rust-lang/rust — pull request #19539 by cmr (2014-12-04 21:51:43+00:00)
Title: rustc: check supertraits for object safety

Closes #18959

Technically, this causes code that once compiled to no longer compile, but
that code probably never ran.

[breaking-change]

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Not quite sure the error message is good enough, I feel like it ought to tell you "because it inherits from non-object-safe trait Foo", so I've opened up a follow-up issue #19538