无法推断一个封闭一辈子返回盒装特征包含参考

Question

我试图编译下面的代码（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, 
    }; 
} 

而且我得到的编译错误：

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]>> 
    |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 

我试图添加各种生命提示，但我无法获得此代码进行编译。在SO这个

我以前的相关问题：

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

• How can multiple struct fields be generics that use the same higher-kinded lifetime?：关于试图解决这个问题没有框。答案表明，现在我将不得不使用Box>。

请注意，我用的根据建议，我在问题2拿到了辅助函数constrain_handler;它允许我克服不同的编译错误。

Answer 1

看来，你已经错过了你以前的问题，它们的副本的关键外卖：

• 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

通过在封闭声明类型论点，你停止执行类型推断的参数。这会导致封闭生成新的隐式生命周期，这与您的要求不符。只是不要声明类型。

接下来，你需要指出你的闭包要采取一些字节的引用，并返回一个盒装特质的对象，将返回相同的寿命的一些字节，并包含相同的寿命的参考：

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, 
    }; 
} 

---

整个事情可以变得更简单，如果你只是需要一个通用的关闭直接：

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 }), 
    }; 
}