Implement a module attribute 'optional' to allow missing headers.

This deals with the fact that our modulemaps include headers which can vary
across library versions and that attribute is a way to express this.

Author: vgvassilev

clang/include/clang/Basic/Module.h

@@ -371,6 +371,9 @@ class alignas(8) Module {
   LLVM_PREFERRED_TYPE(bool)
   unsigned NoUndeclaredIncludes : 1;
 
+  /// Whether the submodule is allowed to have missing headers.
+  unsigned IsOptional: 1;
+
   /// Whether this module came from a "private" module map, found next
   /// to a regular (public) module map.
   LLVM_PREFERRED_TYPE(bool)

clang/include/clang/Lex/ModuleMap.h

@@ -248,9 +248,12 @@ class ModuleMap {
     LLVM_PREFERRED_TYPE(bool)
     unsigned NoUndeclaredIncludes : 1;
 
+    /// Whether we can have a submodule with missing header files.
+    unsigned IsOptional : 1;
+
     Attributes()
         : IsSystem(false), IsExternC(false), IsExhaustive(false),
-          NoUndeclaredIncludes(false) {}
+          NoUndeclaredIncludes(false), IsOptional(false) {}
   };
 
   /// A directory for which framework modules can be inferred.

clang/lib/Basic/Module.cpp

@@ -43,14 +43,15 @@ Module::Module(StringRef Name, SourceLocation DefinitionLoc, Module *Parent,
       IsSystem(false), IsExternC(false), IsInferred(false),
       InferSubmodules(false), InferExplicitSubmodules(false),
       InferExportWildcard(false), ConfigMacrosExhaustive(false),
-      NoUndeclaredIncludes(false), ModuleMapIsPrivate(false),
+      NoUndeclaredIncludes(false), IsOptional(false), ModuleMapIsPrivate(false),
       NamedModuleHasInit(true), NameVisibility(Hidden) {
   if (Parent) {
     IsAvailable = Parent->isAvailable();
     IsUnimportable = Parent->isUnimportable();
     IsSystem = Parent->IsSystem;
     IsExternC = Parent->IsExternC;
     NoUndeclaredIncludes = Parent->NoUndeclaredIncludes;
+    IsOptional = Parent->IsOptional;
     ModuleMapIsPrivate = Parent->ModuleMapIsPrivate;
 
     Parent->SubModuleIndex[Name] = Parent->SubModules.size();

clang/lib/Lex/ModuleMap.cpp

@@ -317,6 +317,8 @@ void ModuleMap::resolveHeader(Module *Mod,
     // this was supposed to modularize the builtin header alone.
   } else if (Header.Kind == Module::HK_Excluded) {
     // Ignore missing excluded header files. They're optional anyway.
+  } else if (Mod->IsOptional) {
+     // Optional submodules can have missing headers.
   } else {
     // If we find a module that has a missing header, we mark this module as
     // unavailable and store the header directive for displaying diagnostics.
@@ -1099,6 +1101,7 @@ Module *ModuleMap::inferFrameworkModule(DirectoryEntryRef FrameworkDir,
   Result->IsExternC |= Attrs.IsExternC;
   Result->ConfigMacrosExhaustive |= Attrs.IsExhaustive;
   Result->NoUndeclaredIncludes |= Attrs.NoUndeclaredIncludes;
+  Result->IsOptional |= Attrs.IsOptional;
   Result->Directory = FrameworkDir;
 
   // Chop off the first framework bit, as that is implied.
@@ -1841,7 +1844,10 @@ namespace {
     AT_exhaustive,
 
     /// The 'no_undeclared_includes' attribute.
-    AT_no_undeclared_includes
+    AT_no_undeclared_includes,
+
+    /// The 'optional' attribute.
+    AT_optional
   };
 
 } // namespace
@@ -2118,6 +2124,8 @@ void ModuleMapParser::parseModuleDecl() {
     ActiveModule->IsExternC = true;
   if (Attrs.NoUndeclaredIncludes)
     ActiveModule->NoUndeclaredIncludes = true;
+  if (Attrs.IsOptional)
+    ActiveModule->IsOptional = true;
   ActiveModule->Directory = Directory;
 
   StringRef MapFileName(ModuleMapFile.getName());
@@ -3020,6 +3028,7 @@ bool ModuleMapParser::parseOptionalAttributes(Attributes &Attrs) {
           .Case("exhaustive", AT_exhaustive)
           .Case("extern_c", AT_extern_c)
           .Case("no_undeclared_includes", AT_no_undeclared_includes)
+          .Case("optional", AT_optional)
           .Case("system", AT_system)
           .Default(AT_unknown);
     switch (Attribute) {
@@ -3043,6 +3052,10 @@ bool ModuleMapParser::parseOptionalAttributes(Attributes &Attrs) {
     case AT_no_undeclared_includes:
       Attrs.NoUndeclaredIncludes = true;
       break;
+
+    case AT_optional:
+      Attrs.IsOptional = true;
+      break;
     }
     consumeToken();