error for multiple impls in scope should say why

[nikomatsakis]

Right now, if there are multiple applicable impls in scope for a given function call, you get a singularly unhelpful error message. It would be nice if it would say which impls are conflicting and where they were imported from (rather like what gcc does for failures resolving overloaded methods)

[nikomatsakis]

Fixed (for now) in f112963. I wanted it to list the imports but that seemed hard, and I didn't want to interfere with the ongoing rewrite of resolve.