SILGen: Add a SILGen test for the @_backDeploy attribute and fix parsing for the attribute so that -verify-syntax-tree passes. The SILGen test doesn't verify anything special yet since we're not emitting thunks yet at call sites.

Author: tshortli

lib/Parse/ParseDecl.cpp

@@ -2927,7 +2927,8 @@ bool Parser::parseNewDeclAttribute(DeclAttributes &Attributes, SourceLoc AtLoc,
     StringRef AttrName = "@_backDeploy";
     ParserStatus Status = parseList(tok::r_paren, LeftLoc, RightLoc, false,
                                     diag::attr_back_deploy_missing_rparen,
-                                    SyntaxKind::Unknown, [&]() -> ParserStatus {
+                                    SyntaxKind::AvailabilitySpecList,
+                                    [&]() -> ParserStatus {
       ParserStatus ListItemStatus =
           parsePlatformVersionInList(AttrName, PlatformAndVersions);
       if (ListItemStatus.isErrorOrHasCompletion())

test/SILGen/back_deploy_attribute.swift

@@ -0,0 +1,36 @@
+// RUN: %target-swift-emit-silgen %s | %FileCheck %s
+// RUN: %target-swift-emit-silgen %s -target %target-cpu-apple-macosx10.50 | %FileCheck %s
+// RUN: %target-swift-emit-silgen %s -target %target-cpu-apple-macosx10.60 | %FileCheck %s
+// REQUIRES: OS=macosx
+
+// CHECK-LABEL: sil [serialized] [available 10.51] [ossa] @$s21back_deploy_attribute0A12DeployedFuncyyF : $@convention(thin) () -> ()
+@available(macOS 10.51, *)
+@_backDeploy(macOS 10.50)
+public func backDeployedFunc() {}
+
+@available(macOS 10.50, *)
+public struct TopLevelStruct {
+  // CHECK-LABEL: sil [serialized] [available 10.51] [ossa] @$s21back_deploy_attribute14TopLevelStructV0a8DeployedF4FuncyyF : $@convention(method) (TopLevelStruct) -> ()
+  @available(macOS 10.51, *)
+  @_backDeploy(macOS 10.50)
+  public func backDeployedStructFunc() {}
+}
+
+
+// FIXME(backDeploy): Verify SIL in a caller that requires back deployment
+
+
+// CHECK-LABLEL: sil hidden [available 10.51] [ossa] @$s21back_deploy_attribute21alwaysAvailableCalleryyAA14TopLevelStructVF : $@convention(thin) (TopLevelStruct) -> ()
+// CHECK: bb0(%0 : $TopLevelStruct):
+@available(macOS 10.51, *)
+func alwaysAvailableCaller(_ s: TopLevelStruct) {
+  /// This function's availability meets the minimum availability of the APIs, so
+  /// no back deployment logic is required.
+
+  // CHECK: %2 = function_ref @$s21back_deploy_attribute0A12DeployedFuncyyF : $@convention(thin) () -> ()
+  // CHECK: %3 = apply %2() : $@convention(thin) () -> ()
+  backDeployedFunc()
+  // CHECK: %4 = function_ref @$s21back_deploy_attribute14TopLevelStructV0a8DeployedF4FuncyyF : $@convention(method) (TopLevelStruct) -> ()
+  // CHECK: %5 = apply %4(%0) : $@convention(method) (TopLevelStruct) -> ()
+  s.backDeployedStructFunc()
+}