我的结构ReadingState
在其recv_dgram
方法中将函数new()
作为参数。 recv_dgram
将带有生命周期'r
的缓冲区作为参数,并返回某种类型的Future
。此未来的Item
包含作为参数提供的缓冲区,具有相同的生命周期'r
。
这就是ReadingState
的样子:
struct FragMsgReceiver<'a, A, FUNC: 'a>
where
FUNC: for<'r> FnMut(&'r [u8])
-> Future<Item = (&'r [u8], usize, A), Error = io::Error>,
{
frag_state_machine: FragStateMachine,
recv_dgram: &'a mut FUNC,
get_cur_instant: &'a mut FnMut() -> Instant,
}
struct ReadingState<'a, 'c, A, FUNC: 'a, F>
where
F: Future<Item = (&'c mut [u8], usize, A), Error = io::Error>,
FUNC: for<'r> FnMut(&'r [u8])
-> Future<Item = (&'r [u8], usize, A), Error = io::Error>,
{
frag_msg_receiver: FragMsgReceiver<'a, A, FUNC>,
temp_buff: Vec<u8>,
res_buff: &'c mut [u8],
opt_read_future: Option<F>,
}
FUNC
的返回类型与F
不同,因为它们使用不同的生命周期。
结构本身可以编译,但我不能在我的其余代码中正确使用它们。例如,当我尝试拨打frag_msg_receiver.recv_dgram
并将结果分配给opt_read_future
的字段ReadingState
时,会发生这种情况:
error[E0308]: match arms have incompatible types
--> src/frag_msg_receiver.rs:80:30
|
80 | let mut fdgram = match mem::replace(&mut reading_state.opt_read_future, None) {
| ______________________________^
81 | | Some(read_future) => read_future,
82 | | None => (*reading_state.frag_msg_receiver.recv_dgram)(
83 | | &mut reading_state.temp_buff),
84 | | };
| |_____________^ expected type parameter, found trait frag_msg_receiver::futures::Future
|
= note: expected type `F`
found type `frag_msg_receiver::futures::Future<Item=(&[u8], usize, A), Error=std::io::Error> + 'static`
一个梦想的解决方案(这不是有效的Rust代码)将是以下形式:
struct ReadingState<'a, 'c, A, FUNC: 'a, F>
where for <'r> {
F: Future<Item = (&'r mut [u8], usize, A), Error = io::Error>,
FUNC: FnMut(&'r [u8]) -> F,
}
{
// ...
}
我不知道如何使用现有语法实现这一目标。
编辑:我做了尽可能小的自包含示例,但它不会因为不同的原因而编译。我把它包括在这里(playground):
trait MockFutureTrait {
type Item;
fn get_item(self) -> Self::Item;
}
type FnTraitObject = FnMut(&mut [u8]) -> MockFutureTrait<Item=&mut [u8]>;
struct MockFuture<T> {
item: T,
}
impl<T> MockFutureTrait for MockFuture<T> {
type Item=T;
fn get_item(self) -> Self::Item {
self.item
}
}
struct FragMsgReceiver<'a> {
recv_dgram: &'a mut FnTraitObject,
}
struct RecvMsg<'a,'c,F>
where F: MockFutureTrait<Item=&'c mut [u8]> {
frag_msg_receiver: FragMsgReceiver<'a>,
res_buff: &'c mut [u8],
read_future: F,
}
fn main() {
let mut recv_dgram = |buf: &mut [u8]| {
MockFuture {
item: buf,
}
};
let fmr = FragMsgReceiver {
recv_dgram: &mut recv_dgram,
};
}
我得到的编译错误:
error[E0271]: type mismatch resolving `for<'r> <[closure@src/main.rs:33:26: 37:6] as std::ops::FnOnce<(&'r mut [u8],)>>::Output == MockFutureTrait<Item=&'r mut [u8]> + 'static`
--> src/main.rs:40:21
|
40 | recv_dgram: &mut recv_dgram,
| ^^^^^^^^^^^^^^^ expected struct `MockFuture`, found trait MockFutureTrait
|
= note: expected type `MockFuture<&mut [u8]>`
found type `MockFutureTrait<Item=&mut [u8]> + 'static`
= note: required for the cast to the object type `for<'r> std::ops::FnMut(&'r mut [u8]) -> MockFutureTrait<Item=&'r mut [u8]> + 'static + 'static`
error: aborting due to previous error
error: Could not compile `noncompiling_lifetime_trait`.
我不确定我知道自己在做什么,以及为什么编译问题会发生变化。你可能有个主意。
答案 0 :(得分:1)
从Rust 1.20开始,你所希望做的事情是不可能的。您需要generic associated types才能在类型参数F
上绑定正确的生命周期。解决方案看起来像这样(显然,我无法测试它,因为通用关联类型尚未实现):
use std::marker::PhantomData;
trait FragFutureFamily<A> {
type F<'a>: Future<Item = (&'a [u8], usize, A), Error = io::Error>;
}
struct FragMsgReceiver<'a, A, FUNC: 'a, FF>
where
FF: FragFutureFamily<A>,
FUNC: for<'r> FnMut(&'r [u8]) -> FF::F<'r>,
{
frag_state_machine: FragStateMachine,
recv_dgram: &'a mut FUNC,
get_cur_instant: &'a mut FnMut() -> Instant,
_phantom_future_family: PhantomData<FF>,
}
struct ReadingState<'a, 'c, A, FUNC: 'a, F, FF>
where
F: Future<Item = (&'c mut [u8], usize, A), Error = io::Error>,
FF: FragFutureFamily<A>,
FUNC: for<'r> FnMut(&'r [u8]) -> FF::F<'r>,
{
frag_msg_receiver: FragMsgReceiver<'a, A, FUNC, FF>,
temp_buff: Vec<u8>,
res_buff: &'c mut [u8],
opt_read_future: Option<F>,
_phantom_future_family: PhantomData<FF>,
}
注意:我在F
上保留了ReadingState
类型参数,因为该类型与FragFutureFamily::F
略有不同,但如果您可以使类型一致,则可以更改{的类型{1}}至opt_read_future
。
作为一种变通方法,您可以使用Option<FF::F<'c>>
代替Box<Future<...>>
类型的类型参数。