[Observation] Add property definite initialization support

Author: phausler

lib/Macros/Sources/ObservationMacros/ObservableMacro.swift

@@ -206,9 +206,6 @@ extension ObservableMacro: MemberMacro {
     let storedInstanceVariables = declaration.definedVariables.filter { $0.isValidForObservation }
     for property in storedInstanceVariables {
       if property.hasMacroApplication(ObservableMacro.ignoredMacroName) { continue }
-      if property.initializer == nil {
-        context.addDiagnostics(from: DiagnosticsError(syntax: property, message: "@Observable requires property '\(property.identifier?.text ?? "")' to have an initial value", id: .missingInitializer), node: property)
-      }
       let storage = DeclSyntax(property.privatePrefixed("_", addingAttribute: ObservableMacro.ignoredAttribute))
       declaration.addIfNeeded(storage, to: &declarations)
 
@@ -293,6 +290,13 @@ public struct ObservationTrackedMacro: AccessorMacro {
       return []
     }
 
+    let initAccessor: AccessorDeclSyntax =
+      """
+      init(newValue, initializes: _\(identifier)) {
+        _\(identifier) = newValue
+      }
+      """
+
     let getAccessor: AccessorDeclSyntax =
       """
       get {
@@ -310,7 +314,7 @@ public struct ObservationTrackedMacro: AccessorMacro {
       }
       """
 
-    return [getAccessor, setAccessor]
+    return [initAccessor, getAccessor, setAccessor]
   }
 }
 

test/stdlib/Observation/Observable.swift

@@ -140,6 +140,22 @@ protocol Intermediary: Observable { }
 @Observable
 class HasIntermediaryConformance: Intermediary { }
 
+@Observable
+class HasInitialization {
+  var field1: String
+  var field2: Int
+
+  init(field1: String, field2: Int) {
+    self.field1 = field1
+    self.field2 = field2
+  }
+
+  init(a: String, b: Int) {
+    field1 = a
+    field2 = b
+  }
+}
+
 class CapturedState<State>: @unchecked Sendable {
   var state: State