我有一个trait，它有一个返回Box<dyn Iterator<...>>的函数，根据我的理解，根据定义，这个返回的迭代器的生命周期需要匹配trait对象的生命周期，但是在一个函数中，如果在末尾同时删除了这两个参数，我会得到一个生命周期错误，这里是一个最小的例子，并添加了一些假设作为注解：

trait Foo<'a> {
    // The returned iterator's lifetime should match self's
    fn bar(&'a self) -> Box<dyn Iterator<Item = usize> + 'a>;
}

struct FooImpl {
    values: Vec<usize>
}

impl<'a> Foo<'a> for FooImpl {
    fn bar(&'a self) -> Box<dyn Iterator<Item = usize> + 'a> {
        Box::new(self.values.iter().cloned())
    }
}

fn foo_bar_caller<'a>(foo: Box<dyn Foo<'a>>) {
    // `foo` is moved into this fn
    let _iter = foo.bar();
    // `_iter` and `foo` should drop
}

fn main() {
    let foo = FooImpl { values: vec![1, 2, 3, 4, 5] };
    foo_bar_caller(Box::new(foo));
}

（代码为rust playground。）运行此命令会出现以下生存期错误：

error[E0597]: `*foo` does not live long enough
  --> src/main.rs:17:17
   |
16 | fn foo_bar_caller<'a>(foo: Box<dyn Foo<'a>>) {
   |                   -- lifetime `'a` defined here
17 |     let _iter = foo.bar();
   |                 ^^^^^^^^^
   |                 |
   |                 borrowed value does not live long enough
   |                 argument requires that `*foo` is borrowed for `'a`
18 |     // `_iter` and `foo` should drop
19 | }
   | - `*foo` dropped here while still borrowed

For more information about this error, try `rustc --explain E0597`.
error: could not compile `playground` due to previous error