# Solution 3: Typestate Pattern using Marker Types

A yet better solution is to utilize the functionality of Rust to be able to do different implementations on a struct depending on type parameters.

We simply introduce two empty marker types `Locked` and `Unlocked`. Then, we pass the marker type as type parameter to the `CredentialManager` struct. Because Rust enforces the usage of type parameters, we need the type parameter to appear in a field for Rust to accept it. We could use `state: Locked` directly, since `Locked` is zero-sized and this compiles fine and costs nothing at runtime. But [`PhantomData<T>`](https://doc.rust-lang.org/nightly/nomicon/phantom-data.html) is preferred by convention because it decouples the type-level marker from needing an actual constructible value of `T`, doesn't silently grow the struct's size if `T` is later changed to something non-zero-sized, and lets you control drop-check/variance/auto-trait behavior independently.

So in this solution, we get the exact same behavior as with the last solution, but with absolutely no extra cost at runtime, which perfectly utilizes the [zero cost abstraction](https://doc.rust-lang.org/beta/embedded-book/static-guarantees/zero-cost-abstractions.html) principle of Rust.