Type properties:

CanDuplicate.
Copies of a value can be made by simply memcpy'ing its contents.
+ i32
- shared_ptr (must increment ref count)

CanDiscard
A value can be discarded without invoking any special code.
+ i32
- shared_ptr (must decrement ref count)

CanRelocate
A value can be relocated onto a new address without invoking any special code.
+ shared_ptr
- small_vector (may hold a pointer to itself, must update the pointer)