rust-const_skill

Overview

This skill is an expert guide to Rust const generics and compile-time computation, covering const fn, type-level computation, MaybeUninit arrays, and compile-time validation. It distills practical patterns, pitfalls, and verification steps so you can safely move work into compile-time when it improves correctness or performance. The content focuses on concrete examples and decision rules for real projects.

How this skill works

It inspects code for opportunities to use const evaluation and selects patterns: const generics for fixed-size types, const fn for pure computations, MaybeUninit + Box for large arrays, and PhantomData/type-state for compile-time state. The skill maps findings to safe implementations, compile-time assertions, and verification commands to validate behavior during builds and tests.

When to use it

• When a value or size is known at compile time and yields zero runtime cost
• When you need compile-time validation to fail fast (configuration invariants)
• When building fixed-size buffers, arrays, or type-level state machines
• When avoiding runtime allocation for small compile-time constants
• When moving pure computations to compile time for optimization

Best practices

• Prefer const fn for pure, deterministic computations that require no heap or I/O
• Use MaybeUninit + Box for very large arrays to avoid stack overflow
• Assert configuration invariants in const contexts to produce compile-time errors
• Keep const generics bounded and consistent to avoid confusing type mismatches
• Avoid overusing const where readability and maintainability suffer

Example use cases

• Defining a Buffer<T, const N: usize> for fixed-size data to avoid runtime size checks
• Computing table values (e.g., fibonacci) at compile time using const fn for fast lookup
• Allocating a 1MB byte array on the heap via Box<[u8; N]> or MaybeUninit to prevent stack overflow
• Enforcing config constraints with const assertions (size > 0, alignment is power of two)
• Implementing a type-level state machine with PhantomData and const generics to restrict APIs