// requires class _javax

static boolean isSafeCompilable(String src) {
  File srcDir = _javax.TempDirMaker_make();
  File classesDir = _javax.TempDirMaker_make();
  _javax.saveTextFile(new File(srcDir, "main.java").getPath(), src);
  new List<File> libraries;
  _javax.safeTranslate = true;
  File transpiledDir = _javax.topLevelTranslate(srcDir, libraries);
  try {
    _javax.compileJava(transpiledDir, libraries, classesDir);
  } catch (RuntimeException e) { // TODO: check if it's actually a compile fail, not any other error
    return false;
  }
  return true;
}

Began life as a copy of #1000590