Add a new diagnostic group and documentation for the module-not-found failure which specifies that a missing module dependency can be found on a search path serialized in another Swift binary module dependency.

include/swift/AST/DiagnosticGroups.def

@@ -50,6 +50,7 @@ GROUP(ExistentialAny, "existential-any")
 GROUP(ExistentialMemberAccess, "existential-member-access-limitations")
 GROUP(IsolatedConformances, "isolated-conformances")
 GROUP(MemberImportVisibility, "member-import-visibility")
+GROUP(MissingModuleOnKnownPaths, "missing-module-on-known-paths")
 GROUP(MultipleInheritance, "multiple-inheritance")
 GROUP(MutableGlobalVariable, "mutable-global-variable")
 GROUP(NominalTypes, "nominal-types")

include/swift/AST/DiagnosticsSema.def

@@ -2332,6 +2332,8 @@ ERROR(alignment_not_power_of_two,none,
 
 // Dependency Scanning
 ERROR(dependency_scan_module_not_found, none, "Unable to find module dependency: '%0'", (StringRef))
+GROUPED_ERROR(dependency_scan_module_not_found_on_specified_search_paths, MissingModuleOnKnownPaths, none,
+              "Compilation search paths unable to resolve module dependency: '%0'", (StringRef))
 NOTE(unresolved_import_location,none,
      "also imported here", ())
 NOTE(dependency_as_imported_by_main_module,none,
@@ -2339,8 +2341,9 @@ NOTE(dependency_as_imported_by_main_module,none,
 NOTE(dependency_as_imported_by, none,
      "a dependency of %select{Swift|Clang}2 module '%0': '%1'", (StringRef, StringRef, bool))
 NOTE(inherited_search_path_resolves_module,none,
-     "'%0' can be found on a search path used to build module '%1': '%2'. "
+     "'%0' can be found using a search path that was specified when building module '%1' ('%2'). "
-     "These search paths are not inherited by the current compilation.", (StringRef, StringRef, StringRef))
+     "This search path was not explicitly specified on the current compilation.", (StringRef, StringRef, StringRef))
+
 ERROR(clang_dependency_scan_error, none, "Clang dependency scanner failure: %0", (StringRef))
 ERROR(clang_header_dependency_scan_error, none, "Bridging header dependency scan failure: %0", (StringRef))
 

include/swift/DependencyScan/ModuleDependencyScanner.h

@@ -203,8 +203,10 @@ private:
   /// Assuming the \c `moduleImport` failed to resolve,
   /// iterate over all binary Swift module dependencies with serialized
   /// search paths and attempt to diagnose if the failed-to-resolve module
-  /// can be found on any of them.
+  /// can be found on any of them. Returns the path containing
-  void attemptToFindResolvingSerializedSearchPath(
+  /// the module, if one is found.
+  std::optional<std::pair<ModuleDependencyID, std::string>>
+  attemptToFindResolvingSerializedSearchPath(
       const ScannerImportStatementInfo &moduleImport,
       const ModuleDependenciesCache &cache, const SourceLoc &importLoc);
 

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -1656,8 +1656,27 @@ void ModuleDependencyScanner::diagnoseScannerFailure(
         locInfo.bufferIdentifier, locInfo.lineNumber, locInfo.columnNumber);
   }
 
-  Diagnostics.diagnose(importLoc, diag::dependency_scan_module_not_found,
+  // Attempt to determine if any of the binary Swift module dependencies contain
-                       moduleImport.importIdentifier);
+  // serialized search paths where the missing module may be found. If yes,
+  // emit a specialized diagnostic letting the user know which search path
+  // is missing in current compilation.
+  auto resolvedModuleDefiningPath = attemptToFindResolvingSerializedSearchPath(
+      moduleImport, cache, importLoc);
+  if (resolvedModuleDefiningPath) {
+    Diagnostics.diagnose(
+        importLoc,
+        diag::dependency_scan_module_not_found_on_specified_search_paths,
+        moduleImport.importIdentifier);
+    Diagnostics.diagnose(importLoc, diag::inherited_search_path_resolves_module,
+                         moduleImport.importIdentifier,
+                         resolvedModuleDefiningPath->first.ModuleName,
+                         resolvedModuleDefiningPath->second);
+  } else
+    Diagnostics.diagnose(importLoc, diag::dependency_scan_module_not_found,
+                         moduleImport.importIdentifier);
+
+  // Emit notes for every link in the dependency chain from the root
+  // module-under-scan to the module whose import failed to resolve.
   if (dependencyOf.has_value()) {
     auto path = findPathToDependency(dependencyOf.value(), cache);
     // We may fail to construct a path in some cases, such as a Swift overlay of
@@ -1703,6 +1722,8 @@ void ModuleDependencyScanner::diagnoseScannerFailure(
     }
   }
 
