rust - 神秘的冲突实现

Question

我试图在固定时间测试我的代码。所以我写了这样的东西:

use std::time::{SystemTime, UNIX_EPOCH};

trait Clock {
    fn now(&self) -> SystemTime;
}

trait MixInClock {
    type Impl;
}

struct RealClock;
impl<T: MixInClock<Impl = RealClock>> Clock for T {
    fn now(&self) -> SystemTime {
        SystemTime::now()
    }
}

struct FakeClock;
impl <T: MixInClock<Impl = FakeClock>> Clock for T {
    fn now(&self) -> SystemTime {
        UNIX_EPOCH
    }
}

struct DIContainer;
impl MixInClock for DIContainer {
    type Impl = FakeClock;
}

这段代码给了我一个错误:

error[E0119]: conflicting implementations of trait `Clock`:
  --> src/lib.rs:19:1
   |
12 | impl<T: MixInClock<Impl = RealClock>> Clock for T {
   | ------------------------------------------------- first implementation here
...
19 | impl <T: MixInClock<Impl = FakeClock>> Clock for T {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation

为什么？ T 不可能工具 MixInClock<Impl = RealClock>和 MixInClock<Impl = FakeClock>在同一时间。对？

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=95e3647b30ae0e37c45ac18da495a3c5

Answer 1

您遇到了 long-standing issue with Rust's coherence rules .有如何解决这个问题的建议，但目前工作已被推迟。报价Jonas Schievink's most recent comment on the bug :

To give an update on this: In 2016, RFC 1672 was proposed to fix this, but was postponed until the Chalk integration is done. Additionally, according to rust-lang/rfcs#1672 (comment), allowing these kinds of impls would allow users to express mutually exclusive traits, which is a very powerful feature that needs more in-depth consideration by the language team (hence marking as blocked on an RFC as well).