`unsafe` is the part where the compiler trusts you to uphold your own invariants, necessary to prevent Unsoundness. For example:
- unsafe fn get_unchecked(index) - compiler believes you will ensure index < length.
- unsafe fn set_capacity(capacity) - compiler trusts you will not set capacity to value that will cause UB. Even if its code boils essentially to set a field - which is safe according to Rust, but may invalidate other invariants preserving soundness.
