Zero-Cost 'Tagless Final' in Rust with GADT-style Enums

The article discusses how to implement the Tagless Final pattern in Rust while maintaining zero-cost performance, using Generalized Algebraic Data Types (GADTs) in place of Haskell's GADTs. By leveraging Rust's never type (!), it's possible to create a GADT-like enum where only one variant is constructible for any given type, eliminating the need for a runtime tag.

This enables the compiler to fully optimize the code, resulting in a performance comparable to direct assembly instructions. The key to this optimization lies in the Eval trait, which defines the evaluation logic as a simple match statement over the GADT, and the Attic and Cursor traits, which act as a type-level configuration system.

The article also illustrates how breaking the invariant, by replacing the never type with a concrete type, results in a loss of optimization due to the need for runtime checks and discriminants. The example code provided in the article demonstrates this technique in action, showcasing the ability to build expressive domain-specific languages (DSLs) in Rust with the performance guarantees expected of a systems language.