Rust 为什么引用上的特征会引发不能borrow 为可变的，因为它也被borrow 为不可变的，而对象上的特征却没有

考虑一个函数

fn clear_non_empty<T>(collection: &mut Vec<T>) {
    if !collection.is_empty() {
        collection.clear()
    }
}

它编译得很好，如果我试着把它推广到一些集合中

trait IsEmpty {
    fn is_empty(&self) -> bool;
}

trait Clear {
    fn clear(&mut self);
}

fn clear_non_empty<'a, Collection: Clear + IsEmpty>(collection: &'a mut Collection) {
    if !collection.is_empty() {
        collection.clear()
    }
}

我也没有问题.但是如果我把特征改成

trait IsEmpty {
    fn is_empty(self) -> bool;
}

trait Clear {
    fn clear(self);
}

fn clear_non_empty<'a, Collection>(collection: &'a mut Collection)
where
    &'a mut Collection: Clear,
    &'a Collection: IsEmpty,
{
    if !collection.is_empty() {
        collection.clear()
    }
}

我正在

error[E0502]: cannot borrow `*collection` as mutable because it is also borrowed as immutable
  --> src/lib.rs:15:9
   |
9  | fn clear_non_empty<'a, Collection>(collection: &'a mut Collection)
   |                    -- lifetime `'a` defined here
...
14 |     if !collection.is_empty() {
   |         ---------------------
   |         |
   |         immutable borrow occurs here
   |         argument requires that `*collection` is borrowed for `'a`
15 |         collection.clear()
   |         ^^^^^^^^^^^^^^^^^^ mutable borrow occurs here

For more information about this error, try `rustc --explain E0502`.

我不明白，为什么特征的方法接受了self个引用，而特征的实现方法接受了self个引用却没有？