將結構轉換為具有生存期的特征得到“由於需求沖突，無法為生存期參數'a'推斷適當的生存期”

Question

trait BT {
    fn get_a(&self) -> &A;
}

#[derive(Debug)]
struct A {
    v: i32,
}

impl A {
    fn nb(&self) -> Box<BT> {
        Box::new(B { a: self })
    }
}

#[derive(Debug)]
struct B<'a> {
    a: &'a A,
}

impl<'a> BT for B<'a> {
    fn get_a(&self) -> &A {
        return self.a;
    }
}

fn main() {
    println!("{:?}", A { v: 32 }.nb().get_a());
}

A必須產生的方法B與參考實例A和B可能有許多方法來訪問Ba （A在B參考）。 如果讓A.nb()返回B而不是BT ，則代碼將運行良好。

我是Rust的新手。 這個問題整日困擾着我。 我應該怎么做才能使此代碼起作用？ 謝謝！

整個錯誤報告：

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
  --> src\socket\msg\message.rs:53:26
   |
53 |                 Box::new(B{a: self})
   |                          ^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 52:13...
  --> src\socket\msg\message.rs:52:13
   |
52 | /             fn nb(&self) -> Box<BT> {
53 | |                 Box::new(B{a: self})
54 | |             }
   | |_____________^
note: ...so that reference does not outlive borrowed content
  --> src\socket\msg\message.rs:53:31
   |
53 |                 Box::new(B{a: self})
   |                               ^^^^
   = note: but, the lifetime must be valid for the static lifetime...
   = note: ...so that the expression is assignable:
           expected std::boxed::Box<socket::msg::message::test::test::BT + 'static>
              found std::boxed::Box<socket::msg::message::test::test::BT>

Answer 1

特征對象的默認生存期為'static 。 您需要添加綁定到nb()函數返回的trait對象的顯式生命周期：

impl A {
    fn nb<'s>(&'s self) -> Box<BT+'s> {
        Box::new(B{a: self})
    }
}

特性對象壽命推斷