For developer mode, this seems to be the same as safe_strcpy: we clobber the specified region at runtime. Otherwise, it skips the static CHECK_STRING_SIZE call.
I think this is meant to allow you to call it passing the address of an array whose size is less than the maxlength passed to safe_strcpy. CHECK_STRING_SIZE would normally trap on this because it expects either a string pointer, or an exact fit? Is that right? If so I'll add a comment to this effect -- and perhaps a plea not to use it in new code. -- Martin