RtlInitUnicodeString - NtDoc

#ifndef _NTRTL_H
#ifndef PHNT_NO_INLINE_INIT_STRING

FORCEINLINE VOID RtlInitUnicodeString(
    _Out_ PUNICODE_STRING DestinationString,
    _In_opt_z_ PCWSTR SourceString
    )
{
    if (SourceString)
        DestinationString->MaximumLength = (DestinationString->Length = (USHORT)(wcslen(SourceString) * sizeof(WCHAR))) + sizeof(UNICODE_NULL);
    else
        DestinationString->MaximumLength = DestinationString->Length = 0;

    DestinationString->Buffer = (PWCH)SourceString;
}

#endif
#endif
View code on GitHub

#ifndef _NTRTL_H
#ifndef PHNT_NO_INLINE_INIT_STRING
// ...
#else

NTSYSAPI
VOID
NTAPI
RtlInitUnicodeString(
    _Out_ PUNICODE_STRING DestinationString,
    _In_opt_z_ PCWSTR SourceString
    );

#endif
#endif
View code on GitHub