@Ichoran, the hint here corresponds to the actual test failures predicted by said comment at def units in Global.scala:
```
    /* An iterator returning all the units being compiled in this run */
    /* !!! Note: changing this to unitbuf.toList.iterator breaks a bunch
       of tests in tests/res.  This is bad, it means the resident compiler
       relies on an iterator of a mutable data structure reflecting changes
       made to the underlying structure (in whatever accidental way it is
       currently depending upon.)
     */
    def units: Iterator[CompilationUnit] = unitbuf.iterator
```