[swift-dev] Definite initialization implementation for property behaviors

[Joe Groff]

Hey Chris (and interested onlookers). I wanted your feedback on how to handle definite initialization with property behaviors. Behavior properties behave a lot like computed properties, so it's tricky to track their initialization behavior with our traditional storage-based analysis. For initialization purposes, the behavior property acts kind of like a proxy for the instantiated storage from the behavior declaration. For instance, if you have an `NSCopying` behavior, then an assignment that initializes the behavior property should trigger the `init` logic for the underlying storage, invoking `.copy()` on the initial value. I'm thinking we can represent this in raw SIL with a `mark_uninitialized_behavior` instruction that abstractly represents the behavior property, with references to its storage and its initialization/setter logic:

  // Build references to the behavior's initializer and setter
  %init = function_ref @NSCopying.init
  %set = function_ref @NSCopying.set
  // Get the address of the storage
  %storage = ref_element_addr %self, #Type.property.[NSCopying].storage
  // Build the initialization marker
  %property = mark_uninitialized_behavior %self, %storage, init %init, set %set
  ...
  // Initialization
  assign %a to %property
  // Reassignment
  assign %b to %property

DI would then turn `assign`-ments that initialize %property into `apply %init(%storage)`, and reassignments into `apply %set(%self)`. Does that seem like a reasonable implementation approach?

-Joe