最近在看《Asynchronous Programming in Rust》中第2.3章《Applied: Build an Executor》里面关于Spawner的实现：

impl Spawner {
    fn spawn(&self, future: impl Future<Output = ()> + 'static + Send) {
        let future = future.boxed();
        let task = Arc::new(Task {
            future: Mutex::new(Some(future)),
            task_sender: self.task_sender.clone(),
        });
        self.task_sender.send(task).expect("too many tasks queued");
    }
}

use {
    futures::{
        future::{BoxFuture, FutureExt},
        task::{waker_ref, ArcWake},
    },
    std::{
        future::Future,
        sync::mpsc::{sync_channel, Receiver, SyncSender},
        sync::{Arc, Mutex},
        task::{Context, Poll},
        time::Duration,
    },
    // The timer we wrote in the previous section:
    timer_future::TimerFuture,
};

pub trait FutureExt: Future {
...

Spawner 实现的第三行future.boxed();明明future只是实现Future的类型，但boxed方法却是futures_util::future::future::FutureExt的方法，请问为什么程序还能运行，编译器也不会报错？

解答： 关键在于

impl<T: ?Sized> FutureExt for T where T: Future {}