design-patterns-rust

Use a consuming builder where each method takes self and returns Self, ending with a build(self) method that consumes the builder. This prevents reuse bugs via move semantics; a non-consuming &mut self variant allows reconfiguration but requires clone() in build().

Use enums when all variants are known at compile time — they are roughly 10x faster with no vtable and contiguous memory. Use Box<dyn Trait> when third-party extensibility is needed, since new types can implement the trait anywhere.

Use LazyLock for initialization on first access (Rust 1.80+) or OnceLock for explicit set-then-get semantics. Since a static requires Sync, wrap mutable state in Mutex or RwLock; prefer dependency injection where possible.

Not directly for broadcast — mpsc is multi-producer, single-consumer, so each event reaches only one receiver. Keep one Sender per observer for fan-out, or use tokio::sync::broadcast or crossbeam-channel for true pub/sub.

Cloning to appease the borrow checker hides underlying ownership problems and adds runtime cost. The fix is to restructure borrows, decompose structs into independent parts, or use mem::take and mem::replace to move values without cloning.

Typestate encodes valid state transitions in the type system using PhantomData<State> marker types and per-state impl blocks. Invalid transitions become compile errors with zero runtime cost, since PhantomData is zero-sized.