Module recursion 

Unified recursion guard for cycle detection, depth limiting, and iteration bounding in recursive type computations.

Design

RecursionGuard replaces the scattered in_progress / visiting / depth / total_checks fields that were manually reimplemented across SubtypeChecker, TypeEvaluator, PropertyAccessEvaluator, and others.

It combines three safety mechanisms:

1. Cycle detection via a visiting set (FxHashSet<K>)
2. Depth limiting to prevent stack overflow
3. Iteration bounding to prevent infinite loops

Profiles

RecursionProfile provides named presets that eliminate magic numbers and make the intent of each guard clear at the call site:

// Before (what does 50, 100_000 mean?)
let guard = RecursionGuard::new(50, 100_000);

// After (intent is clear, limits are centralized)
let guard = RecursionGuard::with_profile(RecursionProfile::TypeEvaluation);

Safety

• Debug leak detection: In debug builds, dropping a guard with active entries triggers a panic, catching forgotten leave() calls.
• Debug double-leave detection: In debug builds, leaving a key that isn’t in the visiting set triggers a panic.
• Overflow protection: Iteration counting uses saturating arithmetic.
• Encapsulated exceeded state: The exceeded flag is private; use is_exceeded() and mark_exceeded().

Structs

DepthCounter

A lightweight depth counter for stack overflow protection.

RecursionGuard

Tracks recursion state for cycle detection, depth limiting, and iteration bounding.

Enums

RecursionProfile

Named recursion limit presets.

RecursionResult

Result of attempting to enter a recursive computation.