ice when compiling rust-css tests

[metajack]

After attempting an udpate of the rust-css tests, I get the following compiler ICE. I'm not sure what to do as the stack trace is very small.

Breakpoint 1, upcall_fail (expr=0x103500420 "explicit failure", 
    file=0x103500470 "/Users/jack/src/servo/src/rust/src/librustc/rustc.rc", line=357) at rust_upcall.cpp:121
121     rust_task *task = rust_try_get_current_task();
(gdb) where
#0  upcall_fail (expr=0x103500420 "explicit failure", file=0x103500470 "/Users/jack/src/servo/src/rust/src/librustc/rustc.rc", 
    line=357) at rust_upcall.cpp:121
#1  0x000000010010e2c7 in sys::rustrt::rust_upcall_fail::_8284db702cc2a8c5::_06 ()
   from /Users/jack/src/servo/build/src/rust/x86_64-apple-darwin/stage2/lib/libcore-c3ca5d77d81b46c1-0.6.dylib
#2  0x000000010010e557 in sys::begin_unwind_::_9873ff47b9982218::_06 ()
   from /Users/jack/src/servo/build/src/rust/x86_64-apple-darwin/stage2/lib/libcore-c3ca5d77d81b46c1-0.6.dylib
#3  0x000000010010e4ec in sys::begin_unwind::anon::anon::expr_fn_14532 ()
   from /Users/jack/src/servo/build/src/rust/x86_64-apple-darwin/stage2/lib/libcore-c3ca5d77d81b46c1-0.6.dylib
#4  0x000000010010e376 in str::as_buf_14527::_224e753a5468732e::_06 ()
   from /Users/jack/src/servo/build/src/rust/x86_64-apple-darwin/stage2/lib/libcore-c3ca5d77d81b46c1-0.6.dylib
#5  0x000000010010e42b in sys::begin_unwind::anon::expr_fn_14530 ()
   from /Users/jack/src/servo/build/src/rust/x86_64-apple-darwin/stage2/lib/libcore-c3ca5d77d81b46c1-0.6.dylib
#6  0x000000010010e376 in str::as_buf_14527::_224e753a5468732e::_06 ()
   from /Users/jack/src/servo/build/src/rust/x86_64-apple-darwin/stage2/lib/libcore-c3ca5d77d81b46c1-0.6.dylib
#7  0x00000001001829c3 in __morestack ()
   from /Users/jack/src/servo/build/src/rust/x86_64-apple-darwin/stage2/lib/libcore-c3ca5d77d81b46c1-0.6.dylib
#8  0x0000000100004a3b in sys::begin_unwind::_61fe198059b9e3fc::_06 ()
   from /Users/jack/src/servo/build/src/rust/x86_64-apple-darwin/stage2/lib/libcore-c3ca5d77d81b46c1-0.6.dylib
#9  0x0000000101a743ff in monitor::_8fb4df4bea51b84e::_06 ()
   from /Users/jack/src/servo/build/src/rust/x86_64-apple-darwin/stage2/lib/librustc-c84825241471686d-0.6.dylib
#10 0x0000000101a8d46f in __morestack ()
   from /Users/jack/src/servo/build/src/rust/x86_64-apple-darwin/stage2/lib/librustc-c84825241471686d-0.6.dylib
#11 0x0000000101a8d166 in main::_c4de63b748e03d::_06 ()
   from /Users/jack/src/servo/build/src/rust/x86_64-apple-darwin/stage2/lib/librustc-c84825241471686d-0.6.dylib
#12 0x0000000100000d3e in main::_7f44549ce68f3f14::_00 ()
#13 0x0000000100000d6e in _rust_main ()
#14 0x0000000103275ee5 in task_start_wrapper(spawn_args*) ()
   from /Users/jack/src/servo/build/src/rust/x86_64-apple-darwin/stage2/lib/librustrt.dylib
#15 0x0000000000000000 in ?? ()

To reproduce, try make check on Servo with rust-css at https://github.com/metajack/rust-css/tree/test-ice

[catamorphism]

When I tried building servo just now, it failed for an unrelated reason (rust-core-foundation not building). I'm going to close this since I can't reproduce it, but feel free to reopen, @metajack , if you can create a self-contained version of the test that won't change when servo gets updated. (The backtrace looks like a stack overflow... not sure, though. Perhaps if you see this again, it would be helpful to look at the backtraces in gdb for any other running threads...)