已经发布了很多类似的错误:
- "Overflow evaluating the requirement" but that kind of recursion should not happen at all
- What does "Overflow evaluating the requirement" mean and how can I fix it?
My case更简单,看起来无辜:
extern crate tokio_core;
extern crate tokio_io;
use std::{borrow::Borrow, rc::Rc};
use tokio_core::net::TcpStream;
use tokio_io::io::read_exact;
fn read_one(conn: Rc<TcpStream>) {
read_exact(conn.borrow(), [0u8]);
}
它给出了这个错误:
error[E0275]: overflow evaluating the requirement `_: std::marker::Sized`
--> src/main.rs:9:5
|
9 | read_exact(conn.borrow(), [0u8]);
| ^^^^^^^^^^
|
= help: consider adding a `#![recursion_limit="128"]` attribute to your crate
= note: required because of the requirements on the impl of `std::io::Read` for `&tokio_core::reactor::poll_evented2::PollEvented<_>`
= note: required because of the requirements on the impl of `std::io::Read` for `&tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<_>>`
= note: required because of the requirements on the impl of `std::io::Read` for `&tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<_>>>`
[... snip ...]
= note: required because of the requirements on the impl of `std::io::Read` for `&tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<_>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>`
= note: required because of the requirements on the impl of `tokio_io::AsyncRead` for `&tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<tokio_core::reactor::poll_evented2::PollEvented<_>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>`
= note: required by `tokio_io::io::read_exact`
这是怎么回事?
我知道下面的作品,它比上面的更简单:
read_exact(&*conn, [0u8]);
我相信 conn.borrow
应该也能正常工作,我只是不明白为什么我们会出现这个错误。
最佳答案
&*conn
之间的区别和 conn.borrow()
是一个类型可能有多个 Borrow impls。
use std::borrow::Borrow;
fn main() {
let input = vec![1, 2, 3];
let _slice: &[u8] = input.borrow(); // ok
let _vec_ref: &Vec<u8> = input.borrow(); // also ok
let _slice: &[u8] = &*input; // ok
// let _vec_ref: &Vec<u8> = &*input; // error!
}
&*conn
表达式使用 Deref
特征,其中每种类型只能有一个 Deref
执行。但是,一个类型可以有多个 Borrow<X>
不同的实现 X
秒。
当你写作时
read_exact(conn.borrow(), [0u8]);
编译器需要解决以下义务:
-
Rc<TcpStream>: Borrow<X1>
由于使用borrow()
-
&X1: AsyncRead
由于read_exact
请注意 X1
是未知类型。编译器需要找出所有潜在的 X1
看看是否有人可以同时履行这两项义务。义务 2 以某种方式首先被评估,最终有这些候选人:
-
impl<X2> AsyncRead for &PollEvented<X2> where &X2: Read
-
impl AsyncRead for &TcpStream
-
impl AsyncRead for &[u8]
可能还有更不重要的候选人……
同样,候选人 1 在候选人 2 之前以某种方式被选中。这导致在候选人 1 被选中后出现以下新的义务集:
-
Rc<TcpStream>: Borrow<PollEvented<X2>>
&PollEvented<X2>: AsyncRead
-
&X2: Read
然后导致 impl<X3> Read for &PollEvented<X3> where &X3: Read
被选中,从这一点开始,求解器陷入无限循环并最终放弃。
有关编译器如何求解这些方程的详细信息,请参阅 Rust Compiler Guide .
好消息是特征系统正在revamped使用允许正确推断 OP 程序的标准逻辑推理技术(如 Prolog)。
但是,在新的trait引擎实现之前,如果必须使用borrow
你可以通过告诉编译器什么来帮助编译器 X1
应该是:
read_exact::<&TcpStream, _>(conn.borrow(), [u8]);
// ^~~~~~~~~~ forces &X1 = &TcpStream
如果您有兴趣,以下 chalk程序证明新求解器可以对 OP 的示例进行类型检查
trait Borrow<T> {}
trait AsyncRead {}
trait Read {}
struct Ref<T> {} // meaning &T
struct Rc<T> {}
impl<T> Borrow<T> for Rc<T> {}
struct TcpStream {}
impl Read for TcpStream {}
impl AsyncRead for TcpStream {}
impl Read for Ref<TcpStream> {}
impl AsyncRead for Ref<TcpStream> {}
struct PollEvented<E> {}
impl<E> AsyncRead for Ref<PollEvented<E>> where Ref<E>: Read {}
impl<E> Read for Ref<PollEvented<E>> where Ref<E>: Read {}
// Verify:
//
// ?- exists<X> { Ref<X>: AsyncRead, Rc<TcpStream>: Borrow<X> }
// Unique; substitution [?0 := TcpStream], lifetime constraints []
关于rust - 为什么我使用 Rc<TcpStream> 得到 "overflow evaluating the requirement ` Sized `"when using tokio_io' s read_exact?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/50189976/