What is @frozen and why does it matter for enums?

@frozen is part of Swift’s library evolution / resilience model — the tradeoff between binary compatibility and performance for types shipped in a library with a stable ABI.

The problem: resilient types by default

When a library is built with library evolution enabled, every public type is resilient by default — clients can’t assume a fixed memory layout, because the library author might add fields/cases in a future version without breaking binary compatibility. For a resilient enum, clients cannot exhaustively switch without @unknown default.

switch error {
case .timeout: ...
case .notFound: ...
// ❌ compiler requires @unknown default here
}

What @frozen does

@frozen is the library author’s promise: this type’s shape is fixed forever. With that promise, the compiler can:

1. Drop the @unknown default requirement.
2. Bake the concrete layout directly into client code.
3. Enable more aggressive optimization/inlining.

Why this specifically matters for enums

A resilient struct mainly costs indirect field access. A resilient enum actively forces different source code — every switch needs a fallback case. Adding a new case to a @frozen enum is a binary-breaking, source-breaking change for every client that assumed exhaustiveness.

The tradeoff

	Non-frozen (resilient, default)	@frozen
Can add cases/fields later?	Yes	No — permanent commitment
Client must handle @unknown default?	Yes	No
Access performance	Indirect	Direct
Appropriate for	Types likely to evolve	Types conceptually closed forever

Where you’d actually see/use this

• Almost never write it yourself unless authoring a library with library evolution enabled.
• The standard library uses it extensively (Optional, Bool, Int).
• @unknown default is what you interact with as an app developer regularly.

One-liner if pressed: “@frozen is a library author’s binary-compatibility promise that a type’s shape is fixed forever, which lets the compiler drop resilient indirection and, for enums specifically, drop the @unknown default requirement; the cost is that adding a case later becomes a breaking change, so it’s reserved for types genuinely closed by design.”