Introduce separately-imported stdlib overlays

The `_Concurrency` and `_StringProcessing` modules are implementation details of the standard library; to developers, their contents should behave as though they are declared directly within module `Swift`. This is the exact same behavior we expect of cross-import overlays, so treat these modules as though they are cross-import overlays with no bystanding module.

Because these modules don’t re-export the standard library, it’s also necessary to treat `Swift` as a separately imported overlay of itself; do so and make that actually work.

Author: beccadax

CHANGELOG.md

@@ -3,6 +3,19 @@
 > [!NOTE]
 > This is in reverse chronological order, so newer entries are added to the top.
 
+## Swift (next)
+
+* Concurrency-related APIs like `Task` and string-processing-related APIs like `Regex` can now be qualified by the name
+  `Swift`, just like other standard library APIs:
+  
+  ```swift
+  Swift.Task { ... }
+  func match(_ regex: Swift.Regex<(Substring)>) { ... }
+  ```
+  
+  The old `_Concurrency` and `_StringProcessing` names are still supported for backwards compatibility, and Embedded
+  Swift projects must still explicitly `import _Concurrency` to access councurrency APIs.
+
 ## Swift 6.2
 
 * [SE-0472][]:

include/swift/AST/ASTContext.h

@@ -1294,6 +1294,10 @@ class ASTContext final {
     return const_cast<ASTContext *>(this)->getStdlibModule(false);
   }
 
+  /// Names of underscored modules whose contents, if imported, should be
+  /// treated as separately-imported overlays of the standard library module.
+  Identifier StdlibOverlayNames[2];
+
   /// Insert an externally-sourced module into the set of known loaded modules
   /// in this context.
   void addLoadedModule(ModuleDecl *M);

lib/AST/ASTContext.cpp

@@ -839,6 +839,9 @@ ASTContext::ASTContext(
   // Initialize all of the known identifiers.
 #define IDENTIFIER_WITH_NAME(Name, IdStr) Id_##Name = getIdentifier(IdStr);
 #include "swift/AST/KnownIdentifiers.def"
+  
+  StdlibOverlayNames[0] = Id_Concurrency;
+  StdlibOverlayNames[1] = Id_StringProcessing;
 
   // Record the initial set of search paths.
   for (const auto &path : SearchPathOpts.getImportSearchPaths())

lib/AST/DeclContext.cpp

@@ -447,6 +447,16 @@ DeclContext *DeclContext::getModuleScopeContext() const {
 
 void DeclContext::getSeparatelyImportedOverlays(
     ModuleDecl *declaring, SmallVectorImpl<ModuleDecl *> &overlays) const {
+  if (declaring->isStdlibModule()) {
+    auto &ctx = getASTContext();
+    for (auto overlayName: ctx.StdlibOverlayNames) {
+      if (auto module = ctx.getLoadedModule(overlayName))
+        overlays.push_back(module);
+    }
+    overlays.push_back(declaring);
+    return;
+  }
+
   if (auto SF = getOutermostParentSourceFile())
     SF->getSeparatelyImportedOverlays(declaring, overlays);
 }

lib/AST/Module.cpp

@@ -2432,6 +2432,12 @@ ModuleDecl::getDeclaringModuleAndBystander() {
   if (!hasUnderscoredNaming())
     return *(declaringModuleAndBystander = {nullptr, Identifier()});
 
+  // If this is one of the stdlib overlays, indicate as much.
+  auto &ctx = getASTContext();
+  if (llvm::is_contained(ctx.StdlibOverlayNames, getRealName()))
+    return *(declaringModuleAndBystander = { ctx.getStdlibModule(),
+                                             Identifier() });
+
   // Search the transitive set of imported @_exported modules to see if any have
   // this module as their overlay.
   SmallPtrSet<ModuleDecl *, 16> seen;
@@ -2511,7 +2517,8 @@ bool ModuleDecl::getRequiredBystandersIfCrossImportOverlay(
   auto *clangModule = declaring->getUnderlyingModuleIfOverlay();
   auto current = std::make_pair(this, Identifier());
   while ((current = current.first->getDeclaringModuleAndBystander()).first) {
-    bystanderNames.push_back(current.second);
+    if (!current.second.empty())
+      bystanderNames.push_back(current.second);
     if (current.first == declaring || current.first == clangModule)
       return true;
   }

lib/AST/ModuleNameLookup.cpp

@@ -163,11 +163,18 @@ void ModuleNameLookup<LookupStrategy>::lookupInModule(
       moduleOrFile->getParentModule(), overlays);
   if (!overlays.empty()) {
     // If so, look in each of those overlays.
-    for (auto overlay : overlays)
-      lookupInModule(decls, overlay, accessPath, moduleScopeContext, options);
+    bool selfOverlay = false;
+    for (auto overlay : overlays) {
+      if (overlay == moduleOrFile->getParentModule())
+        selfOverlay = true;
+      else
+        lookupInModule(decls, overlay, accessPath, moduleScopeContext,
+                       options);
+    }
     // FIXME: This may not work gracefully if more than one of these lookups
     // finds something.
-    return;
+    if (!selfOverlay)
+      return;
   }
 
   const size_t initialCount = decls.size();

lib/IDE/CompletionLookup.cpp

@@ -2182,11 +2182,9 @@ bool CompletionLookup::tryModuleCompletions(Type ExprType,
     // If the module is shadowed by a separately imported overlay(s), look up
     // the symbols from the overlay(s) instead.
     SmallVector<ModuleDecl *, 1> ShadowingOrOriginal;
-    if (auto *SF = CurrDeclContext->getParentSourceFile()) {
-      SF->getSeparatelyImportedOverlays(M, ShadowingOrOriginal);
-      if (ShadowingOrOriginal.empty())
-        ShadowingOrOriginal.push_back(M);
-    }
+    CurrDeclContext->getSeparatelyImportedOverlays(M, ShadowingOrOriginal);
+    if (ShadowingOrOriginal.empty())
+      ShadowingOrOriginal.push_back(M);
     for (ModuleDecl *M : ShadowingOrOriginal) {
       RequestedResultsTy Request =
           RequestedResultsTy::fromModule(M, Filter).needLeadingDot(needDot());

test/Concurrency/concurrency_module_shadowing.swift

@@ -17,3 +17,8 @@ func f(_ t : UnsafeCurrentTask) -> Bool {
 
 @available(SwiftStdlib 5.1, *)
 func g(_: _Concurrency.UnsafeCurrentTask) {}
+
+// Should also be allowed since _Concurrency is a separately-imported overlay of
+// the standard library.
+@available(SwiftStdlib 5.1, *)
+func h(_: Swift.UnsafeCurrentTask) {}

test/Concurrency/global_actor_function_types.swift

@@ -408,7 +408,7 @@ struct GlobalType {}
 
 @_Concurrency.MainActor
 extension GlobalType {
-  @_Concurrency.MainActor static func ==(
+  @Swift.MainActor static func ==(
     lhs: GlobalType,
     rhs: GlobalType
   ) -> Bool { true }