rust - 如何从更高级别的特征绑定特征返回关联类型?
问题描述
我有一个特征,它具有反序列化关联类型的功能。然而,该关联类型需要有一个由调用者决定的生命周期,所以我有一个单独的特征,我使用更高级别的特征绑定,以便它可以在任何生命周期内反序列化。
我需要使用返回此关联类型的闭包。
我有以下代码可以做到这一点:
#![allow(unreachable_code)]
use std::marker::PhantomData;
trait Endpoint: for<'a> EndpointBody<'a> {}
trait EndpointBody<'a> {
type Out: 'a;
fn serialize(body: &Self::Out) -> Vec<u8>;
fn deserialize(raw_body: &'a [u8]) -> Self::Out;
}
// /////////////////////////////////////////////////////////
/// Trait object compatible handler
trait Handler {
fn execute(&self, raw_body: &[u8]) -> Vec<u8>;
}
/// Wraps a function for an endpoint, convertint it to a Handler
struct FnHandler<EP, F>
where
EP: Endpoint,
F: 'static + for<'a> Fn(&'a [u8]) -> <EP as EndpointBody<'a>>::Out,
{
func: F,
_ph: PhantomData<EP>,
}
impl<EP, F> FnHandler<EP, F>
where
EP: Endpoint,
F: 'static + for<'a> Fn(&'a [u8]) -> <EP as EndpointBody<'a>>::Out,
{
pub fn new(func: F) -> Self {
Self {
func,
_ph: PhantomData,
}
}
}
impl<EP, F> Handler for FnHandler<EP, F>
where
EP: Endpoint,
F: 'static + for<'a> Fn(&'a [u8]) -> <EP as EndpointBody<'a>>::Out,
{
fn execute(&self, in_raw_body: &[u8]) -> Vec<u8> {
let body = (self.func)(in_raw_body);
let serialized_body = unimplemented!();
return serialized_body;
}
}
// /////////////////////////////////////////////////////////
/// Collection of handlers
struct Handlers(Vec<Box<dyn Handler>>);
impl Handlers {
pub fn new() -> Self {
Self(vec![])
}
pub fn handle<EP: 'static, F>(&mut self, func: F)
where
EP: Endpoint,
F: 'static + for<'a> Fn(&'a [u8]) -> <EP as EndpointBody<'a>>::Out,
{
self.0.push(Box::new(FnHandler::<EP, F>::new(func)));
}
}
// /////////////////////////////////////////////////////////
struct MyEndpoint;
struct MyEndpointBody<'a> {
pub string: &'a str,
}
impl Endpoint for MyEndpoint {}
impl<'a> EndpointBody<'a> for MyEndpoint {
type Out = MyEndpointBody<'a>;
fn serialize(body: &Self::Out) -> Vec<u8> {
unimplemented!()
}
fn deserialize(raw_body: &'a [u8]) -> Self::Out {
unimplemented!()
}
}
// /////////////////////////////////////////////////////////
fn main() {
let mut handlers = Handlers::new();
handlers.handle::<MyEndpoint, _>(|_body| MyEndpointBody {
string: "test string",
});
handlers.0[1].execute(&[]);
}
我认为这应该可行,但是当我检查它时,我得到一个类型错误:
error[E0271]: type mismatch resolving `for<'a> <[closure@src/main.rs:92:38: 94:6] as std::ops::FnOnce<(&'a [u8],)>>::Output == <MyEndpoint as EndpointBody<'a>>::Out`
--> src/main.rs:92:14
|
92 | handlers.handle::<MyEndpoint, _>(|_body| MyEndpointBody {
| ^^^^^^ expected struct `MyEndpointBody`, found associated type
|
= note: expected struct `MyEndpointBody<'_>`
found associated type `<MyEndpoint as EndpointBody<'_>>::Out`
= note: consider constraining the associated type `<MyEndpoint as EndpointBody<'_>>::Out` to `MyEndpointBody<'_>`
= note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
这很令人困惑,因为MyEndpoint::Out
是 a MyEndpointBody
,我从闭包中返回它,但 Rust 认为它们不是同一类型。我猜这是因为 Rust 为该MyEndpointBody
类型选择了不兼容的匿名生命周期,但我不知道如何解决这个问题。
我怎样才能让这个代码工作,以便我可以使用带有 HRTB 关联类型的闭包?
解决方案
让闭包将返回类型包装在一个新类型中解决了这个问题:
#![allow(unreachable_code)]
use std::marker::PhantomData;
trait Endpoint: for<'a> EndpointBody<'a> {}
trait EndpointBody<'a> {
type Out: 'a;
fn serialize(body: &Self::Out) -> Vec<u8>;
fn deserialize(raw_body: &'a [u8]) -> Self::Out;
}
struct EPOut<'a, EP: Endpoint>(<EP as EndpointBody<'a>>::Out);
// /////////////////////////////////////////////////////////
/// Trait object compatible handler
trait Handler {
fn execute(&self, raw_body: &[u8]) -> Vec<u8>;
}
/// Wraps a function for an endpoint, convertint it to a Handler
struct FnHandler<EP, F>
where
EP: Endpoint,
F: 'static + for<'a> Fn(&'a [u8]) -> EPOut<'a, EP>,
{
func: F,
_ph: PhantomData<EP>,
}
impl<EP, F> FnHandler<EP, F>
where
EP: Endpoint,
F: 'static + for<'a> Fn(&'a [u8]) -> EPOut<'a, EP>,
{
pub fn new(func: F) -> Self {
Self {
func,
_ph: PhantomData,
}
}
}
impl<EP, F> Handler for FnHandler<EP, F>
where
EP: Endpoint,
F: 'static + for<'a> Fn(&'a [u8]) -> EPOut<'a, EP>,
{
fn execute(&self, in_raw_body: &[u8]) -> Vec<u8> {
let body = (self.func)(in_raw_body);
let serialized_body = unimplemented!();
return serialized_body;
}
}
// /////////////////////////////////////////////////////////
/// Collection of handlers
struct Handlers(Vec<Box<dyn Handler>>);
impl Handlers {
pub fn new() -> Self {
Self(vec![])
}
pub fn handle<EP: 'static, F>(&mut self, func: F)
where
EP: Endpoint,
F: 'static + for<'a> Fn(&'a [u8]) -> EPOut<'a, EP>,
{
self.0.push(Box::new(FnHandler::<EP, F>::new(func)));
}
}
// /////////////////////////////////////////////////////////
struct MyEndpoint;
struct MyEndpointBody<'a> {
pub string: &'a str,
}
impl Endpoint for MyEndpoint {}
impl<'a> EndpointBody<'a> for MyEndpoint {
type Out = MyEndpointBody<'a>;
fn serialize(body: &Self::Out) -> Vec<u8> {
unimplemented!()
}
fn deserialize(raw_body: &'a [u8]) -> Self::Out {
unimplemented!()
}
}
// /////////////////////////////////////////////////////////
fn main() {
let mut handlers = Handlers::new();
handlers.handle::<MyEndpoint, _>(|_body| EPOut(MyEndpointBody {
string: "test string",
}));
handlers.0[1].execute(&[]);
}
我很想说这是一个 Rust 编译器错误,考虑到新类型应该与关联类型几乎相同。似乎还有一些与使用 HRTB 相关类型相关的 ICE:https ://github.com/rust-lang/rust/issues/62529
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