Cross-crate coherence failures can produce unhelpful spans

[aturon]

Consider the following example.

Crate A

pub trait To<T> {
    fn convert_to(&self) -> T;
}

pub trait Into<T>: Sized {
    fn convert_into(self) -> T;
}

pub trait From<T> {
    fn from(T) -> Self;
}

impl<'a, T: ?Sized, U> Into<U> for &'a T where T: To<U> {
    fn convert_into(self) -> U {
        self.convert_to()
    }
}

impl<T, U> From<T> for U where T: Into<U> {
    fn from(t: T) -> U {
        t.convert_into()
    }
}

Crate B

struct LocalType<T>(Option<T>);

impl<'a, T> From<&'a [T]> for LocalType<T> {
    fn from(_: &'a [T]) -> LocalType<T> { LocalType(None) }
}

impl<T> Into<LocalType<T>> for LocalType<T> {
    fn convert_into(self) -> LocalType<T> {
        self
    }
}

impl To<LocalType<u8>> for [u8] {
    fn convert_to(&self) -> LocalType<u8> {
        LocalType(None)
    }
}

You get an error like:

conv-downstream.rs:6:1: 8:2 error: conflicting implementations for trait `conv_crate::From` [E0119]
conv-downstream.rs:6 impl<'a, T> From<&'a [T]> for LocalType<T> {
conv-downstream.rs:7     fn from(_: &'a [T]) -> LocalType<T> { LocalType(None) }
conv-downstream.rs:8 }
conv-downstream.rs:6:1: 8:2 note: conflicting implementation in crate `conv_crate`
conv-downstream.rs:6 impl<'a, T> From<&'a [T]> for LocalType<T> {
conv-downstream.rs:7     fn from(_: &'a [T]) -> LocalType<T> { LocalType(None) }
conv-downstream.rs:8 }

where the conflicting impls are identical ones, both drawn from the downstream crate.

[aturon]

cc @nikomatsakis

[nikomatsakis]

thanks for filing, been meaning to clean this up -- it's not just the spans, i'd like it to tell you more. For example, I would like it to print out something like "there are two impls applicable to the type Foo, and the following conditions are all satisfiable"

[steveklabnik]

Triage: here's a way to reproduce this:

In a.rs:

pub trait LolTo<T> {
    fn convert_to(&self) -> T;
}

pub trait LolInto<T>: Sized {
    fn convert_into(self) -> T;
}

pub trait LolFrom<T> {
    fn from(T) -> Self;
}

impl<'a, T: ?Sized, U> LolInto<U> for &'a T where T: LolTo<U> {
    fn convert_into(self) -> U {
        self.convert_to()
    }
}

impl<T, U> LolFrom<T> for U where T: LolInto<U> {
    fn from(t: T) -> U {
        t.convert_into()
    }
}

In b.rs:

extern crate a;

use a::LolFrom;
use a::LolInto;
use a::LolTo;

struct LocalType<T>(Option<T>);

impl<'a, T> LolFrom<&'a [T]> for LocalType<T> {
    fn from(_: &'a [T]) -> LocalType<T> { LocalType(None) }
}

impl<T> LolInto<LocalType<T>> for LocalType<T> {
    fn convert_into(self) -> LocalType<T> {
        self
    }
}

impl LolTo<LocalType<u8>> for [u8] {
    fn convert_to(&self) -> LocalType<u8> {
        LocalType(None)
    }
}

then:

$ rustc a.rs --crate-type=lib
$ rustc b.rs --crate-type=lib --extern a=liba.rlib
error[E0119]: conflicting implementations of trait `a::LolFrom<&[_]>` for type `LocalType<_>`:
  --> b.rs:9:1
   |
9  | impl<'a, T> LolFrom<&'a [T]> for LocalType<T> {
   | ^
   |
   = note: conflicting implementation in crate `a`

error: aborting due to previous error

(this is with 1.13.0)