Mysterious conflicting implementations

user_13025505 0
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I was trying to test my code with fixed time. so I wrote something like this:

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;
}

This code gives me an error:


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

Why? There's no possibility that T implements MixInClock<Impl = RealClock> and MixInClock<Impl = FakeClock> at same time. right?

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

rust
2 Answers

You ran into a long-standing issue with Rust's coherence rules. There are proposals how to resolve this, but the work has been postponed for now. Quoting 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).

user_2796270

While Sven's answer is correct, that this is an issue with Rust's coherence rules, you can solve this problem by just generalizing your code a bit. In praticular, when you have two different types that should have different behavior but a similar interface, it's often useful to create a trait for that.

In this case, you could create a trait for RealClock and FakeClock that each can implement, and use that in your generic Clock implementation instead of adding the behavior of each directly:

// Common interface for RealClock and FakeClock, implemented separated for each
trait ClockImpl {
    fn now() -> SystemTime;
}

impl ClockImpl for RealClock {
    fn now() -> SystemTime {
        SystemTime::now()
    }
}

impl ClockImpl for FakeClock {
    fn now() -> SystemTime {
        UNIX_EPOCH
    }
}

// MixInClock only needs to now that Self::Impl implements ClockImpl
trait MixInClock {
    type Impl: ClockImpl;
}

// We call the T::Impl::now() method for the specific behavior of RealClock and FakeClock
impl<T: MixInClock> Clock for T {
    fn now(&self) -> SystemTime {
        <T::Impl as ClockImpl>::now()
    }
}

Code in playground

user_4231700
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