Merge in ABI changes while not breaking the Windows build

[catamorphism]

I reverted #4590 because it breaks Windows. Figure out why, and unrevert.

[catamorphism]

I'll investigate; running a try build right now so I can see what the error is.

[catamorphism]

Build output is: http://buildbot.rust-lang.org/builders/try-win/builds/210/steps/compile/logs/stdio -- I have no idea where to begin and don't have a working Windows setup.

[catamorphism]

Done in 41adf9d