ModuleInterface: Print imports with @preconcurrency in swiftinterface files

include/swift/AST/FileUnit.h

@@ -126,6 +126,13 @@ class FileUnit : public DeclContext, public ASTAllocated<FileUnit> {
                             const ModuleDecl *importedModule,
                             llvm::SmallSetVector<Identifier, 4> &spiGroups) const {};
 
+  /// Returns true if any import of \p importedModule has the `@preconcurrency`
+  /// attribute.
+  virtual bool
+  isModuleImportedPreconcurrency(const ModuleDecl *importedModule) const {
+    return false;
+  };
+
   /// Find all availability domains defined in this module with the given
   /// identifier.
   ///

include/swift/AST/Module.h

@@ -965,6 +965,10 @@ class ModuleDecl
                          const ModuleDecl *importedModule,
                          llvm::SmallSetVector<Identifier, 4> &spiGroups) const;
 
+  /// Returns true if any import of \p importedModule has the `@preconcurrency`
+  /// attribute.
+  bool isModuleImportedPreconcurrency(const ModuleDecl *importedModule) const;
+
   /// Finds the custom availability domain defined by this module with the
   /// given identifier and if one exists adds it to results.
   void

include/swift/AST/SourceFile.h

@@ -471,6 +471,11 @@ class SourceFile final : public FileUnit {
                 const ModuleDecl *importedModule,
                 llvm::SmallSetVector<Identifier, 4> &spiGroups) const override;
 
+  /// Returns true if any import of \p importedModule has the `@preconcurrency`
+  /// attribute.
+  virtual bool isModuleImportedPreconcurrency(
+      const ModuleDecl *importedModule) const override;
+
   // Is \p targetDecl accessible as an explicitly imported SPI from this file?
   bool isImportedAsSPI(const ValueDecl *targetDecl) const;
 

include/swift/Serialization/SerializedModuleLoader.h

@@ -466,6 +466,9 @@ class SerializedASTFile final : public LoadedFile {
                 const ModuleDecl *importedModule,
                 llvm::SmallSetVector<Identifier, 4> &spiGroups) const override;
 
+  virtual bool isModuleImportedPreconcurrency(
+      const ModuleDecl *importedModule) const override;
+
   std::optional<CommentInfo> getCommentForDecl(const Decl *D) const override;
 
   bool hasLoadedSwiftDoc() const override;

lib/AST/Module.cpp

@@ -1044,6 +1044,20 @@ void ModuleDecl::lookupImportedSPIGroups(
   FORWARD(lookupImportedSPIGroups, (importedModule, spiGroups));
 }
 
+bool ModuleDecl::isModuleImportedPreconcurrency(
+    const ModuleDecl *importedModule) const {
+  for (const FileUnit *file : getFiles()) {
+    if (file->isModuleImportedPreconcurrency(importedModule))
+      return true;
+
+    if (auto *synth = file->getSynthesizedFile()) {
+      if (synth->isModuleImportedPreconcurrency(importedModule))
+        return true;
+    }
+  }
+  return false;
+}
+
 void ModuleDecl::lookupAvailabilityDomains(
     Identifier identifier,
     llvm::SmallVectorImpl<AvailabilityDomain> &results) const {
@@ -3050,6 +3064,20 @@ void SourceFile::lookupImportedSPIGroups(
   }
 }
 
+bool SourceFile::isModuleImportedPreconcurrency(
+    const ModuleDecl *importedModule) const {
+  auto &imports = getASTContext().getImportCache();
+  for (auto &import : *Imports) {
+    if (import.options.contains(ImportFlags::Preconcurrency) &&
+        (importedModule == import.module.importedModule ||
+         imports.isImportedByViaSwiftOnly(importedModule,
+                                          import.module.importedModule))) {
+      return true;
+    }
+  }
+  return false;
+}
+
 bool shouldImplicitImportAsSPI(ArrayRef<Identifier> spiGroups) {
   for (auto group : spiGroups) {
     if (group.empty())

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -271,6 +271,9 @@ static void printImports(raw_ostream &out,
       ModuleDecl::ImportFilterKind::Default,
       ModuleDecl::ImportFilterKind::ShadowedByCrossImportOverlay};
 
+  // FIXME: Scan over all imports in the module once to build up the attribute
+  // set for printed imports, instead of repeatedly doing linear scans for each
+  // kind of attribute.
   using ImportSet = llvm::SmallSet<ImportedModule, 8, ImportedModule::Order>;
   auto getImports = [M](ModuleDecl::ImportFilter filter) -> ImportSet {
     SmallVector<ImportedModule, 8> matchingImports;
@@ -355,6 +358,9 @@ static void printImports(raw_ostream &out,
         out << "@_spi(" << spiName << ") ";
     }
 
+    if (M->isModuleImportedPreconcurrency(importedModule))
+      out << "@preconcurrency ";
+
     if (Opts.printPackageInterface() &&
         !publicImportSet.count(import) &&
         packageOnlyImportSet.count(import))

lib/Serialization/SerializedModuleLoader.cpp

@@ -1915,6 +1915,14 @@ void SerializedASTFile::lookupImportedSPIGroups(
   }
 }
 
+bool SerializedASTFile::isModuleImportedPreconcurrency(
+    const ModuleDecl *importedModule) const {
+  // This method should only be queried during `-merge-modules` jobs, which are
+  // deprecated, and thus no effort has been made to answer this query correctly
+  // (@preconcurrency is not encoded on imports in serialized modules).
+  return false;
+}
+
 std::optional<CommentInfo>
 SerializedASTFile::getCommentForDecl(const Decl *D) const {
   return File.getCommentForDecl(D);

test/ModuleInterface/preconcurrency_imports.swift

@@ -0,0 +1,37 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %target-swift-frontend -emit-module %t/PreconcurrencyLib.swift -module-name PreconcurrencyLib -swift-version 5 -enable-library-evolution -emit-module-path %t/PreconcurrencyLib.swiftmodule -emit-module-interface-path %t/PreconcurrencyLib.swiftinterface
+// RUN: %target-swift-frontend -emit-module %t/OtherLib.swift -module-name OtherLib -swift-version 5 -enable-library-evolution -emit-module-path %t/OtherLib.swiftmodule -emit-module-interface-path %t/OtherLib.swiftinterface
+
+// RUN: %target-swift-emit-module-interface(%t/ClientLib.swiftinterface) -swift-version 6 %t/ClientLib_file1.swift %t/ClientLib_file2.swift -module-name ClientLib -I %t
+// RUN: %target-swift-typecheck-module-from-interface(%t/ClientLib.swiftinterface) -module-name ClientLib -I %t
+// RUN: %FileCheck %s < %t/ClientLib.swiftinterface
+
+// CHECK: {{^}}@preconcurrency import OtherLib
+// CHECK: {{^}}@preconcurrency import PreconcurrencyLib
+// CHECK: public struct Struct1 : Swift.Sendable
+// CHECK: public struct Struct2
+
+//--- PreconcurrencyLib.swift
+
+public class C {}
+
+//--- OtherLib.swift
+// Intentionally empty
+
+//--- ClientLib_file1.swift
+
+@preconcurrency public import PreconcurrencyLib
+public import OtherLib
+
+public struct Struct1: Sendable {
+  public var c: C
+}
+
+//--- ClientLib_file2.swift
+
+internal import PreconcurrencyLib
+@preconcurrency internal import OtherLib
+
+public struct Struct2 {}