AST: Allow empty protocols to be @rethrows

Author: slavapestov

lib/AST/Effects.cpp

@@ -41,9 +41,8 @@ ProtocolDecl::getRethrowingRequirements() const {
     ProtocolRethrowsRequirementList());
 }
 
-bool 
-ProtocolDecl::isRethrowingProtocol() const {
-  return getRethrowingRequirements().size() > 0;
+bool ProtocolDecl::isRethrowingProtocol() const {
+  return getAttrs().hasAttribute<swift::AtRethrowsAttr>();
 }
 
 FunctionRethrowingKind AbstractFunctionDecl::getRethrowingKind() const {

test/attr/attr_rethrows_protocol.swift

@@ -113,4 +113,12 @@ func rethrowsWithThrowsClosure<T : ThrowsClosure>(_ t: T) rethrows {
   try t.doIt() {}
 }
 
-try rethrowsWithThrowsClosure(ThrowsClosureWitness())
+try rethrowsWithThrowsClosure(ThrowsClosureWitness())
+
+// Empty protocol
+@rethrows protocol Empty {}
+struct EmptyWitness : Empty {}
+
+func takesEmpty<T : Empty>(_: T) rethrows {}
+
+takesEmpty(EmptyWitness())