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Here's an example of the bug:
It seems that libraries re-export the ifaces/impls of libraries they're using, which triggers a duplicate symbol error when libraries are used in a diamond shaped pattern.
Test for #2242 (xfailed for now)
This is no longer a duplicate symbol error, but has progressed to being a type error. The problem is that resolve adds an extra _impl into the iscopes list for b's re-export of to_str. I think this is a resolve bug: resolve shouldn't add an _impl twice in two different scopes for an impl with the same def_id. I was able to fix it by making typeck::check::method::lookup::add_candidates_from_scope remove duplicate entries from the iscopes -- but that was a terrible performance regression, and anyway, resolve shouldn't add the same def_id twice.
I checked in the test case as src/test/run-pass/issue-2242-d.rs. Reassigning to @pcwalton , who can hopefully make sure the resolve rewrite doesn't reproduce this bug :-)
test: Modernize and un-XFAIL issue-2242-d.rs (issue #2242)
Some combination of resolve3 and coherence fixed this. Un-XFAILed in 793c0a1.