RtlUnicodeStringToAnsiString - NtDoc

Native API online documentation, based on the System Informer (formerly Process Hacker) phnt headers
#ifndef _NTRTL_H

NTSYSAPI
NTSTATUS
NTAPI
RtlUnicodeStringToAnsiString(
    _Inout_ PANSI_STRING DestinationString,
    _In_ PUNICODE_STRING SourceString,
    _In_ BOOLEAN AllocateDestinationString
    );

#endif

View code on GitHub

This function is documented in Windows Driver Kit.