无法推断返回包含引用的盒装特征的闭包的生命周期

Question

我正在尝试编译以下代码 (playground)：

trait MockFutureTrait {
    type Item;
}

struct MockFuture<T> {
    item: T,
}

impl<T> MockFutureTrait for MockFuture<T> {
    type Item = T;
}

struct FragMsgReceiver<'a, 'c: 'a> {
    recv_dgram: &'a FnMut(&mut [u8])
        -> Box<MockFutureTrait<Item = &mut [u8]> + 'c>,
}

fn constrain_handler<F>(f: F) -> F
where
    F: FnMut(&mut [u8]) -> Box<MockFutureTrait<Item = &mut [u8]>>,
{
    f
}


fn main() {
    let mut recv_dgram = constrain_handler(|buf: &mut [u8]| {
        Box::new(MockFuture { item: buf }) as Box<MockFutureTrait<Item = &mut [u8]>>
    });

    let ref_recv_dgram = &mut recv_dgram;
    let fmr = FragMsgReceiver {
        recv_dgram: ref_recv_dgram,
    };
}

我得到了编译错误：

error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
  --> src/main.rs:28:37
   |
28 |         Box::new(MockFuture { item: buf }) as Box<MockFutureTrait<Item = &mut [u8]>>
   |                                     ^^^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #2 defined on the body at 27:44...
  --> src/main.rs:27:44
   |
27 |       let mut recv_dgram = constrain_handler(|buf: &mut [u8]| {
   |  ____________________________________________^
28 | |         Box::new(MockFuture { item: buf }) as Box<MockFutureTrait<Item = &mut [u8]>>
29 | |     });
   | |_____^
note: ...so that expression is assignable (expected &mut [u8], found &mut [u8])
  --> src/main.rs:28:37
   |
28 |         Box::new(MockFuture { item: buf }) as Box<MockFutureTrait<Item = &mut [u8]>>
   |                                     ^^^
   = note: but, the lifetime must be valid for the static lifetime...
note: ...so that expression is assignable (expected std::boxed::Box<MockFutureTrait<Item=&mut [u8]> + 'static>, found std::boxed::Box<MockFutureTrait<Item=&mut [u8]>>)
  --> src/main.rs:28:9
   |
28 |         Box::new(MockFuture { item: buf }) as Box<MockFutureTrait<Item = &mut [u8]>>
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

我尝试添加各种生命周期提示，但无法编译此代码。

我之前关于 SO 的相关问题：

• Cannot infer a lifetime for a struct containing a reference to a closure：当返回值是一个简单的结构而不是一个特征时，解决同样的问题。

• How can multiple struct fields be generics that use the same higher-kinded lifetime?：关于尝试在没有 Box 的情况下解决此问题。答案表明，现在我将不得不使用 Box>。

请注意，根据我在问题 2 中得到的建议，我正在使用辅助函数 constrain_handler；它允许我克服不同的编译错误。

Answer 1

您似乎错过了以前问题及其重复问题的关键要点：

• Lifetime annotation for closure argument
• Cannot infer a lifetime for a struct containing a reference to a closure
• How to declare a lifetime for a closure argument

通过在闭包参数上声明类型，您停止对参数执行类型推断。这会导致闭包生成一个新的隐式生命周期，它与您的要求不匹配。根本不声明类型。

接下来，您需要声明您的闭包将引用一些字节并返回一个装箱的 trait 对象，该对象将返回相同生命周期的一些字节并包含相同生命周期的引用：

struct FragMsgReceiver<'a> {
    recv_dgram: &'a for<'b> FnMut(&'b mut [u8])
        -> Box<MockFutureTrait<Item = &'b mut [u8]> + 'b>,
}

有关+ 'a 语法的更多详细信息，请参阅Why is Box<Iterator<Item = &Foo> + 'a> needed?。

然后更新constrain_handler 以匹配：

struct FragMsgReceiver<'a> {
    recv_dgram: &'a for<'b> FnMut(&'b mut [u8])
        -> Box<MockFutureTrait<Item = &'b mut [u8]> + 'b>,
}

fn constrain_handler<F>(f: F) -> F
where
    F: for<'b> FnMut(&'b mut [u8])
        -> Box<MockFutureTrait<Item = &'b mut [u8]> + 'b>,
{
    f
}

fn main() {
    let mut recv_dgram = constrain_handler(|buf| Box::new(MockFuture { item: buf }));

    let fmr = FragMsgReceiver {
        recv_dgram: &mut recv_dgram,
    };
}

---

如果你直接采用通用闭包，整个事情可以变得更简单：

struct FragMsgReceiver<R>
where
    R: for<'b> FnMut(&'b mut [u8])
        -> Box<MockFutureTrait<Item = &'b mut [u8]> + 'b>,
{
    recv_dgram: R,
}

fn main() {
    let fmr = FragMsgReceiver {
        recv_dgram: |buf| Box::new(MockFuture { item: buf }),
    };
}