Rust doens't have copy constructors, to be clear.

> A language with copy constructors doesn't prevent you from putting a bug in them.

If you don't override it, it will copy everything properly.

> Rust might have put a 0 in the uninitialized member

Rust does not do that. If a member is not initialised, it's uninitialised and in this case the structure will be rejected. Here even if you could not just have derived Clone (struct bio looks non-trivial) your hand-rolled implementation would have complained that you had not properly initialised the member.

> If you don't override it, it will copy everything properly.

Sure, so does memcpy or bare struct assignment in C. The problem is that trivial copy constructors are relatively rare.

> Here even if you could not just have derived Clone (struct bio looks non-trivial) your hand-rolled implementation would have complained that you had not properly initialised the member.

Right, but this is more similar to an assignment operator. DerefMut in Rust I think wouldn't have caught the bug.

> Right, but this is more similar to an assignment operator.

Only if you don't zoom out. If you do, this is alloc/init/fill which in C++ or Rust would use a regular copy and RVO.