Add @_requiresConcreteImplementation attribute

Author: xwu

include/swift/AST/Attr.def

@@ -552,6 +552,12 @@ DECL_ATTR(transpose, Transpose,
   ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove,
   99)
 
+SIMPLE_DECL_ATTR(_requiresConcreteImplementation, RequiresConcreteImplementation,
+  OnFunc | OnAccessor | OnVar | OnSubscript | OnConstructor |
+  LongAttribute | UserInaccessible | NotSerialized |
+  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  100)
+
 #undef TYPE_ATTR
 #undef DECL_ATTR_ALIAS
 #undef CONTEXTUAL_DECL_ATTR_ALIAS

include/swift/AST/DiagnosticsSema.def

@@ -2116,8 +2116,8 @@ NOTE(declared_protocol_conformance_here,none,
      (Type, unsigned, DeclName, DeclName))
 
 ERROR(witness_unavailable,none,
-        "unavailable %0 %1 was used to satisfy a requirement of protocol %2",
-        (DescriptiveDeclKind, DeclName, DeclName))
+      "unavailable %0 %1 was used to satisfy a requirement of protocol %2",
+      (DescriptiveDeclKind, DeclName, DeclName))
 
 ERROR(redundant_conformance,none,
       "redundant conformance of %0 to protocol %1", (Type, DeclName))
@@ -2900,6 +2900,19 @@ ERROR(implements_attr_non_protocol_type,none,
 ERROR(implements_attr_protocol_not_conformed_to,none,
       "containing type %0 does not conform to protocol %1",
       (DeclName, DeclName))
+      
+// @_requiresConcreteImplementation
+ERROR(requires_concrete_implementation_attr_wrong_decl_context,none,
+      "'@_requiresConcreteImplementation' can only be specified on protocol "
+      "requirements", ())
+ERROR(witness_must_be_concrete_implementation,none,
+      "protocol requirement marked '@_requiresConcreteImplementation' cannot "
+      "be satisfied by %0 %1 declared in protocol extension",
+      (DescriptiveDeclKind, DeclName))
+ERROR(override_not_requiring_concrete_implementation,none,
+      "override of protocol requirement marked "
+      "'@_requiresConcreteImplementation' should also be marked "
+      "'@_requiresConcreteImplementation'", ())
 
 // @differentiable
 ERROR(differentiable_attr_no_vjp_or_jvp_when_linear,none,

lib/Sema/TypeCheckAttr.cpp

@@ -254,6 +254,9 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
 
   void visitDifferentiableAttr(DifferentiableAttr *attr);
   void visitDerivativeAttr(DerivativeAttr *attr);
+  
+  void visitRequiresConcreteImplementationAttr(
+                                     RequiresConcreteImplementationAttr *attr);
 };
 } // end anonymous namespace
 
@@ -4551,3 +4554,10 @@ void AttributeChecker::visitDerivativeAttr(DerivativeAttr *attr) {
   if (typeCheckDerivativeAttr(Ctx, D, attr))
     attr->setInvalid();
 }
+
+void AttributeChecker::visitRequiresConcreteImplementationAttr(
+                                     RequiresConcreteImplementationAttr *attr) {
+  if (!isa<ProtocolDecl>(D->getDeclContext()))
+    diagnoseAndRemoveAttr(attr,
+                diag::requires_concrete_implementation_attr_wrong_decl_context);
+}

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1539,6 +1539,21 @@ namespace  {
         .fixItInsert(Base->getAttributeInsertionLoc(false),
                      "@objc ");
     }
+
+    void visitRequiresConcreteImplementationAttr(
+                                    RequiresConcreteImplementationAttr *attr) {
+      // Require '@_requiresConcreteImplementation' on the override if it was
+      // there on the base.
+      if (!Base->getAttrs().hasAttribute<RequiresConcreteImplementationAttr>())
+        return;
+
+      if (Override->getAttrs().hasAttribute<RequiresConcreteImplementationAttr>())
+        return;
+
+      Diags.diagnose(Override,
+                     diag::override_not_requiring_concrete_implementation);
+      Diags.diagnose(Base, diag::overridden_here);
+    }
   };
 } // end anonymous namespace
 

lib/Sema/TypeCheckProtocol.cpp

@@ -1377,6 +1377,13 @@ RequirementCheck WitnessChecker::checkWitness(ValueDecl *requirement,
     return CheckKind::WitnessUnavailable;
   }
 
+  // If a requirement has the @_requiresConcreteImplementation attribute, it
+  // cannot be satisfied by a default implementation in a protocol extension.
+  if (requirement->getAttrs().hasAttribute<RequiresConcreteImplementationAttr>()
+      && match.Witness->getDeclContext()->getExtendedProtocolDecl()) {
+    return CheckKind::RequiresConcreteImplementation;
+  }
+
   return CheckKind::Success;
 }
 
@@ -3433,8 +3440,7 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
 
     case CheckKind::WitnessUnavailable:
       diagnoseOrDefer(requirement, /*isError=*/true,
-        [witness, requirement](
-                                    NormalProtocolConformance *conformance) {
+        [witness, requirement](NormalProtocolConformance *conformance) {
           auto &diags = witness->getASTContext().Diags;
           SourceLoc diagLoc = getLocForDiagnosingWitness(conformance, witness);
           diags.diagnose(diagLoc,
@@ -3448,6 +3454,22 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
                          requirement->getFullName());
         });
       break;
+
+    case CheckKind::RequiresConcreteImplementation:
+      diagnoseOrDefer(requirement, false,
+        [witness, requirement](NormalProtocolConformance *conformance) {
+          auto &diags = witness->getASTContext().Diags;
+          SourceLoc diagLoc = getLocForDiagnosingWitness(conformance, witness);
+          diags.diagnose(diagLoc,
+                         diag::witness_must_be_concrete_implementation,
+                         witness->getDescriptiveKind(),
+                         witness->getFullName());
+          emitDeclaredHereIfNeeded(diags, diagLoc, witness);
+          diags.diagnose(requirement, diag::kind_declname_declared_here,
+                         DescriptiveDeclKind::Requirement,
+                         requirement->getFullName());
+        });
+      break;
     }
 
     if (auto *classDecl = Adoptee->getClassOrBoundGenericClass()) {

lib/Sema/TypeCheckProtocol.h

@@ -278,6 +278,10 @@ enum class CheckKind : unsigned {
 
   /// The witness itself is inaccessible.
   WitnessUnavailable,
+
+  /// The requirement was marked '@_requiresConcreteImplementation', but the
+  /// witness is a default implementation in a protocol extension.
+  RequiresConcreteImplementation,
 };
 
 /// Describes an optional adjustment made to a witness.