[Sema] Intro @_spiOnly attribute and import filter

Introduce the attribute and basic import filter logic.

include/swift/AST/Attr.def

@@ -768,6 +768,11 @@ SIMPLE_DECL_ATTR(typeWrapper, TypeWrapper,
   ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
   134)
 
+SIMPLE_DECL_ATTR(_spiOnly, SPIOnly,
+  OnImport | UserInaccessible |
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  135)
+
 // If you're adding a new underscored attribute here, please document it in
 // docs/ReferenceGuides/UnderscoredAttributes.md.
 

include/swift/AST/Import.h

@@ -88,7 +88,11 @@ enum class ImportFlags {
   WeakLinked = 0x40,
 
   /// Used for DenseMap.
-  Reserved = 0x80
+  Reserved = 0x80,
+
+  /// The imported module can only be referenced from SPI decls, or
+  /// implementation details.
+  SPIOnly = 0x100
 };
 
 /// \see ImportFlags

include/swift/AST/Module.h

@@ -696,11 +696,13 @@ public:
     Default = 1 << 1,
     /// Include imports declared with `@_implementationOnly`.
     ImplementationOnly = 1 << 2,
-    /// Include imports of SPIs declared with `@_spi`
+    /// Include imports of SPIs declared with `@_spi`.
     SPIAccessControl = 1 << 3,
+    /// Include imports declared with `@_spiOnly`.
+    SPIOnly = 1 << 4,
     /// Include imports shadowed by a cross-import overlay. Unshadowed imports
     /// are included whether or not this flag is specified.
-    ShadowedByCrossImportOverlay = 1 << 4
+    ShadowedByCrossImportOverlay = 1 << 5
   };
   /// \sa getImportedModules
   using ImportFilter = OptionSet<ImportFilterKind>;

lib/AST/ImportCache.cpp

@@ -195,6 +195,7 @@ ImportSet &ImportCache::getImportSet(const DeclContext *dc) {
     file->getImportedModules(imports,
                              {ModuleDecl::ImportFilterKind::Default,
                               ModuleDecl::ImportFilterKind::ImplementationOnly,
+                              ModuleDecl::ImportFilterKind::SPIOnly,
                               ModuleDecl::ImportFilterKind::SPIAccessControl});
   }
 
@@ -279,6 +280,7 @@ ImportCache::getAllAccessPathsNotShadowedBy(const ModuleDecl *mod,
     file->getImportedModules(stack,
                              {ModuleDecl::ImportFilterKind::Default,
                               ModuleDecl::ImportFilterKind::ImplementationOnly,
+                              ModuleDecl::ImportFilterKind::SPIOnly,
                               ModuleDecl::ImportFilterKind::SPIAccessControl});
   }
 

lib/AST/Module.cpp

@@ -1596,6 +1596,8 @@ SourceFile::getImportedModules(SmallVectorImpl<ImportedModule> &modules,
       requiredFilter |= ModuleDecl::ImportFilterKind::Exported;
     else if (desc.options.contains(ImportFlags::ImplementationOnly))
       requiredFilter |= ModuleDecl::ImportFilterKind::ImplementationOnly;
+    else if (desc.options.contains(ImportFlags::SPIOnly))
+      requiredFilter |= ModuleDecl::ImportFilterKind::SPIOnly;
     else if (desc.options.contains(ImportFlags::SPIAccessControl))
       requiredFilter |= ModuleDecl::ImportFilterKind::SPIAccessControl;
     else
@@ -1904,6 +1906,7 @@ SourceFile::collectLinkLibraries(ModuleDecl::LinkLibraryCallback callback) const
 
   ModuleDecl::ImportFilter topLevelFilter = filter;
   topLevelFilter |= ModuleDecl::ImportFilterKind::ImplementationOnly;
+  topLevelFilter |= ModuleDecl::ImportFilterKind::SPIOnly;
   topLevel->getImportedModules(stack, topLevelFilter);
 
   // Make sure the top-level module is first; we want pre-order-ish traversal.
@@ -2600,6 +2603,7 @@ canBeUsedForCrossModuleOptimization(DeclContext *ctxt) const {
   // @_implementationOnly or @_spi.
   ModuleDecl::ImportFilter filter = {
     ModuleDecl::ImportFilterKind::ImplementationOnly,
+    ModuleDecl::ImportFilterKind::SPIOnly,
     ModuleDecl::ImportFilterKind::SPIAccessControl
   };
   SmallVector<ImportedModule, 4> results;

lib/Sema/ImportResolution.cpp

@@ -536,6 +536,9 @@ UnboundImport::UnboundImport(ImportDecl *ID)
   if (ID->getAttrs().hasAttribute<ImplementationOnlyAttr>())
     import.options |= ImportFlags::ImplementationOnly;
 
+  if (ID->getAttrs().hasAttribute<SPIOnlyAttr>())
+    import.options |= ImportFlags::SPIOnly;
+
   if (auto *privateImportAttr =
           ID->getAttrs().getAttribute<PrivateImportAttr>()) {
     import.options |= ImportFlags::PrivateImport;

lib/Sema/TypeCheckAttr.cpp

@@ -296,6 +296,7 @@ public:
   void visitResultBuilderAttr(ResultBuilderAttr *attr);
 
   void visitImplementationOnlyAttr(ImplementationOnlyAttr *attr);
+  void visitSPIOnlyAttr(SPIOnlyAttr *attr);
   void visitNonEphemeralAttr(NonEphemeralAttr *attr);
   void checkOriginalDefinedInAttrs(ArrayRef<OriginallyDefinedInAttr *> Attrs);
 
@@ -3908,6 +3909,10 @@ AttributeChecker::visitImplementationOnlyAttr(ImplementationOnlyAttr *attr) {
   // it won't necessarily be able to say why.
 }
 
+void
+AttributeChecker::visitSPIOnlyAttr(SPIOnlyAttr *attr) {
+}
+
 void AttributeChecker::visitTypeSequenceAttr(TypeSequenceAttr *attr) {
   if (!isa<GenericTypeParamDecl>(D)) {
     attr->setInvalid();

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1582,6 +1582,7 @@ namespace  {
     UNINTERESTING_ATTR(Frozen)
     UNINTERESTING_ATTR(HasInitialValue)
     UNINTERESTING_ATTR(ImplementationOnly)
+    UNINTERESTING_ATTR(SPIOnly)
     UNINTERESTING_ATTR(Custom)
     UNINTERESTING_ATTR(PropertyWrapper)
     UNINTERESTING_ATTR(TypeWrapper)