+  // Emit notes for every other location where the failed-to-resolve
+  // module is imported.
   if (moduleImport.importLocations.size() > 1) {
     for (size_t i = 1; i < moduleImport.importLocations.size(); ++i) {
       auto locInfo = moduleImport.importLocations[i];
@@ -1711,8 +1732,6 @@ void ModuleDependencyScanner::diagnoseScannerFailure(
       Diagnostics.diagnose(importLoc, diag::unresolved_import_location);
     }
   }
-
-  attemptToFindResolvingSerializedSearchPath(moduleImport, cache, importLoc);
 }
 
 static std::string getModuleDefiningPath(const ModuleDependencyInfo &info) {
@@ -1737,31 +1756,17 @@ static std::string getModuleDefiningPath(const ModuleDependencyInfo &info) {
 
   // Relative to the `module.modulemap` or `.swiftinterface` or `.swiftmodule`,
   // the defininig path is the parent directory of the file.
-<<<<<<< Updated upstream
-  path = llvm::sys::path::parent_path(path);
-
-  // If the defining path is the top-level `.swiftmodule` directory,
-  // take one more step up.
-  if (llvm::sys::path::extension(path) == ".swiftmodule")
-    path = llvm::sys::path::parent_path(path);
-
-  // If the defining path is under `.framework/Modules/` directory,
-  // return the parent path containing the framework.
-  if (llvm::sys::path::filename(path) == "Modules" && llvm::sys::path::extension(llvm::sys::path::parent_path(path)) == ".framework")
-    path = llvm::sys::path::parent_path(llvm::sys::path::parent_path(path));
-
-  return path;
-=======
   return llvm::sys::path::parent_path(path).str();
->>>>>>> Stashed changes
 }
 
-void ModuleDependencyScanner::attemptToFindResolvingSerializedSearchPath(
+std::optional<std::pair<ModuleDependencyID, std::string>>
+ModuleDependencyScanner::attemptToFindResolvingSerializedSearchPath(
     const ScannerImportStatementInfo &moduleImport,
     const ModuleDependenciesCache &cache, const SourceLoc &importLoc) {
   std::set<ModuleDependencyID> binarySwiftModuleDepIDs =
       collectBinarySwiftDeps(cache);
 
+  std::optional<std::pair<ModuleDependencyID, std::string>> result;
   for (const auto &binaryDepID : binarySwiftModuleDepIDs) {
     auto binaryModInfo =
         cache.findKnownDependency(binaryDepID).getAsSwiftBinaryModule();
@@ -1772,9 +1777,10 @@ void ModuleDependencyScanner::attemptToFindResolvingSerializedSearchPath(
     // Note: this will permanently mutate this worker with additional search
     // paths. That's fine because we are diagnosing a scan failure here, but
     // worth being aware of.
-    withDependencyScanningWorker(
+    result = withDependencyScanningWorker(
-        [&binaryModInfo, &moduleImport, &cache, &binaryDepID, &importLoc,
+        [&binaryModInfo, &moduleImport, &cache, this,
-         this](ModuleDependencyScanningWorker *ScanningWorker) {
+         &binaryDepID](ModuleDependencyScanningWorker *ScanningWorker)
+            -> std::optional<std::pair<ModuleDependencyID, std::string>> {
           ModuleDependencyVector result;
           for (const auto &sp : binaryModInfo->serializedSearchPaths)
             ScanningWorker->workerASTContext->addSearchPath(
@@ -1784,31 +1790,22 @@ void ModuleDependencyScanner::attemptToFindResolvingSerializedSearchPath(
               getModuleImportIdentifier(moduleImport.importIdentifier),
               cache.getModuleOutputPath(), cache.getSDKModuleOutputPath(),
               cache.getScanService().getPrefixMapper());
-          if (!result.empty()) {
+          if (!result.empty())
-            Diagnostics.diagnose(
+            return std::make_pair(binaryDepID,
-                importLoc, diag::inherited_search_path_resolves_module,
+                                  getModuleDefiningPath(result[0].second));
-                moduleImport.importIdentifier, binaryDepID.ModuleName,
-                getModuleDefiningPath(result[0].second));
-          }
 
