There is nothing difficult with that as long as the GC knows everything about strings and their pointers so it can detect that a particular string is reachable

There is nothing difficult with that as long as the GC knows everything about strings and their pointers so it can detect that a particular string is reachable only through single another string. Right, but that is a specific case where a built-in data type is optimized. It's not difficult, because the GC knows the semantics of a specific data type. But this does not work in the general case of slicing of library/used-defined data types.