[Caching] Support index while building when caching on

Model indexing output as an optional output from the swift compiler
as the build system has no knowledge about them and they can be
regenerated by indexer. Make sure the indexing store output is produced
when cache hit so the compilation is done for the module. If cache hit,
no indexing data is produced since no compilation is done.

rdar://123331335

Author: cachemeifyoucan

include/swift/Option/Options.td

@@ -1454,7 +1454,7 @@ def index_file_path : Separate<["-"], "index-file-path">,
   MetaVarName<"<path>">;
 
 def index_store_path : Separate<["-"], "index-store-path">,
-  Flags<[FrontendOption, ArgumentIsPath]>, MetaVarName<"<path>">,
+  Flags<[FrontendOption, ArgumentIsPath, CacheInvariant]>, MetaVarName<"<path>">,
   HelpText<"Store indexing data to <path>">;
 
 def index_unit_output_path : Separate<["-"], "index-unit-output-path">,

lib/ClangImporter/ClangImporter.cpp

@@ -1110,6 +1110,10 @@ std::optional<std::vector<std::string>> ClangImporter::getClangCC1Arguments(
     // the knowledge about clang command-line options.
     if (ctx.CASOpts.EnableCaching)
       CI->getCASOpts() = ctx.CASOpts.CASOpts;
+
+    // Forward the index store path. That information is not passed to scanner
+    // and it is cached invariant so we don't want to re-scan if that changed.
+    CI->getFrontendOpts().IndexStorePath = ctx.ClangImporterOpts.IndexStorePath;
   } else {
     // Otherwise, create cc1 arguments from driver args.
     auto driverArgs = getClangDriverArguments(ctx, ignoreClangTarget);
@@ -3979,6 +3983,9 @@ ClangImporter::getSwiftExplicitModuleDirectCC1Args() const {
   FEOpts.IncludeTimestamps = false;
   FEOpts.ModuleMapFiles.clear();
 
+  // IndexStorePath is forwarded from swift.
+  FEOpts.IndexStorePath.clear();
+
   // PreprocessorOptions.
   // Cannot clear macros as the main module clang importer doesn't have clang
   // include tree created and it has to be created from command-line. However,

test/CAS/index-store.swift

@@ -0,0 +1,55 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %target-swift-frontend -emit-module %t/A.swift -I %t -module-name A \
+// RUN:   -disable-implicit-string-processing-module-import -disable-implicit-concurrency-module-import -parse-stdlib \
+// RUN:   -enable-library-evolution -swift-version 5 \
+// RUN:   -emit-module-interface-path %t/A.swiftinterface \
+// RUN:   -o %t/A.swiftmodule
+
+// RUN: %target-swift-frontend -scan-dependencies -module-name Test -O -module-cache-path %t/clang-module-cache \
+// RUN:   -disable-implicit-string-processing-module-import -disable-implicit-concurrency-module-import -parse-stdlib \
+// RUN:   %t/test.swift -I %t -o %t/deps.json -cache-compile-job -cas-path %t/cas
+
+// RUN: %{python} %S/Inputs/BuildCommandExtractor.py %t/deps.json clang:B > %t/B.cmd
+// RUN: %swift_frontend_plain @%t/B.cmd
+
+// RUN: %{python} %S/Inputs/BuildCommandExtractor.py %t/deps.json A > %t/A.cmd
+// RUN: %swift_frontend_plain @%t/A.cmd
+
+// RUN: %{python} %S/Inputs/GenerateExplicitModuleMap.py %t/deps.json > %t/map.json
+// RUN: llvm-cas --cas %t/cas --make-blob --data %t/map.json > %t/map.casid
+// RUN: %{python} %S/Inputs/BuildCommandExtractor.py %t/deps.json Test > %t/MyApp.cmd
+
+// RUN: %target-swift-frontend -cache-compile-job -Rcache-compile-job %t/test.swift -O -emit-module -o %t/Test.swiftmodule \
+// RUN:  -disable-implicit-string-processing-module-import -disable-implicit-concurrency-module-import -parse-stdlib \
+// RUN:  -disable-implicit-swift-modules -explicit-swift-module-map-file @%t/map.casid \
+// RUN:  -module-name Test -cas-path %t/cas @%t/MyApp.cmd -index-system-modules -index-store-path %t/db 2>&1 | %FileCheck --check-prefix=CACHE-MISS %s
+// RUN: ls %t/db
+
+/// Cache hit with a different index-store-path. Note cache hit will skip replay index data.
+// RUN: %target-swift-frontend -cache-compile-job -Rcache-compile-job %t/test.swift -O -emit-module -o %t/Test.swiftmodule \
+// RUN:  -disable-implicit-string-processing-module-import -disable-implicit-concurrency-module-import -parse-stdlib \
+// RUN:  -disable-implicit-swift-modules -explicit-swift-module-map-file @%t/map.casid \
+// RUN:  -module-name Test -cas-path %t/cas @%t/MyApp.cmd -index-system-modules -index-store-path %t/db2 2>&1 | %FileCheck --check-prefix=CACHE-HIT %s
+// RUN: not ls %t/db2
+
+// CACHE-MISS: remark: cache miss for input
+// CACHE-HIT: remark: replay output file
+
+//--- test.swift
+import A
+import B
+func test() {}
+
+//--- A.swift
+func a() {}
+
+//--- module.modulemap
+module B {
+  header "B.h"
+  export *
+}
+
+//--- B.h
+void b(void);