Provide additional guidance about the py2 problem with genrule.
@@ -594,7 +594,8 @@ public final class PyCommon {
+ ": "
+ "This target is being built for Python %s but (transitively) includes Python %s-only "
+ "sources. You can get diagnostic information about which dependencies introduce this "
- + "version requirement by running the `find_requirements` aspect. For more info see "
+ + "version requirement by running the `find_requirements` aspect. If this is used in a "
+ + "genrule, you may need to migrate from tools to exec_tools. For more info see "
+ "the documentation for the `srcs_version` attribute: "
+ semantics.getSrcsVersionDocURL();
