Coherence error messages can be inscrutable

[nikomatsakis]

In particular, when the overlap check fails, it doesn't give much clarity, particularly when the overlap is between impls in different crates. It should tell you:

1. Precisely what type overlaps
2. For each case of negative reasoning it was not able to make, why it was not able to make it.
   bind's glue function should tail-call its target  #2 might be a bit tricky to do, not sure, but we can certainly improve on current state of affairs.

[nikomatsakis]

triage: P-high (1.0)

[nikomatsakis]

@flaper87 is going to look into this as well. As a concrete first step, we want to print out the overlapping types, and not just cite impls. For example:

// crate A
trait Foo { }
impl<T> Foo for &T { }

// crate B
struct MyType;
impl Foo for &MyType { } // X

This would ideally print something like:

• Multiple impls can be applied to &MyType
  • First candidate is impl X, on line 123
  • Second candidate is in crate A: impl<T> Foo for &T

However, the last part is a bit tricky since the impl must be reconstructed from metadata, but we can start with even just printing &MyType. It's "obvious" from the impl decl (sort of...) once you know what's going on, but helpful to see it printed when you don't.

[nikomatsakis]

I'm encouraging @aturon to take this on :)