从 trait 方法返回对结构字段的可变引用时，如何修复生命周期不匹配？

Question

这是我尝试返回对结构字段的可变引用。

pub trait Objective {
    fn get_children<'a>(&'a mut self) -> &'_ mut Vec<&'_ mut Box<dyn Objective>>;
    fn get_parent(&'_ mut self) -> &'_ mut Box<dyn Objective>;
    fn update(&'_ self) -> ();
}

// #[derive(Objective)]
pub struct Object<'a> {
    children: Vec<&'a mut Box<dyn Objective>>,
    parent: &'a mut Box<dyn Objective>,
}

impl<'a> Objective for Object<'a> {
    fn get_children(&'a mut self) -> &'_ mut Vec<&'_ mut Box<dyn Objective>> {
        return &mut self.children;
    }

    fn get_parent(&'_ mut self) -> &'_ mut Box<dyn Objective> {
        return self.parent;
    }

    fn update(&'_ self) -> () {}
}

寿命有问题。 这是编译器给出的错误：

error[E0308]: method not compatible with trait
  --> src/lib.rs:14:5
   |
14 |     fn get_children(&'a mut self) -> &'_ mut Vec<&'_ mut Box<dyn Objective>> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
   |
   = note: expected fn pointer `fn(&'a mut Object<'a>) -> &'a mut std::vec::Vec<&'a mut std::boxed::Box<(dyn Objective + 'static)>>`
              found fn pointer `fn(&'a mut Object<'a>) -> &'a mut std::vec::Vec<&'a mut std::boxed::Box<(dyn Objective + 'static)>>`
note: the lifetime `'a` as defined on the method body at 14:5...
  --> src/lib.rs:14:5
   |
14 |     fn get_children(&'a mut self) -> &'_ mut Vec<&'_ mut Box<dyn Objective>> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...does not necessarily outlive the lifetime `'a` as defined on the impl at 13:6
  --> src/lib.rs:13:6
   |
13 | impl<'a> Objective for Object<'a> {
   |      ^^

error[E0308]: method not compatible with trait
  --> src/lib.rs:14:5
   |
14 |     fn get_children(&'a mut self) -> &'_ mut Vec<&'_ mut Box<dyn Objective>> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
   |
   = note: expected fn pointer `fn(&'a mut Object<'a>) -> &'a mut std::vec::Vec<&'a mut std::boxed::Box<(dyn Objective + 'static)>>`
              found fn pointer `fn(&'a mut Object<'a>) -> &'a mut std::vec::Vec<&'a mut std::boxed::Box<(dyn Objective + 'static)>>`
note: the lifetime `'a` as defined on the impl at 13:6...
  --> src/lib.rs:13:6
   |
13 | impl<'a> Objective for Object<'a> {
   |      ^^
note: ...does not necessarily outlive the lifetime `'a` as defined on the method body at 14:5
  --> src/lib.rs:14:5
   |
14 |     fn get_children(&'a mut self) -> &'_ mut Vec<&'_ mut Box<dyn Objective>> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

这是我之前的尝试，我没有明确指定生命周期：

pub trait Objective {
    fn get_children(&'_ mut self) -> &'_ mut Vec<&'_ mut Box<dyn Objective>>;
    fn get_parent(&'_ mut self) -> &'_ mut Box<dyn Objective>;
    fn update(&'_ self) -> ();
}

// #[derive(Objective)]
pub struct Object<'a> {
    children: Vec<&'a mut Box<dyn Objective>>,
    parent: &'a mut Box<dyn Objective>,
}

impl Objective for Object<'_> {
    fn get_children(&'_ mut self) -> &'_ mut Vec<&'_ mut Box<dyn Objective>> {
        return &mut self.children;
    }

    fn get_parent(&'_ mut self) -> &'_ mut Box<dyn Objective> {
        return self.parent;
    }

    fn update(&'_ self) -> () {}
}

我得到了一个类似的错误：

error[E0308]: mismatched types
  --> src/lib.rs:15:16
   |
15 |         return &mut self.children;
   |                ^^^^^^^^^^^^^^^^^^ lifetime mismatch
   |
   = note: expected mutable reference `&mut std::vec::Vec<&mut std::boxed::Box<(dyn Objective + 'static)>>`
              found mutable reference `&mut std::vec::Vec<&mut std::boxed::Box<(dyn Objective + 'static)>>`
note: the anonymous lifetime #1 defined on the method body at 14:5...
  --> src/lib.rs:14:5
   |
14 | /     fn get_children(&'_ mut self) -> &'_ mut Vec<&'_ mut Box<dyn Objective>> {
15 | |         return &mut self.children;
16 | |     }
   | |_____^
note: ...does not necessarily outlive the lifetime `'_` as defined on the impl at 13:27
  --> src/lib.rs:13:27
   |
13 | impl Objective for Object<'_> {
   |                           ^^

我该如何解决？

Answer 1

指定特征的生命周期，它可以工作：

pub trait Objective<'a> {
    fn get_children(&mut self) -> &mut Vec<&'a mut Box<dyn Objective<'a>>>;
    fn get_parent(&mut self) -> &mut Box<dyn Objective<'a>>;
    fn update(&self) -> ();
}

// #[derive(Objective)]
pub struct Object<'a> {
    children: Vec<&'a mut Box<dyn Objective<'a>>>,
    parent: Box<dyn Objective<'a>>,
}

impl<'a> Objective<'a> for Object<'a> {
    fn get_children(&mut self) -> &mut Vec<&'a mut Box<dyn Objective<'a>>> {
        &mut self.children
    }

    fn get_parent(&'_ mut self) -> &mut Box<dyn Objective<'a>> {
        &mut self.parent
    }

    fn update(&'_ self) -> () {}
}

通过在Objective上指定生命周期，我们可以确保实现者将返回一个可变引用的Vec ，它满足与 trait 所需的生命周期相同的生命周期要求。