Reasons behind `--force-single-frontend-invocation`?

Development Standard Library
1

What's the technical constraint that made stdlib builds require -force-single-frontend-invocation?

2

I'm interested in this also.

The reason is probably recorded in <rdar://problem/15972329> according to this comment in cmake/modules/SwiftSource.cmake. I don't think the rdar is publicly accessible.

    # FIXME: We shouldn't /have/ to build things in a single process.
    # <rdar://problem/15972329>
    list(APPEND swift_compile_flags "-force-single-frontend-invocation")