VMS Help DCE_IDL, DCL Command Interface, QUALIFIERS, /DIAGNOSTICS  *Conan The Librarian (sorry for the slow response - running on an old VAX)

      /DIAGNOSTICS [=filename]
      /NODIAGNOSTICS (default)

        Specifies that a diagnostic file listing the errors reported
        by a compilation should be generated for LSE. If you do not
        specify a filename, the compiler uses the basename of the IDL
        file and appends the .DIA extension to it.