Rust's affine typing solves precisely this problem as well. Your state change methods consume the previous state variable, making it a compile-time error to access it afterwards. Given code like in the article and assuming connect_unauthenticated takes self by value:

  let i1 = socks5_socket();
  let i2 = i1.connect_unauthenticated(proxy_addr);
  let i3 = i1.connect_tcp(addr);

Rust will complain:

  error[E0382]: use of moved value: `i1`
    |
  2 |     let i2 = i1.connect_unauthenticated(proxy_addr);
    |             - value moved here
  3 |     let i3 = i1.connect_tcp(addr);
    |              ^ value used here after move
    |
    = note: move occurs because `i1` has type `socks5_socket`, which does not implement the `Copy` trait

Yes, between traits and alias control, you have everything required to solve OP's problem safely and efficiently. The only remaining aspect, I feel, is aesthetic, as the errors show; they show aliasing/ownership errors, but not indicative of operations on a single state machine.

That is really marvelous. How is this kind of thing implemented under the hood?

That new wizkid language Zig by Andrew Kelly - I wonder if it has anything similar?

You can actually do the type-checking part of this even in C, it's just uglier:

  struct sock5_blank i1 = socks5_socket();
  struct sock5_authed i2 = connect_unauthenticated(i1,proxy_addr);
  struct sock5_tcp i3 = connect_tcp(i1,addr); /* error: expected struct sock5_authed, got struct sock5_blank */

Rust adds the constraint that any (non-copyable,non-dropable) variable has to be passed to a function (or operator, or return statement) exactly once:

  Noncopyable a = ...;
  Copyable b = ...;
  f(a,b); //=> f(a,copy(&b)) // a is no longer in scope after this
  g(b); // this is fine, we took a reference to b and made a copy without changing the original
  h(a); // error: a is no longer a live variable

The problem with the C solution is that you'll get hanging pointers. Try using i1 after the code above executes and you'll get a runtime error.

Er, yes; that's what I said (or rather, hanging various things including but not limited to pointers). C only supported the typechecking part of this because it doesn't have any facilities for compile-time linearity checking.

Linearity[0] and dependency[1] checking is what makes Rust a improvement over C, rather than yet another "let's reinvent C++, but slightly less awful" project.

0: "error: no implementation for copy〈T〉","error: no implementation for drop〈T〉"[2]

1: "error: cannot move/destroy x (of type T); y (of type T&'x) outlives it"

You can use C++ unique_ptr and turn on Clang warnings about "use after move", if you want that level of protection.

I think this is a feature of Rust's ownership model.