That would be the unsoundness of `Vec` itself, but if the abstraction of `Vec` is sound, there would be no way to use `Vec` outside of `unsafe` that can cause memory unsafety.

The point coming back to abstraction and responsibility, in Rust, you can build abstraction that is sound and guarantee memory safety from there. There can be soundness bug inside your abstraction, but it will be a massively smaller surface for auditing and expert required to write such abstraction. Also, when soundness bug appears, the responsibility is solely on the abstraction writer, not the user.

Whereas in C, without those safe abstraction, the surface of doing thing right to avoid memory safety issue is your entire codebase, and responsibility of "holding the knife correctly" is on the user.