Optionally build the compiler and _Concurrency library for back deployment.

When enabling the build of the back-deployable concurrency library via
the build-script option `--back-deploy-concurrency`, also build the
compiler and (main) concurrency library to support older deployment targets.

Building the compiler for older deployment targets is effectively the
same as implicitly passing
`-Xfrontend -enable-experimental-back-deploy-concurrency`. That option
should probably go away.

Building the primary _Concurrency library for back-deployment means
setting the "SwiftStdlib 5.5" availability back to the earlier
deployment targets. This should have no effect on how the _Concurrency
library binary is built, but it does ensure that the right
availability annotations are in the _Concurrency module.

Author: DougGregor

include/swift/Basic/LangOptions.h

@@ -302,7 +302,8 @@ namespace swift {
     bool EnableExperimentalConcurrency = false;
 
     /// Enable experimental back-deployment of the concurrency model.
-    bool EnableExperimentalBackDeployConcurrency = false;
+    bool EnableExperimentalBackDeployConcurrency =
+      SWIFT_ALLOW_BACK_DEPLOY_CONCURRENCY;
 
     /// Enable experimental support for named opaque result types, e.g.
     /// `func f() -> <T> T`.

include/swift/Config.h.in

@@ -14,4 +14,6 @@
 
 #cmakedefine01 SWIFT_ENABLE_GLOBAL_ISEL_ARM64
 
+#cmakedefine01 SWIFT_ALLOW_BACK_DEPLOY_CONCURRENCY
+
 #endif // SWIFT_CONFIG_H

stdlib/public/BackDeployConcurrency/CMakeLists.txt

@@ -41,6 +41,5 @@ add_compile_definitions(SWIFT_CONCURRENCY_BACK_DEPLOYMENT)
 set(swift_concurrency_swift_flags -Xfrontend -enable-experimental-back-deploy-concurrency)
 set(swift_concurrency_install_component back-deployment)
 set(swift_concurrency_options BACK_DEPLOYMENT_LIBRARY 5.5)
-set(swift_concurrency_availability "macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0")
 
 add_subdirectory(../Concurrency stdlib/public/BackDeployConcurrency)

stdlib/public/Concurrency/CMakeLists.txt

@@ -13,7 +13,9 @@
 set(LLVM_OPTIONAL_SOURCES
   ${swift_concurrency_objc_sources})
 
-if(NOT swift_concurrency_availability)
+if (SWIFT_ALLOW_BACK_DEPLOY_CONCURRENCY OR BUILD_SWIFT_CONCURRENCY_BACK_DEPLOYMENT_LIBRARIES)
+  set(swift_concurrency_availability "macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0")
+else()
   set(swift_concurrency_availability "macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0")
 endif()
 

utils/swift_build_support/swift_build_support/build_script_invocation.py

@@ -233,6 +233,8 @@ def convert_to_impl_arguments(self):
 
         if args.swift_disable_dead_stripping:
             args.extra_cmake_options.append('-DSWIFT_DISABLE_DEAD_STRIPPING:BOOL=TRUE')
+        if args.build_backdeployconcurrency:
+            args.extra_cmake_options.append('-DSWIFT_ALLOW_BACK_DEPLOY_CONCURRENCY:BOOL=TRUE')
 
         # Then add subproject install flags that either skip building them /or/
         # if we are going to build them and install_all is set, we also install