That doesn't work for two reasons.

• disable diagnostics; only affects run-time warnings. "used only once" is a compile-time warning.
• disable diagnostics; doesn't disable the warning, just the diagnostic. So while disable diagnostics; would remove the diagnostic message, the basic warning message would still be displayed.

The solution is to eliminate the warning (using no warnings 'once'; for example), since there's no diagnostic message if there's no warning.