为什么helper访问器方法会导致生存期错误(与内联访问器相比)?

游乐场连接 https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=f9217c8f4c6fe708fec852c9515d9fd6

version1() 不起作用, version2() 确实如此,即使它应该是等效的(只是内联 get_x_mut )

在我的实际代码中, get_x_mut 正在返回一个嵌套字段,因此使用访问器抽象出访问权限会非常方便。

我也很困惑为什么循环迭代之间会发生冲突——我没有存储任何引用。

struct Bar<'a> {
    foo: Option<&'a Foo<'a>>
}

impl<'a> Bar<'a> {
    fn bar(&mut self) {}
}

struct Foo<'a> {
    x: Bar<'a>,
    y: Option<Bar<'a>>
}

impl<'a> Foo<'a> {
    fn get_x_mut(&'a mut self) -> &'a mut Bar {
        &mut self.x
    }
    
    fn get_y(&self) -> &Option<Bar> {
        &self.y
    }

    fn version1(&'a mut self) {
        loop {
            if let Some(y) = self.get_y() {
                let x = self.get_x_mut();
                x.bar();
            }
        }
    }
    
    fn version2(&'a mut self) {
        loop {
            if let Some(y) = self.get_y() {
                let x = &mut self.x;
                x.bar();
            }
        }
    }
}

错误消息为

error[E0502]: cannot borrow `*self` as immutable because it is also borrowed as mutable
  --> src/lib.rs:25:30
   |
14 | impl<'a> Foo<'a> {
   |      -- lifetime `'a` defined here
...
25 |             if let Some(y) = self.get_y() {
   |                              ^^^^^^^^^^^^ immutable borrow occurs here
26 |                 let x = self.get_x_mut();
   |                         ----------------
   |                         |
   |                         mutable borrow occurs here
   |                         argument requires that `*self` is borrowed for `'a`

error[E0499]: cannot borrow `*self` as mutable more than once at a time
  --> src/lib.rs:26:25
   |
14 | impl<'a> Foo<'a> {
   |      -- lifetime `'a` defined here
...
26 |                 let x = self.get_x_mut();
   |                         ^^^^^^^^^^^^^^^^
   |                         |
   |                         `*self` was mutably borrowed here in the previous iteration of the loop
   |                         argument requires that `*self` is borrowed for `'a`