无法推断闭包的寿命，该闭包返回包含引用的盒装特征 [英] Cannot infer a lifetime for a closure returning a boxed trait that contains a reference
问题描述
我正在尝试编译以下代码(游乐场 ):
I am trying to compile the following code (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，但是无法编译此代码.
I tried to add various lifetime hints, but I couldn't get this code to compile.
我以前在SO上与此相关的问题:
My previous related questions on SO about this:
-
无法推断包含引用闭包的结构的生存期:解决相同的问题，当返回值是简单的结构而不是特征.
Cannot infer a lifetime for a struct containing a reference to a closure: Solving the same problem, when the return value is a simple struct and not a trait.
如何将多个结构字段变成使用相同寿命的泛型?:关于尝试解决这一问题没有盒子的问题.答案表明，现在我将不得不使用Box>.
How can multiple struct fields be generics that use the same higher-kinded lifetime?: About trying to solve this problem without Boxes. The answer suggests that for now I will have to use Box>.
请注意，根据问题2的建议，我正在使用辅助功能constrain_handler.它使我能够克服另一个编译错误.
Note that I am using the helper function constrain_handler according to a suggestion I got in question 2; it allows me to overcome a different compilation error.
推荐答案
您似乎错过了先前问题及其重复内容的关键要点:
It appears that you've missed a key takeaway of your previous questions and their duplicates:
- 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，您停止对参数进行类型推断.这将导致闭包生成新的隐式生存期，这与您的要求不符.只是根本不声明类型.
By declaring a type on the closure argument, you stop performing type inference for the arguments. This causes a new implicit lifetime to be generated by the closure, one which does not match your requirements. Just don't declare the type at all.
接下来，您需要声明您的闭包将引用一些字节，并返回一个装箱的特征对象，该对象将返回一些具有相同生存期的字节并包含相同生存期的引用:
Next, you need to state that your closure is going to take a reference to some bytes and return a boxed trait object that will return some bytes of the same lifetime and contains a reference of that same lifetime:
struct FragMsgReceiver<'a> {
recv_dgram: &'a for<'b> FnMut(&'b mut [u8])
-> Box<MockFutureTrait<Item = &'b mut [u8]> + 'b>,
}
请参见为什么Box< Iterator< Item =& Foo> +'a> 以获取有关+ 'a语法的更多详细信息.
See Why is Box<Iterator<Item = &Foo> + 'a> needed? for more details about the + 'a syntax.
然后更新constrain_handler进行匹配:
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,
};
}
如果您直接采用通用闭包，则整个过程可以变得更简单:
The whole thing can be made simpler if you just take a generic closure directly:
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 }),
};
}
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