Pattern 14: Compile-Time Validation

Context

Invariants (tick budgets, guard limits, feature combinations) should fail fast during compilation.

Problem

Runtime checks add overhead and can be bypassed in rarely executed code paths. Missing an invariant leads to subtle production bugs.

Solution

Push validation to compile time with const generics, type-level markers, and const_assert!. When runtime validation is unavoidable, encapsulate it in constructors that return Result, making misuse impossible through types.

Forces

• Safety vs. flexibility: some values remain runtime, but defaults should be encoded in types
• Compile time vs. ergonomics: const generics expose parameters without macros
• Diagnostics vs. noise: compile errors must explain the invariant succinctly

Examples

#![allow(unused)]
fn main() {
pub const fn assert_tick_budget(ticks: u64) {
    const_assert!(ticks <= 8);
}

pub struct SizeValidatedArray<const SIZE: usize, const MAX: usize> {
    data: [u8; SIZE],
    _marker: PhantomData<[u8; MAX]>,
}
}

Related Patterns

• Pattern 11: Zero-Cost Abstractions
• Pattern 15: Type State Enforcement
• Pattern 18: Timeout Defense in Depth