Type state pattern for compile-time state machines

The typestate pattern uses Rust's type system to encode state machines, making invalid states unrepresentable. Each state is a separate type, and transitions consume self and return a new state. The compiler prevents calling methods that aren't valid for the current state. I use this for protocols, workflows, and resource initialization. For example, a Connection can be Closed, Open, or Authenticated, and only Authenticated can send messages. The pattern is zero-cost: states are phantom types that disappear at runtime. It's more verbose than runtime state machines, but it catches logic errors at compile time. This is especially valuable for security-critical code where state transitions must be enforced.