There's nothing special about String other than it's in the standard library. It doesn't even have special cased syntax (&str does have special syntax, it doesn

There's nothing special about String other than it's in the standard library. It doesn't even have special cased syntax (&str does have special syntax, it doesn't allocate). String does need to know how to deallocate itself, so the compiler has to know what allocated it. It can be moved between threads, so a thread local allocator doesn't work. If you want a String type which is allocated differently, or which stores a pointer to its deallocate, you can easily implement that as a new type by hand. (The standard library types might some day become generic over allocator, which would allow statically using a different allocator with the same api. The compiler would then force you to keep track of which allocator your string was allocated with).