[Parse] Add an attribute for typeEraser.

include/swift/AST/Attr.def

@@ -519,6 +519,11 @@ SIMPLE_DECL_ATTR(_inheritsConvenienceInitializers,
   APIStableToAdd | ABIStableToAdd | APIBreakingToRemove | ABIBreakingToRemove,
   93)
 
+DECL_ATTR(_typeEraser, TypeEraser,
+  OnProtocol | UserInaccessible | NotSerialized |
+  ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove,
+  94)
+
 SIMPLE_DECL_ATTR(IBSegueAction, IBSegueAction,
   OnFunc |
   ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,

include/swift/AST/Attr.h

@@ -1108,6 +1108,21 @@ class DynamicReplacementAttr final
   }
 };
 
+/// The \c @_typeEraser(TypeEraserType) attribute.
+class TypeEraserAttr final : public DeclAttribute {
+  TypeLoc TypeEraserLoc;
+public:
+  TypeEraserAttr(SourceLoc atLoc, SourceRange range, TypeLoc typeEraserLoc)
+      : DeclAttribute(DAK_TypeEraser, atLoc, range, /*Implicit=*/false),
+        TypeEraserLoc(typeEraserLoc) {}
+
+  const TypeLoc &getTypeEraserLoc() const { return TypeEraserLoc; }
+
+  static bool classof(const DeclAttribute *DA) {
+    return DA->getKind() == DAK_TypeEraser;
+  }
+};
+
 /// Represents any sort of access control modifier.
 class AbstractAccessControlAttr : public DeclAttribute {
 protected:

include/swift/AST/DiagnosticsParse.def

@@ -1488,6 +1488,11 @@ ERROR(attr_dynamic_replacement_expected_for,none,
 ERROR(attr_dynamic_replacement_expected_colon,none,
       "expected ':' after @_dynamicReplacement(for", ())
 
+ERROR(attr_type_eraser_expected_type_name,none,
+      "expected a type name in @_typeEraser()", ())
+ERROR(attr_type_eraser_expected_rparen,none,
+      "expected ')' after type name for @_typeEraser", ())
+
 ERROR(attr_private_import_expected_rparen,none,
       "expected ')' after function name for @_private", ())
 ERROR(attr_private_import_expected_sourcefile, none,

lib/AST/Attr.cpp

@@ -1094,6 +1094,8 @@ StringRef DeclAttribute::getAttrName() const {
     return "objc";
   case DAK_DynamicReplacement:
     return "_dynamicReplacement";
+  case DAK_TypeEraser:
+    return "_typeEraser";
   case DAK_PrivateImport:
     return "_private";
   case DAK_RestatedObjCConformance:

lib/Parse/ParseDecl.cpp

@@ -2175,6 +2175,37 @@ bool Parser::parseNewDeclAttribute(DeclAttributes &Attributes, SourceLoc AtLoc,
     break;
   }
 
+  case DAK_TypeEraser: {
+    // Parse leading '('
+    if (Tok.isNot(tok::l_paren)) {
+      diagnose(Loc, diag::attr_expected_lparen, AttrName,
+               DeclAttribute::isDeclModifier(DK));
+      return false;
+    }
+
+    SourceLoc LParenLoc = consumeToken(tok::l_paren);
+    ParserResult<TypeRepr> ErasedType;
+    bool invalid = false;
+    {
+      // Parse type-eraser type
+      SyntaxParsingContext ContentContext(SyntaxContext, SyntaxKind::Type);
+      ErasedType = parseType(diag::attr_type_eraser_expected_type_name);
+      invalid = ErasedType.hasCodeCompletion() || ErasedType.isNull();
+    }
+
+    // Parse matching ')'
+    SourceLoc RParenLoc;
+    invalid |= parseMatchingToken(tok::r_paren, RParenLoc,
+                                  diag::attr_type_eraser_expected_rparen,
+                                  LParenLoc);
+    if (invalid)
+      return false;
+
+    Attributes.add(new (Context)
+        TypeEraserAttr(AtLoc, {Loc, RParenLoc}, ErasedType.get()));
+    break;
+  }
+
   case DAK_Specialize: {
     if (Tok.isNot(tok::l_paren)) {
       diagnose(Loc, diag::attr_expected_lparen, AttrName,

lib/Sema/TypeCheckAttr.cpp

@@ -108,6 +108,7 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
   IGNORED_ATTR(StaticInitializeObjCMetadata)
   IGNORED_ATTR(SynthesizedProtocol)
   IGNORED_ATTR(Testable)
+  IGNORED_ATTR(TypeEraser)
   IGNORED_ATTR(WeakLinked)
   IGNORED_ATTR(PrivateImport)
   IGNORED_ATTR(DisfavoredOverload)

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1434,6 +1434,7 @@ namespace  {
     UNINTERESTING_ATTR(Convenience)
     UNINTERESTING_ATTR(Semantics)
     UNINTERESTING_ATTR(SetterAccess)
+    UNINTERESTING_ATTR(TypeEraser)
     UNINTERESTING_ATTR(HasStorage)
     UNINTERESTING_ATTR(UIApplicationMain)
     UNINTERESTING_ATTR(UsableFromInline)

lib/Serialization/ModuleFormat.h

@@ -1823,6 +1823,12 @@ namespace decls_block {
     BCArray<IdentifierIDField>
   >;
 
+  using TypeEraserDeclAttrLayout = BCRecordLayout<
+    TypeEraser_DECL_ATTR,
+    BCFixed<1>, // implicit flag
+    TypeIDField // type eraser type
+  >;
+
   using CustomDeclAttrLayout = BCRecordLayout<
     Custom_DECL_ATTR,
     BCFixed<1>,  // implicit flag

lib/Serialization/Serialization.cpp

@@ -2108,6 +2108,7 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
     case DAK_RestatedObjCConformance:
     case DAK_ClangImporterSynthesizedType:
     case DAK_PrivateImport:
+    case DAK_TypeEraser:
       llvm_unreachable("cannot serialize attribute");
 
     case DAK_Count:

test/attr/typeEraser.swift

@@ -0,0 +1,19 @@
+// RUN: %target-swift-frontend -typecheck %s -verify
+
+class AnyP: P1 {}
+
+@_typeEraser(AnyP) // okay
+protocol P1 {}
+
+@_typeEraser // expected-error {{expected '(' in '_typeEraser' attribute}}
+protocol P2 {}
+
+@_typeEraser() // expected-error {{expected a type name in @_typeEraser()}}
+protocol P3 {}
+
+@_typeEraser(AnyP // expected-note {{to match this opening '('}}
+protocol P4 {} // expected-error {{expected ')' after type name for @_typeEraser}}
+
+@_typeEraser(AnyP) // expected-error {{@_typeEraser may only be used on 'protocol' declarations}}
+func notAProtocol() {}
+