Servo compilation fails with multiple codegen units enabled

[bholley]

STR (on linux64):
(1) Clone git@github.com:servo/servo.git
(2) Check out revision 183c4772e7b93bd6cd9b48a729a043421eb6bc65
(3) export RUSTFLAGS=-Ccodegen-units=16 (I'm on a 16-cpu linux machine, presumably the bug reproduces with other numbers of codegen units)
(4) ./mach build -d

On my machine, this produces the link error in servo/servo#14916

Clearing the env var makes the build succeed.

CC @Manishearth

[Manishearth]

In particular, it's interesting that it's trying to link to <Root<T>>::downcast(), which is a generic function and should never find its way to the linker without being monomorphized.

[est31]

This looks quite similar to #38254

[Manishearth]

cc @nikomatsakis

[michaelwoerister]

PR #38822 (which has landed only recently) might help here.

[SimonSapin]

I could reproduce this issue with rustc 6f1ae66 2017-01-06, but not in 2782e8f 2017-01-12. So I think this is fixed, possibly by #38822.

[Mark-Simulacrum]

Closing since it wasn't reproducible a while back (see above comment).