AST: Support '@_usableFromInline import'

Author: slavapestov

include/swift/AST/Attr.def

@@ -372,6 +372,9 @@ SIMPLE_DECL_ATTR(_frozen, Frozen,
   OnEnum |
   UserInaccessible,
   76)
+SIMPLE_DECL_ATTR(_usableFromInline, UsableFromInlineImport,
+  OnImport | UserInaccessible,
+  77)
 
 #undef TYPE_ATTR
 #undef DECL_ATTR_ALIAS

include/swift/AST/Decl.h

@@ -1549,10 +1549,19 @@ class ImportDecl final : public Decl,
     return static_cast<ImportKind>(Bits.ImportDecl.ImportKind);
   }
 
+  // An exported import is visible to name lookup from other modules, and is
+  // autolinked when the containing module is autolinked.
   bool isExported() const {
     return getAttrs().hasAttribute<ExportedAttr>();
   }
 
+  // A usable from inline import is autolinked but not visible to name lookup.
+  // This attribute is inferred when type checking inlinable and default
+  // argument bodies.
+  bool isUsableFromInline() const {
+    return getAttrs().hasAttribute<UsableFromInlineImportAttr>();
+  }
+
   ModuleDecl *getModule() const { return Mod; }
   void setModule(ModuleDecl *M) { Mod = M; }
 

include/swift/AST/Module.h

@@ -732,13 +732,17 @@ class SourceFile final : public FileUnit {
   };
 
   /// Possible attributes for imports in source files.
-  enum class ImportFlags {
+  enum class ImportFlags : uint8_t {
     /// The imported module is exposed to anyone who imports the parent module.
     Exported = 0x1,
 
     /// This source file has access to testable declarations in the imported
     /// module.
-    Testable = 0x2
+    Testable = 0x2,
+
+    /// Modules that depend on the module containing this source file will
+    /// autolink this dependency.
+    UsableFromInline = 0x4,
   };
 
   /// \see ImportFlags

lib/Sema/NameBinding.cpp

@@ -235,6 +235,8 @@ void NameBinder::addImport(
   ImportOptions options;
   if (ID->isExported())
     options |= SourceFile::ImportFlags::Exported;
+  if (ID->isUsableFromInline())
+    options |= SourceFile::ImportFlags::UsableFromInline;
   if (testableAttr)
     options |= SourceFile::ImportFlags::Testable;
   imports.push_back({ { ID->getDeclPath(), M }, options });

lib/Sema/TypeCheckAttr.cpp

@@ -112,6 +112,7 @@ class AttributeEarlyChecker : public AttributeVisitor<AttributeEarlyChecker> {
   IGNORED_ATTR(UIApplicationMain)
   IGNORED_ATTR(UnsafeNoObjCTaggedPointer)
   IGNORED_ATTR(UsableFromInline)
+  IGNORED_ATTR(UsableFromInlineImport)
   IGNORED_ATTR(WeakLinked)
 #undef IGNORED_ATTR
 
@@ -840,6 +841,7 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
     IGNORED_ATTR(SynthesizedProtocol)
     IGNORED_ATTR(Testable)
     IGNORED_ATTR(Transparent)
+    IGNORED_ATTR(UsableFromInlineImport)
     IGNORED_ATTR(WarnUnqualifiedAccess)
     IGNORED_ATTR(WeakLinked)
 #undef IGNORED_ATTR

lib/Sema/TypeCheckDecl.cpp

@@ -5775,6 +5775,8 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     UNINTERESTING_ATTR(ClangImporterSynthesizedType)
     UNINTERESTING_ATTR(WeakLinked)
     UNINTERESTING_ATTR(Frozen)
+    UNINTERESTING_ATTR(UsableFromInlineImport)
+
 #undef UNINTERESTING_ATTR
 
     void visitAvailableAttr(AvailableAttr *attr) {