The small vector class contains a static array of a few bytes. And as long as your data fits in it, no extra heap allocation is needed. A vector always does heap allocation, even if you only use a few bytes.

`SmallVec` allocates a contiguous array in the struct holding the vector itself (so it can be located on the stack), then if the capacity is insufficent, fallbacks to a heap-allocated `Vec` equivalent.

The `Vec` struct contains the capacity and used length integers, as well as a pointer to the heap-allocated storage.