Add diagnostics for ownership modifiers on borrow/mutate accessors

Author: meg-gupta

include/swift/AST/DiagnosticsSema.def

@@ -1798,9 +1798,16 @@ ERROR(mutating_invalid_classes,none, "%0 is not valid on %kindonly1s in "
 ERROR(functions_mutating_and_not,none,
       "method must not be declared both %0 and %1",
       (SelfAccessKind, SelfAccessKind))
+
 ERROR(static_functions_not_mutating,none,
       "static functions must not be declared mutating", ())
 
+ERROR(consuming_invalid_borrow_mutate_accessor, none,
+      "%0 cannot be declared consuming", (StringRef))
+
+ERROR(ownership_modifier_unsupported_borrow_mutate_accessor, none,
+      "%0 ownership modifier is not yet supported on %1", (StringRef, StringRef))
+
 ERROR(readwriter_mutatingness_differs_from_reader_or_writer_mutatingness,none,
       "%0 cannot be %1 when "
       "%select{both the %3 is %4 and the %5 is not %6|either the %3 is not %4 or the %5 is %6|the %3 is not %4|the %5 is %6}2",

lib/AST/Decl.cpp

@@ -7595,7 +7595,7 @@ StringRef swift::getAccessorNameForDiagnostic(AccessorKind accessorKind,
   case AccessorKind::Init:
     return article ? "an init accessor" : "init accessor";
   case AccessorKind::Borrow:
-    return article ? " a 'borrow' accessor" : "borrow accessor";
+    return article ? "a 'borrow' accessor" : "borrow accessor";
   case AccessorKind::Mutate:
     return article ? "a 'mutate' accessor" : "mutate accessor";
   }

lib/Sema/TypeCheckAttr.cpp

@@ -708,6 +708,31 @@ void AttributeChecker::visitMutationAttr(DeclAttribute *attr) {
   // Verify that we don't have a static function.
   if (FD->isStatic())
     diagnoseAndRemoveAttr(attr, diag::static_functions_not_mutating);
+
+  auto *accessor = dyn_cast<AccessorDecl>(FD);
+  if (accessor &&
+      (accessor->isBorrowAccessor() || accessor->isMutateAccessor())) {
+    if (attrModifier == SelfAccessKind::Consuming ||
+        attrModifier == SelfAccessKind::LegacyConsuming) {
+      diagnoseAndRemoveAttr(
+          attr, diag::consuming_invalid_borrow_mutate_accessor,
+          getAccessorNameForDiagnostic(accessor, /*article*/ true));
+    }
+    if (attrModifier == SelfAccessKind::NonMutating &&
+        accessor->isMutateAccessor()) {
+      diagnoseAndRemoveAttr(
+          attr, diag::ownership_modifier_unsupported_borrow_mutate_accessor,
+          attr->getAttrName(),
+          getAccessorNameForDiagnostic(accessor, /*article*/ true));
+    }
+    if (attrModifier == SelfAccessKind::Mutating &&
+        accessor->isBorrowAccessor()) {
+      diagnoseAndRemoveAttr(
+          attr, diag::ownership_modifier_unsupported_borrow_mutate_accessor,
+          attr->getAttrName(),
+          getAccessorNameForDiagnostic(accessor, /*article*/ true));
+    }
+  }
 }
 
 void AttributeChecker::visitDynamicAttr(DynamicAttr *attr) {

test/Sema/borrow_and_mutate_accessors.swift

@@ -9,6 +9,34 @@ class Klass {
 
 struct Struct {
   var _i: Int = 0
+  var _k: Klass
+
+  var k1: Klass {
+    consuming borrow { // expected-error{{a 'borrow' accessor cannot be declared consuming}}
+      return _k
+    }
+    consuming mutate { // expected-error{{a 'mutate' accessor cannot be declared consuming}}
+      return &_k
+    }
+  }
+
+  var k2: Klass {
+    mutating borrow { // expected-error{{mutating ownership modifier is not yet supported on a 'borrow' accessor}}
+      return _k
+    }
+    nonmutating mutate { // expected-error{{nonmutating ownership modifier is not yet supported on a 'mutate' accessor}}
+      return &_k // expected-error{{'&' may only be used to pass an argument to inout parameter}}
+    }
+  }
+
+  var k3: Klass {
+    nonmutating borrow {
+      return _k
+    }
+    mutating mutate {
+      return &_k
+    }
+  }
 }
 
 extension Klass {