[Observation] Forward availability and defines to extensions

lib/Macros/Sources/ObservationMacros/Availability.swift

@@ -0,0 +1,120 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+import SwiftSyntax
+import SwiftSyntaxMacros
+import SwiftDiagnostics
+import SwiftOperators
+import SwiftSyntaxBuilder
+
+
+extension AttributeSyntax {
+  var availability: AttributeSyntax? {
+    if attributeName.identifier == "available" {
+      return self
+    } else {
+      return nil
+    }
+  }
+}
+
+extension IfConfigClauseSyntax.Elements {
+  var availability: IfConfigClauseSyntax.Elements? {
+    switch self {
+    case .attributes(let attributes):
+      if let availability = attributes.availability {
+        return .attributes(availability)
+      } else {
+        return nil
+      }
+    default:
+      return nil
+    }
+  }
+}
+
+extension IfConfigClauseSyntax {
+  var availability: IfConfigClauseSyntax? {
+    if let availability = elements?.availability {
+      return with(\.elements, availability)
+    } else {
+      return nil
+    }
+  }
+
+  var clonedAsIf: IfConfigClauseSyntax {
+    detached.with(\.poundKeyword, .poundIfKeyword())
+  }
+}
+
+extension IfConfigDeclSyntax {
+  var availability: IfConfigDeclSyntax? {
+    var elements = [IfConfigClauseListSyntax.Element]()
+    for clause in clauses {
+      if let availability = clause.availability {
+        if elements.isEmpty {
+          elements.append(availability.clonedAsIf)
+        } else {
+          elements.append(availability)
+        }
+      }
+    }
+    if elements.isEmpty {
+      return nil
+    } else {
+      return with(\.clauses, IfConfigClauseListSyntax(elements))
+    }
+
+  }
+}
+
+extension AttributeListSyntax.Element {
+  var availability: AttributeListSyntax.Element? {
+    switch self {
+    case .attribute(let attribute):
+      if let availability = attribute.availability {
+        return .attribute(availability)
+      }
+    case .ifConfigDecl(let ifConfig):
+      if let availability = ifConfig.availability {
+        return .ifConfigDecl(availability)
+      }
+    default:
+      break
+    }
+    return nil
+  }
+}
+
+extension AttributeListSyntax {
+  var availability: AttributeListSyntax? {
+    var elements = [AttributeListSyntax.Element]()
+    for element in self {
+      if let availability = element.availability {
+        elements.append(availability)
+      }
+    }
+    if elements.isEmpty {
+      return nil
+    }
+    return AttributeListSyntax(elements)
+  }
+}
+
+extension DeclGroupSyntax {
+  var availability: AttributeListSyntax? {
+    if let attributes {
+      return attributes.availability
+    } else {
+      return nil
+    }
+  }
+}

lib/Macros/Sources/ObservationMacros/CMakeLists.txt

@@ -11,8 +11,9 @@
 #===----------------------------------------------------------------------===#
 
 add_swift_macro_library(ObservationMacros
-  ObservableMacro.swift
+  Availability.swift
   Extensions.swift
+  ObservableMacro.swift
   SWIFT_DEPENDENCIES
     SwiftSyntax::SwiftDiagnostics
     SwiftSyntax::SwiftOperators

lib/Macros/Sources/ObservationMacros/Extensions.swift

@@ -11,10 +11,9 @@
 
 import SwiftSyntax
 import SwiftSyntaxMacros
-
-@_implementationOnly import SwiftDiagnostics
-@_implementationOnly import SwiftOperators
-@_implementationOnly import SwiftSyntaxBuilder
+import SwiftDiagnostics
+import SwiftOperators
+import SwiftSyntaxBuilder
 
 extension VariableDeclSyntax {
   var identifierPattern: IdentifierPatternSyntax? {

lib/Macros/Sources/ObservationMacros/ObservableMacro.swift

@@ -11,10 +11,9 @@
 
 import SwiftSyntax
 import SwiftSyntaxMacros
-
-@_implementationOnly import SwiftDiagnostics
-@_implementationOnly import SwiftOperators
-@_implementationOnly import SwiftSyntaxBuilder
+import SwiftDiagnostics
+import SwiftOperators
+import SwiftSyntaxBuilder
 
 public struct ObservableMacro {
   static let moduleName = "Observation"
@@ -282,12 +281,15 @@ extension ObservableMacro: ExtensionMacro {
     }
 
     let decl: DeclSyntax = """
-      extension \(raw: type.trimmedDescription): \(raw: qualifiedConformanceName) {}
-      """
+        extension \(raw: type.trimmedDescription): \(raw: qualifiedConformanceName) {}
+        """
+    let ext = decl.cast(ExtensionDeclSyntax.self)
 
-    return [
-      decl.cast(ExtensionDeclSyntax.self)
-    ]
+    if let availability = declaration.availability {
+      return [ext.with(\.attributes, availability)]
+    } else {
+      return [ext]
+    }
   }
 }
 

test/stdlib/Observation/Observable.swift

@@ -202,6 +202,16 @@ class RecursiveOuter {
   }
 }
 
+@Observable
+#if FOO
+@available(SwiftStdlib 5.9, *)
+#elseif BAR
+@available(SwiftStdlib 5.9, *)
+#else
+#endif
+class GuardedAvailability {
+}
+
 @main
 struct Validator {
   @MainActor