On gcc 4.5.1 (fedora 14 i686) rust_test_runtime.cpp fails to compile
with the following error:
rt/test/rust_test_runtime.cpp:57:61: error: passing NULL to non-pointer argument 3 of ‘void rust_task::start(uintptr_t, uintptr_t, uintptr_t, size_t)’
The explicit cast works around the error.