AST: Emit @_backDeploy function bodies into .swiftinterface files.

Resolves rdar://88650341

Author: tshortli

include/swift/AST/DeclContext.h

@@ -201,6 +201,7 @@ struct FragileFunctionKind {
     AlwaysEmitIntoClient,
     DefaultArgument,
     PropertyInitializer,
+    BackDeploy,
     None
   };
 

lib/AST/DeclContext.cpp

@@ -320,6 +320,7 @@ ResilienceExpansion DeclContext::getResilienceExpansion() const {
   case FragileFunctionKind::AlwaysEmitIntoClient:
   case FragileFunctionKind::DefaultArgument:
   case FragileFunctionKind::PropertyInitializer:
+  case FragileFunctionKind::BackDeploy:
     return ResilienceExpansion::Minimal;
   case FragileFunctionKind::None:
     return ResilienceExpansion::Maximal;
@@ -418,6 +419,11 @@ swift::FragileFunctionKindRequest::evaluate(Evaluator &evaluator,
                 /*allowUsableFromInline=*/true};
       }
 
+      if (AFD->getAttrs().hasAttribute<BackDeployAttr>()) {
+        return {FragileFunctionKind::BackDeploy,
+                /*allowUsableFromInline=*/true};
+      }
+
       // If a property or subscript is @inlinable or @_alwaysEmitIntoClient,
       // the accessors are @inlinable or @_alwaysEmitIntoClient also.
       if (auto accessor = dyn_cast<AccessorDecl>(AFD)) {

lib/AST/TypeCheckRequests.cpp

@@ -560,6 +560,9 @@ void swift::simple_display(llvm::raw_ostream &out,
   case FragileFunctionKind::PropertyInitializer:
     out << "propertyInitializer";
     break;
+  case FragileFunctionKind::BackDeploy:
+    out << "backDeploy";
+    break;
   case FragileFunctionKind::None:
     out << "none";
     break;

test/ModuleInterface/back-deploy-attr.swift

@@ -17,27 +17,24 @@
 
 public struct TopLevelStruct {
   // CHECK: @_backDeploy(macOS 11.0)
-  // CHECK: public func backDeployedFunc1_SinglePlatform() -> Swift.Int
+  // FROMSOURCE: public func backDeployedFunc1_SinglePlatform() -> Swift.Int { return 42 }
+  // FROMMODULE: public func backDeployedFunc1_SinglePlatform() -> Swift.Int
   @available(macOS 12.0, *)
   @_backDeploy(macOS 11.0)
-  public func backDeployedFunc1_SinglePlatform() -> Int {
-    return 42
-  }
+  public func backDeployedFunc1_SinglePlatform() -> Int { return 42 }
 
   // CHECK: @_backDeploy(macOS 11.0)
   // CHECK: @_backDeploy(iOS 14.0)
-  // CHECK: public func backDeployedFunc2_MultiPlatform() -> Swift.Int
+  // FROMSOURCE: public func backDeployedFunc2_MultiPlatform() -> Swift.Int { return 43 }
+  // FROMMODULE: public func backDeployedFunc2_MultiPlatform() -> Swift.Int
   @available(macOS 12.0, *)
   @_backDeploy(macOS 11.0, iOS 14.0)
-  public func backDeployedFunc2_MultiPlatform() -> Int {
-    return 43
-  }
+  public func backDeployedFunc2_MultiPlatform() -> Int { return 43 }
 }
 
 // CHECK: @_backDeploy(macOS 11.0)
-// CHECK: public func backDeployTopLevelFunc() -> Swift.Int
+// FROMSOURCE: public func backDeployTopLevelFunc() -> Swift.Int { return 44 }
+// FROMMODULE: public func backDeployTopLevelFunc() -> Swift.Int
 @available(macOS 12.0, *)
 @_backDeploy(macOS 11.0)
-public func backDeployTopLevelFunc() -> Int {
-  return 42
-}
+public func backDeployTopLevelFunc() -> Int { return 44 }