[AST] Treat platform modules as non-user

Modules defined within the SDK are considered
non-user modules, extend this to any module found
within the parent platform directory if there is
one. This ensures we include modules such as
XCTest and Testing.

rdar://131854240

include/swift/AST/SearchPathOptions.h

@@ -380,6 +380,8 @@ private:
 
   std::optional<StringRef> SysRoot = std::nullopt;
 
+  mutable std::optional<std::string> SDKPlatformPath = std::nullopt;
+
 public:
   StringRef getSDKPath() const { return SDKPath; }
 
@@ -398,6 +400,12 @@ public:
     Lookup.searchPathsDidChange();
   }
 
+  /// Retrieves the corresponding parent platform path for the SDK, or
+  /// \c nullopt if there isn't one.
+  /// NOTE: This computes and caches the result, and as such will not respect
+  /// a different FileSystem being passed later.
+  std::optional<StringRef> getSDKPlatformPath(llvm::vfs::FileSystem *FS) const;
+
   std::optional<StringRef> getWinSDKRoot() const { return WinSDKRoot; }
   void setWinSDKRoot(StringRef root) {
     WinSDKRoot = root;

include/swift/AST/TypeCheckRequests.h

@@ -4773,8 +4773,9 @@ void simple_display(llvm::raw_ostream &out, const TypeRepr *TyR);
 void simple_display(llvm::raw_ostream &out, ImplicitMemberAction action);
 
 /// Computes whether a module is part of the stdlib or contained within the
-/// SDK. If no SDK was specified, falls back to whether the module was
-/// specified as a system module (ie. it's on the system search path).
+/// SDK or the platform directory. If no SDK was specified, falls back to
+/// whether the module was specified as a system module (ie. it's on the system
+/// search path).
 class IsNonUserModuleRequest
     : public SimpleRequest<IsNonUserModuleRequest,
                            bool(ModuleDecl *),

lib/AST/Module.cpp

@@ -4009,7 +4009,8 @@ FrontendStatsTracer::getTraceFormatter<const SourceFile *>() {
 bool IsNonUserModuleRequest::evaluate(Evaluator &evaluator, ModuleDecl *mod) const {
   // If there's no SDK path, fallback to checking whether the module was
   // in the system search path or a clang system module
-  SearchPathOptions &searchPathOpts = mod->getASTContext().SearchPathOpts;
+  auto &ctx = mod->getASTContext();
+  SearchPathOptions &searchPathOpts = ctx.SearchPathOpts;
   StringRef sdkPath = searchPathOpts.getSDKPath();
   if (sdkPath.empty() && mod->isSystemModule())
     return true;
@@ -4028,9 +4029,14 @@ bool IsNonUserModuleRequest::evaluate(Evaluator &evaluator, ModuleDecl *mod) con
   if (modulePath.empty())
     return false;
 
+  // If we have a platform path, check against that as it will be a parent of
+  // the SDK path.
+  auto *FS = ctx.SourceMgr.getFileSystem().get();
+  auto sdkOrPlatform = searchPathOpts.getSDKPlatformPath(FS).value_or(sdkPath);
+
   StringRef runtimePath = searchPathOpts.RuntimeResourcePath;
   return (!runtimePath.empty() && pathStartsWith(runtimePath, modulePath)) ||
-      (!sdkPath.empty() && pathStartsWith(sdkPath, modulePath));
+      (!sdkOrPlatform.empty() && pathStartsWith(sdkOrPlatform, modulePath));
 }
 
 version::Version ModuleDecl::getLanguageVersionBuiltWith() const {

lib/AST/SearchPathOptions.cpp

@@ -80,6 +80,34 @@ void ModuleSearchPathLookup::rebuildLookupTable(const SearchPathOptions *Opts,
   State.IsPopulated = true;
 }
 
+static std::string computeSDKPlatformPath(StringRef SDKPath,
+                                          llvm::vfs::FileSystem *FS) {
+  if (SDKPath.empty())
+    return "";
+
+  SmallString<128> platformPath;
+  if (auto err = FS->getRealPath(SDKPath, platformPath))
+    llvm::sys::path::append(platformPath, SDKPath);
+
+  llvm::sys::path::remove_filename(platformPath); // specific SDK
+  llvm::sys::path::remove_filename(platformPath); // SDKs
+  llvm::sys::path::remove_filename(platformPath); // Developer
+
+  if (!llvm::sys::path::filename(platformPath).endswith(".platform"))
+    return "";
+
+  return platformPath.str().str();
+}
+
+std::optional<StringRef>
+SearchPathOptions::getSDKPlatformPath(llvm::vfs::FileSystem *FS) const {
+  if (!SDKPlatformPath)
+    SDKPlatformPath = computeSDKPlatformPath(getSDKPath(), FS);
+  if (SDKPlatformPath->empty())
+    return std::nullopt;
+  return *SDKPlatformPath;
+}
+
 void SearchPathOptions::dump(bool isDarwin) const {
   llvm::errs() << "Module import search paths (non system):\n";
   for (auto Entry : llvm::enumerate(getImportSearchPaths())) {

test/lit.cfg

@@ -1321,6 +1321,9 @@ if run_vendor == 'apple':
         (config.darwin_xcrun_toolchain, config.variant_sdk))
     extra_frameworks_dir = make_path(config.variant_sdk, "..", "..", "..",
                                      "Developer", "Library", "Frameworks")
+    extra_platform_swift_modules = make_path(config.variant_sdk, "..", "..", "..",
+                                             "Developer", "usr", "lib")
+
     target_options = (
         "-target %s %s %s" %
         (config.variant_triple, config.resource_dir_opt, mcp_opt))
@@ -2805,7 +2808,9 @@ config.substitutions.insert(0, ('%test-resource-dir', test_resource_dir))
 
 if run_vendor != 'apple':
     extra_frameworks_dir = ''
+    extra_platform_swift_modules = ''
 config.substitutions.append(('%xcode-extra-frameworks-dir', extra_frameworks_dir))
+config.substitutions.append(('%xcode-extra-platform-swift-modules', extra_platform_swift_modules))
 
 config.substitutions.append(('%target-swiftmodule-name', target_specific_module_triple + '.swiftmodule'))
 config.substitutions.append(('%target-swiftdoc-name', target_specific_module_triple + '.swiftdoc'))

validation-test/IDE/complete_sdk_platform.swift

@@ -0,0 +1,18 @@
+// RUN: %empty-directory(%t)
+// RUN: ln -s %sdk %t/sdk
+
+// RUN: %batch-code-completion -F %xcode-extra-frameworks-dir -I %xcode-extra-platform-swift-modules
+
+// Works if SDK is specified as a symlink too.
+// RUN: %batch-code-completion -sdk %t/sdk -F %xcode-extra-frameworks-dir -I %xcode-extra-platform-swift-modules
+
+// REQUIRES: VENDOR=apple
+
+// rdar://131854240 - Make sure modules found in the platform dir are treated
+// as system.
+
+import XCTest
+
+#^COMPLETE^#
+// COMPLETE: Decl[Module]/None/IsSystem: XCTest[#Module#]; name=XCTest
+// COMPLETE: Decl[FreeFunction]/OtherModule[XCTest]/IsSystem: XCTFail()[#Void#]; name=XCTFail()