understanding-stability-inference

Runtime stable means the class was compiled in a separate module, so the compiler emitted @StabilityInferred with a parameter bitmask and a $stable: Int field. The runtime ANDs that bitmask against the actual type arguments. It is a correct verdict, not a bug or an unstable classification.

Generic classes are classified as Stability.Parameter with a bitmask where bit i set means type argument Ti affects stability. For example Pair uses 0b11 and ImmutableList uses 0b1. At the call site, the bitmask is ANDed against the substituted argument stabilities to produce the final verdict.

The compiler's cycle detection phase conservatively returns unstable for recursive types like Node(children: List<Node>) to guarantee termination. The fix is annotating the recursive class with @Immutable and using ImmutableList, which hits the Known Stable Constructs registry before cycle detection matters.

Unknown means the compiler cannot decide, typically for interfaces and abstract classes, and the runtime falls back to identity (===) equality. Unstable means the compiler proved instability, such as a var property or an unstable field type, which disables skipping outright.

Yes. Both classify as Stability.Certain, but @Immutable promises the value never changes, letting the compiler promote property reads to static expressions and elide equality checks. @Stable only promises change notification, so equality probes are still emitted.

Skip it when no compiler report exists yet, when the fix is mechanical like changing var to val or List to ImmutableList, or when you want CI enforcement of stability. Those cases belong to the diagnosing, stabilizing, and CI-enforcement skills respectively.