDescriptor;

    /// Specifies that the system should run with other exclusive systems at the start of stage.
    fn at_start(self) -> SystemDescriptor;

    /// Specifies that the system should run with other exclusive systems after the parallel
    /// systems and before command buffer application.
    fn before_commands(self) -> SystemDescriptor;

    /// Specifies that the system should run with other exclusive systems at the end of stage.
    fn at_end(self) -> SystemDescriptor;
}

很难理解为什么fn实现了trait
看起来他们两个没有任何关系
谁能解释一下为什么fn满足这个特性？
或者给予我一个关于它的铁 rust 特征的链接