Sealed Traits

The sealed trait pattern creates a closed set of types that external code cannot extend. A public trait requires implementing a private supertrait, so only types within your crate can satisfy the bound:

mod private { pub trait Sealed {} }

pub trait ValidState: private::Sealed { /* ... */ }

struct Active;
struct Suspended;

impl private::Sealed for Active {}
impl private::Sealed for Suspended {}
impl ValidState for Active {}
impl ValidState for Suspended {}

// External code cannot create new states — the set is closed

This is essential for the typestate-pattern when you want to guarantee that only your defined states exist. Combined with From and TryFrom for type-safe conversions, sealed traits let you build compile-time verified domain models where the valid state space is exactly what you define. See type-driven-development for the full methodology.