[NFC] Add a "Main Module" Bit

The constraint system will need this if it wants to disable debug mode for serialized modules

Author: CodaFi

include/swift/AST/Decl.h

@@ -575,7 +575,7 @@ class alignas(1 << DeclAlignInBits) Decl {
     HasAnyUnavailableValues : 1
   );
 
-  SWIFT_INLINE_BITFIELD(ModuleDecl, TypeDecl, 1+1+1+1+1+1+1+1,
+  SWIFT_INLINE_BITFIELD(ModuleDecl, TypeDecl, 1+1+1+1+1+1+1+1+1,
     /// If the module was or is being compiled with `-enable-testing`.
     TestingEnabled : 1,
 
@@ -601,7 +601,10 @@ class alignas(1 << DeclAlignInBits) Decl {
 
     /// Whether the module was imported from Clang (or, someday, maybe another
     /// language).
-    IsNonSwiftModule : 1
+    IsNonSwiftModule : 1,
+
+    /// Whether this module is the main module.
+    IsMainModule : 1
   );
 
   SWIFT_INLINE_BITFIELD(PrecedenceGroupDecl, Decl, 1+2,

include/swift/AST/Module.h

@@ -347,6 +347,13 @@ class ModuleDecl : public DeclContext, public TypeDecl {
     return new (ctx) ModuleDecl(name, ctx, importInfo);
   }
 
+  static ModuleDecl *
+  createMainModule(ASTContext &ctx, Identifier name, ImplicitImportInfo iinfo) {
+    auto *Mod = ModuleDecl::create(name, ctx, iinfo);
+    Mod->Bits.ModuleDecl.IsMainModule = true;
+    return Mod;
+  }
+
   using Decl::getASTContext;
 
   /// Retrieves information about which modules are implicitly imported by
@@ -542,6 +549,10 @@ class ModuleDecl : public DeclContext, public TypeDecl {
     Bits.ModuleDecl.IsNonSwiftModule = flag;
   }
 
+  bool isMainModule() const {
+    return Bits.ModuleDecl.IsMainModule;
+  }
+
   /// Retrieve the top-level module. If this module is already top-level, this
   /// returns itself. If this is a submodule such as \c Foo.Bar.Baz, this
   /// returns the module \c Foo.

lib/AST/Module.cpp

@@ -475,6 +475,16 @@ ModuleDecl::ModuleDecl(Identifier name, ASTContext &ctx,
   setInterfaceType(ModuleType::get(this));
 
   setAccess(AccessLevel::Public);
+
+  Bits.ModuleDecl.TestingEnabled = 0;
+  Bits.ModuleDecl.FailedToLoad = 0;
+  Bits.ModuleDecl.RawResilienceStrategy = 0;
+  Bits.ModuleDecl.HasResolvedImports = 0;
+  Bits.ModuleDecl.PrivateImportsEnabled = 0;
+  Bits.ModuleDecl.ImplicitDynamicEnabled = 0;
+  Bits.ModuleDecl.IsSystemModule = 0;
+  Bits.ModuleDecl.IsNonSwiftModule = 0;
+  Bits.ModuleDecl.IsMainModule = 0;
 }
 
 ArrayRef<ImplicitImport> ModuleDecl::getImplicitImports() const {

lib/Frontend/Frontend.cpp

@@ -717,7 +717,8 @@ ImplicitImportInfo CompilerInstance::getImplicitImportInfo() const {
 ModuleDecl *CompilerInstance::getMainModule() const {
   if (!MainModule) {
     Identifier ID = Context->getIdentifier(Invocation.getModuleName());
-    MainModule = ModuleDecl::create(ID, *Context, getImplicitImportInfo());
+    MainModule = ModuleDecl::createMainModule(*Context, ID,
+                                              getImplicitImportInfo());
     if (Invocation.getFrontendOptions().EnableTesting)
       MainModule->setTestingEnabled();
     if (Invocation.getFrontendOptions().EnablePrivateImports)