为什么在将 tokio_io 的 read_exact 与 Rc<TcpStream> 一起使用时会出现“评估要求‘Sized’的溢出”？

Question

已经贴了不少类似的错误：

• "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，看起来很无辜：

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`

发生了什么事？

我知道以下工作，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会出现这个错误。

Answer 1

&*conn 和conn.borrow() 的区别在于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，其中每个类型只能有一个 Deref 实现。但是，一个类型可以针对不同的Xs 有多个Borrow<X> 实现。

当你写作时

read_exact(conn.borrow(), [0u8]);

编译器需要解决以下义务：

1. Rc<TcpStream>: Borrow<X1> 由于使用了borrow()
2. &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X1s 并查看是否有人可以同时承担这两项义务。义务 2 以某种方式首先得到了评估，最终得到了这些候选人：

1. impl<X2> AsyncRead for &PollEvented<X2> where &X2: Read
2. impl AsyncRead for &TcpStream
3. impl AsyncRead for &[u8] 和可能更不重要的候选人...

再次，不知何故，候选人 1 在候选人 2 之前被选中。这导致在候选人 1 被选中后，会出现以下一组新的义务：

1. Rc<TcpStream>: Borrow<PollEvented<X2>>
2. &PollEvented<X2>: AsyncRead 解决了！
3. &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 trait 系统正在 revamped 使用标准逻辑推理技术（如 Prolog），它允许正确推断 OP 的程序。

但是，在实现新的 trait 引擎之前，如果你必须使用 borrow，你可以告诉编译器 X1 应该是什么：

read_exact::<&TcpStream, _>(conn.borrow(), [u8]);
//           ^~~~~~~~~~ forces &X1 = &TcpStream

---

如果您有兴趣，下面的chalk 程序证明新的求解器可以对 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 []