Rust trait 中self 的显式生命周期似乎导致E0499无法在循环中多次borrow * emits 器作为可变的

我遇到了一个问题，该特征可以返回对以self为单位的数据的引用. 这是一个最小的例子:

trait A<T> {
    fn emit_symbol(&mut self) -> T;
}

fn via_ans<'a>(emitter: &'a mut impl A<char>) -> String {
    let mut rval = String::new();
    for _ in 0..30 {
        let ch = emitter.emit_symbol();
        rval.push(ch)
    }
    rval
}

struct Hippo<T> {
    things: Vec<T>,
}

impl<T> A<&T> for Hippo<T> {
    fn emit_symbol(&mut self) -> &T {
        &self.things[2]
    }
}

给出错误

error: `impl` item signature doesn't match `trait` item signature
  --> src/b.rs:19:5
   |
2  |     fn emit_symbol(&mut self) -> T;
   |     ------------------------------- expected `fn(&'1 mut b::Hippo<T>) -> &'2 T`
...
19 |     fn emit_symbol(&mut self) -> &T {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found `fn(&'1 mut b::Hippo<T>) -> &'1 T`
   |
   = note: expected signature `fn(&'1 mut b::Hippo<T>) -> &'2 T`
              found signature `fn(&'1 mut b::Hippo<T>) -> &'1 T`
help: the lifetime requirements from the `impl` do not correspond to the requirements in the `trait`
  --> src/b.rs:2:34
   |
2  |     fn emit_symbol(&mut self) -> T;
   |                                  ^ consider borrowing this type parameter in the trait

如果我将生命周期添加到trait中为&'s self，那么实现就会工作，但我在使用trait方法的循环中遇到了新错误.

trait A<'s, T> {
    fn emit_symbol(&'s mut self) -> T;
}

fn via_ans<'a: 'b, 'b>(emitter: &'a mut impl A<'b, char>) -> String {
    let mut rval = String::new();
    for _ in 0..30 {
        let ch = emitter.emit_symbol();
        rval.push(ch)
    }
    rval
}

struct Hippo<T> {
    things: Vec<T>,
}

impl<'s, T> A<'s, &'s T> for Hippo<T> {
    fn emit_symbol(&'s mut self) -> &'s T {
        &self.things[2]
    }
}

给出错误

error[E0499]: cannot borrow `*emitter` as mutable more than once at a time
  --> src/main.rs:10:18
   |
7  | fn via_ans<'a: 'b, 'b>(emitter: &'a mut impl A<'b, char>) -> String {
   |                    -- lifetime `'b` defined here
...
10 |         let ch = emitter.emit_symbol();
   |                  ^^^^^^^--------------
   |                  |
   |                  `*emitter` was mutably borrowed here in the previous iteration of the loop
   |                  argument requires that `*emitter` is borrowed for `'b`

这是Rust的borrow 判断器的缺点，还是还有其他问题？

如何调整此特征以在我的应用程序中工作？