[Prebuilt Module Cache] Add an environment variable overload to specify location of the prebuilt module cache

Useful for allowing e.g. CI systems to share this parameter without needing to plumb through a new compiler flag everywhere

Resolves rdar://135033114

Author: artemcm

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -29,6 +29,7 @@
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Option/Option.h"
+#include "llvm/Support/Process.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/LineIterator.h"
 #include "llvm/Support/Path.h"
@@ -66,6 +67,11 @@ bool ArgsToFrontendOptionsConverter::convert(
   if (const Arg *A = Args.getLastArg(OPT_prebuilt_module_cache_path)) {
     Opts.PrebuiltModuleCachePath = A->getValue();
   }
+  if (auto envPrebuiltModuleCachePath =
+      llvm::sys::Process::GetEnv("SWIFT_OVERLOAD_PREBUILT_MODULE_CACHE_PATH")) {
+    Opts.PrebuiltModuleCachePath = *envPrebuiltModuleCachePath;
+  }
+
   if (const Arg *A = Args.getLastArg(OPT_module_cache_path)) {
     Opts.ExplicitModulesOutputPath = A->getValue();
   } else {

test/ModuleInterface/swift_build_sdk_interfaces/compiler-uses-prebuilt-envvar.swift

@@ -0,0 +1,59 @@
+// RUN: %empty-directory(%t)
+// RUN: mkdir -p %t/sdk/usr/lib/swift/Normal.swiftmodule
+// RUN: mkdir -p %t/sdk/System/Library/Frameworks/FMWK.framework/Modules/FMWK.swiftmodule
+
+// RUN: echo 'public func normal() {}' | %target-swift-frontend - -emit-module-interface-path %t/sdk/usr/lib/swift/Normal.swiftmodule/%target-swiftinterface-name -emit-module -o /dev/null -module-name Normal
+// RUN: echo 'public func flat() {}' | %target-swift-frontend - -emit-module-interface-path %t/sdk/usr/lib/swift/Flat.swiftinterface -emit-module -o /dev/null -module-name Flat
+// RUN: echo 'public func fmwk() {}' | %target-swift-frontend - -emit-module-interface-path %t/sdk/System/Library/Frameworks/FMWK.framework/Modules/FMWK.swiftmodule/%target-swiftinterface-name -emit-module -o /dev/null -module-name FMWK
+
+// RUN: %swift_build_sdk_interfaces -sdk %t/sdk -Fsystem %t/sdk/System/Library/Frameworks -v -o %t/prebuilt
+// RUN: ls %t/prebuilt | %FileCheck %s
+// CHECK-DAG: Normal.swiftmodule
+// CHECK-DAG: Flat.swiftmodule
+// CHECK-DAG: FMWK.swiftmodule
+
+// RUN: env SWIFT_OVERLOAD_PREBUILT_MODULE_CACHE_PATH=%t/prebuilt %target-typecheck-verify-swift -sdk %t/sdk -Fsystem %t/sdk/System/Library/Frameworks -I %t/sdk/usr/lib/swift/ -module-cache-path %t/MCP
+// RUN: %{python} %S/../ModuleCache/Inputs/check-is-forwarding-module.py %t/MCP/*.swiftmodule
+
+// Touch a file in the SDK (to make it look like it changed) and try again.
+// This should still be able to use the prebuilt modules because they track
+// content hashes, not just size+mtime.
+// RUN: rm -rf %t/MCP
+// RUN: %{python} %S/../ModuleCache/Inputs/make-old.py %t/sdk/usr/lib/swift/Normal.swiftmodule/%target-swiftinterface-name
+// RUN: env SWIFT_OVERLOAD_PREBUILT_MODULE_CACHE_PATH=%t/prebuilt %target-typecheck-verify-swift -sdk %t/sdk -Fsystem %t/sdk/System/Library/Frameworks -I %t/sdk/usr/lib/swift/ -module-cache-path %t/MCP
+// RUN: ls %t/MCP/*.swiftmodule | %FileCheck -check-prefix CHECK-CACHE %s
+// RUN: %{python} %S/../ModuleCache/Inputs/check-is-forwarding-module.py %t/MCP/*.swiftmodule
+
+// CHECK-CACHE-DAG: Normal-{{.+}}.swiftmodule
+// CHECK-CACHE-DAG: Flat-{{.+}}.swiftmodule
+// CHECK-CACHE-DAG: FMWK-{{.+}}.swiftmodule
+
+// Actually change a file in the SDK, to check that we're tracking dependencies
+// at all.
+// RUN: rm -rf %t/MCP
+// RUN: echo "public func another()" >> %t/sdk/usr/lib/swift/Normal.swiftmodule/%target-swiftinterface-name
+// RUN: env SWIFT_OVERLOAD_PREBUILT_MODULE_CACHE_PATH=%t/prebuilt %target-typecheck-verify-swift -sdk %t/sdk -Fsystem %t/sdk/System/Library/Frameworks -I %t/sdk/usr/lib/swift/ -module-cache-path %t/MCP
+// RUN: ls %t/MCP/*.swiftmodule | %FileCheck -check-prefix CHECK-CACHE %s
+// RUN: not %{python} %S/../ModuleCache/Inputs/check-is-forwarding-module.py %t/MCP/Normal-*.swiftmodule
+// RUN: %{python} %S/../ModuleCache/Inputs/check-is-forwarding-module.py %t/MCP/Flat-*.swiftmodule
+// RUN: %{python} %S/../ModuleCache/Inputs/check-is-forwarding-module.py %t/MCP/FMWK-*.swiftmodule
+
+// Without the prebuilt cache everything should still work; it'll just take time
+// because we have to build the interfaces.
+// RUN: rm -rf %t/MCP
+// RUN: %target-typecheck-verify-swift -sdk %t/sdk -Fsystem %t/sdk/System/Library/Frameworks -I %t/sdk/usr/lib/swift/ -module-cache-path %t/MCP
+// RUN: ls %t/MCP/*.swiftmodule | %FileCheck -check-prefix CHECK-CACHE %s
+// RUN: not %{python} %S/../ModuleCache/Inputs/check-is-forwarding-module.py %t/MCP/Normal-*.swiftmodule
+// RUN: not %{python} %S/../ModuleCache/Inputs/check-is-forwarding-module.py %t/MCP/Flat-*.swiftmodule
+// RUN: not %{python} %S/../ModuleCache/Inputs/check-is-forwarding-module.py %t/MCP/FMWK-*.swiftmodule
+
+
+import Normal
+import Flat
+import FMWK
+
+func test() {
+  normal()
+  flat()
+  fmwk()
+}