           result = ScanningWorker->scanFilesystemForClangModuleDependency(
               getModuleImportIdentifier(moduleImport.importIdentifier),
               cache.getModuleOutputPath(), cache.getSDKModuleOutputPath(), {},
               cache.getScanService().getPrefixMapper());
-<<<<<<< Updated upstream
-          if (!result.empty()) {
-            Diagnostics.diagnose(
-                importLoc, diag::inherited_search_path_resolves_module,
-                moduleImport.importIdentifier, binaryDepID.ModuleName,
-                getModuleDefiningPath(result[0].second));
-          }
-          return result;
-=======
           if (!result.empty())
             return std::make_pair(binaryDepID,
                                   getModuleDefiningPath(result[0].second));
           return std::nullopt;
->>>>>>> Stashed changes
         });
+    if (result)
+      break;
   }
+
+  return result;
 }

test/ScanDependencies/cached-missing-module-found-in-serialized-paths.swift

@@ -21,9 +21,9 @@
 // CHECK: remark: Incremental module scan: Re-using serialized module scanning dependency cache from: '{{.*}}cache.moddepcache'.
 // CHECK: remark: Incremental module scan: Dependency info for module 'C' invalidated due to a modified input since last scan: '{{.*}}deps{{/|\\}}C.swiftinterface'.
 // CHECK: remark: Incremental module scan: Dependency info for module 'deps' invalidated due to an out-of-date dependency.
-// CHECK: error: Unable to find module dependency: 'C'
+// CHECK: error: Compilation search paths unable to resolve module dependency: 'C'
+// CHECK: note: 'C' can be found using a search path that was specified when building module 'B' ('{{.*}}moreDeps'). This search path was not explicitly specified on the current compilation.
 // CHECK: note: a dependency of main module 'deps'
-// CHECK: note: 'C' can be found on a search path used to build module 'B': '{{.*}}moreDeps'. These search paths are not inherited by the current compilation.
 
 //--- moreDeps/C.swiftinterface
 // swift-interface-format-version: 1.0

test/ScanDependencies/diagnose-missing-module-found-in-serialized-paths.swift

@@ -9,10 +9,10 @@
 // RUN: %target-swift-frontend -scan-dependencies -o %t/deps.json %t/client.swift -I %t/deps -disable-implicit-string-processing-module-import -disable-implicit-concurrency-module-import &> %t/output.txt
 // RUN: cat %t/output.txt | %FileCheck %s
 
-// CHECK: error: Unable to find module dependency: 'C'
+// CHECK: error: Compilation search paths unable to resolve module dependency: 'C'
+// CHECK: note: 'C' can be found using a search path that was specified when building module 'B' ('{{.*}}moreDeps'). This search path was not explicitly specified on the current compilation.
 // CHECK: note: a dependency of Swift module 'B': '{{.*}}B.swiftmodule'
 // CHECK: note: a dependency of main module 'deps'
-// CHECK: note: 'C' can be found on a search path used to build module 'B': '{{.*}}moreDeps'. These search paths are not inherited by the current compilation.
 
 //--- moreDeps/C.swiftinterface
 // swift-interface-format-version: 1.0

userdocs/diagnostics/missing-module-on-known-paths.md

@@ -0,0 +1,18 @@
+# Missing Module On Known Path From A Dependency Note (`MissingModuleOnKnownPaths`)
+
+
+This diagnostic group covers notes related to displaying information about a missing module dependency which the compiler is able to locate as present on a search path found in a loaded Swift binary module, but which is not specified to the current compilation.
+
+As one example of a potential missing module diagnostic, suppose an imported module `Foo` is resolved to a Swift binary module which itself depends on module `Bar` and was built with an additional search path where `Bar` is located, and suppose that the client which imports `Foo` does not specify this search path:
+
+```
+import Foo
+```
+
+The Swift compiler would emit a module-not-found error and a note to inform the user of the missing search path containing `Bar` which was found serialized in `Foo`'s binary Swift module:
+
+```
+error: Compilation search paths unable to resolve module dependency: 'Bar' [#MissingModuleOnKnownPaths]
+note: 'Bar' can be found using a search path that was specified when building module 'Foo' ('<Search Path>'). This search path was not specified on the current compilation.
+```
+Some prior versions of the Swift compiler erroneously inherited search paths from loaded binary Swift modules and used them to resolve other, subsequently-encountered module dependencies. All search paths required to resolve direct and transitive module dependencies must be explicitly specified on the compiler invocation which will encounter these dependencies.