procedure InternalUTF8ToCP(const s: string; TargetCodePage: TSystemCodePage;
   SetTargetCodePage: boolean;
   const UTF8CharConvFunc: TUnicodeToCharID;
   out TheResult: RawByteString); inline;
begin
   if not Assigned(UTF8CharConvFunc) then
   begin
     TheResult:=s;
     SetCodePage(TheResult, TargetCodePage, True);
     if not SetTargetCodePage then
       SetCodePage(TheResult, CP_ACP, False);
   end else begin
     TheResult:=UTF8ToSingleByte(s,UTF8CharConvFunc);
     if SetTargetCodePage then
       SetCodePage(TheResult, TargetCodePage, False);
   end;
end;

Func is complex, why it's inline?

It is not supposed to be used directly. It is supposed to be used by a few other functions, which are called a lot.

Mattias