Manish Goregaokar 9eb0bab9de Rollup merge of #23867 - nikomatsakis:issue-23086-take-3, r=pnkfelix
This PR implements rust-lang/rfcs#1023. In the process it fixes #23086 and #23516. A few impls in libcore had to be updated, but the impact is generally pretty minimal. Most of the fallout is in the tests that probed the limits of today's coherence.

I tested and we were able to build the most popular crates along with iron (modulo errors around errors being sendable).

Fixes #23918.
2015-04-02 00:40:38 +05:30
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