[Frontend] Optional remarks on loaded modules location

Passing the frontend flag -Rmodule-loading makes the compiler emit
remarks with the path of every module loaded. The path for Swift modules
is either the swiftinterface file for modules built with library
evolution or the binary swiftmodule otherwise. The path for clangmodules
is always in the cache which could be improved as it may be less useful.

Here's an extract of the output for a simple SwiftUI app:

<unknown>:0: remark: loaded module from
/Users/xymus/Library/Developer/Xcode/DerivedData/ModuleCache.noindex/2VJP7CNCGWRF0/SwiftShims-18ZF6992O9H75.pcm
<unknown>:0: remark: loaded module from
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator14.2.sdk/usr/lib/swift/Swift.swiftmodule/x86_64-apple-ios-simulator.swiftinterface
<unknown>:0: remark: loaded module from
/Users/xymus/Library/Developer/Xcode/DerivedData/ModuleCache.noindex/2VJP7CNCGWRF0/os-1HVC6DNXVU37C.pcm
<unknown>:0: remark: loaded module from
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator14.2.sdk/usr/lib/swift/os.swiftmodule/x86_64-apple-ios-simulator.swiftinterface
<unknown>:0: remark: loaded module from
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator14.2.sdk/System/Library/Frameworks/SwiftUI.framework/Modules/SwiftUI.swiftmodule/x86_64-apple-ios-simulator.swiftinterface

Author: xymus

include/swift/AST/DiagnosticsSema.def

@@ -908,6 +908,10 @@ REMARK(cross_import_added,none,
        "import of %0 and %1 triggered a cross-import of %2",
        (Identifier, Identifier, Identifier))
 
+REMARK(module_loaded,none,
+       "loaded module from %0",
+       (StringRef))
+
 // Operator decls
 ERROR(ambiguous_operator_decls,none,
       "ambiguous operator declarations found for operator", ())

include/swift/Basic/LangOptions.h

@@ -129,6 +129,9 @@ namespace swift {
     /// overlay.
     bool EnableCrossImportRemarks = false;
 
+    /// Emit a remark after loading a module.
+    bool EnableModuleLoadingRemarks = false;
+
     ///
     /// Support for alternate usage modes
     ///

include/swift/Option/Options.td

@@ -336,6 +336,10 @@ def emit_cross_import_remarks : Flag<["-"], "Rcross-import">,
   Flags<[FrontendOption, DoesNotAffectIncrementalBuild]>,
   HelpText<"Emit a remark if a cross-import of a module is triggered.">;
 
+def remark_loading_module : Flag<["-"], "Rmodule-loading">,
+  Flags<[FrontendOption, DoesNotAffectIncrementalBuild]>,
+  HelpText<"Emit a remark and file path of each loaded module">;
+
 def emit_tbd : Flag<["-"], "emit-tbd">,
   HelpText<"Emit a TBD file">,
   Flags<[FrontendOption, NoInteractiveOption, SupplementaryOutput]>;

lib/AST/ASTContext.cpp

@@ -1921,6 +1921,11 @@ ASTContext::getModule(ImportPath::Module ModulePath) {
   auto moduleID = ModulePath[0];
   for (auto &importer : getImpl().ModuleLoaders) {
     if (ModuleDecl *M = importer->loadModule(moduleID.Loc, ModulePath)) {
+      if (LangOpts.EnableModuleLoadingRemarks) {
+        Diags.diagnose(nullptr,
+                       diag::module_loaded,
+                       M->getModuleFilename());
+      }
       return M;
     }
   }

lib/Frontend/CompilerInvocation.cpp

@@ -577,6 +577,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   Opts.EnableCrossImportRemarks = Args.hasArg(OPT_emit_cross_import_remarks);
 
+  Opts.EnableModuleLoadingRemarks = Args.hasArg(OPT_remark_loading_module);
+
   llvm::Triple Target = Opts.Target;
   StringRef TargetArg;
   std::string TargetArgScratch;

test/ModuleInterface/loading-remarks.swift

@@ -0,0 +1,14 @@
+/// Test the -Rmodule-loading flag.
+// RUN: %empty-directory(%t)
+
+/// Create a simple module and interface.
+// RUN: echo 'public func publicFunction() {}' > %t/TestModule.swift
+// RUN: %target-swift-frontend -typecheck %t/TestModule.swift -emit-module-interface-path %t/TestModule.swiftinterface -swift-version 5
+
+/// Use -Rmodule-loading in a client and look for the diagnostics output.
+// RUN: %target-swift-frontend -typecheck %s -I %t -Rmodule-loading 2>&1 | %FileCheck %s
+
+import TestModule
+// CHECK: remark: loaded module from {{.*}}SwiftShims-{{.*}}.pcm
+// CHECK: remark: loaded module from {{.*}}Swift.swiftmodule{{.*}}.swiftmodule
+// CHECK: remark: loaded module from {{.*}}TestModule.swiftinterface