Function CFStringCreateStringWithValidatedFormat 

pub unsafe extern "C" fn CFStringCreateStringWithValidatedFormat(
    alloc: CFAllocatorRef,
    formatOptions: CFDictionaryRef,
    validFormatSpecifiers: CFStringRef,
    format: CFStringRef,
    errorPtr: *mut CFErrorRef,
    ...
) -> CFStringRef

Apple SDK function CFStringCreateStringWithValidatedFormat.