is blocked by:

• SR-260 Add availability information to @_fixed_layout attribute
• SR-256 [Resilience] Resilient case numbering
• SR-258 Resilient class layout
• SR-259 Resiliently-typed global variables
• SR-261 Plumb ResilienceExpansion through SIL type lowering
• SR-262 Write before/after tests for resilience
• SR-264 Sort out lazy emission of accessors for fixed-layout structs
• SR-265 Sort resilient enum cases, protocol requirements, virtual methods, etc by availability
• SR-267 Don't build standard library with -sil-serialize-all
• SR-268 Add build flag for building standard library with resilience enabled
• SR-269 Implement ResilienceDomain abstraction
• SR-263 Implement a general solution to prevent deadlocks when generic types rely on their own metadata recursively

Issue Description:

The goal here is to make client code resilient to changes in the internal representation of library types. In particular, we want the following changes to be resilient by default:

• Adding a public or internal stored property to a struct

• Removing a private or internal stored property from a struct, as long as it is not used from @transparent functions

• Changing a struct stored property into a computed property, or vice versa

• Adding a payload or no-payload case to an enum

• Removing an internal case from an enum (not supported by Sema, but the calling convention changes are independent of that)

• Adding a public or internal stored property to a class

• Removing a private or internal stored property from a class, as long as it is not used from @transparent functions

• Changing a class stored property into a computed property, or vice versa

• Changing a global variable from a stored to computed or vice versa

To allow optimization, the @_fixed_layout attribute communicates the intent that the layout of a type is part of the interface and will not change. The layout is defined as follows:

• For structs, the ordering and types of all stored properties must never change

• For enums, the total set of all payload and no-payload cases must never change

Note that @_fixed_layout types can contain resiliently-typed properties or payloads, and have a dynamic size as a result.