Diagnose borrow/mutate accessors in enums

We don't have support for borrowing switch on Copyable types.
It is supported for ~Copyable types, but the return expression emission for borrow accessors is not yet implemented.
Diagnose instead of crashing the compiler.

Author: meg-gupta

include/swift/AST/DiagnosticsCommon.def

@@ -280,8 +280,8 @@ ERROR(lookup_outputs_dont_match,none,
 // MARK: Accessor diagnostics
 //------------------------------------------------------------------------------
 
-ERROR(accessor_not_supported_in_decl,none,
-      "%0 is supported only on a struct or enum", (StringRef))      
+ERROR(borrow_mutate_accessor_not_supported_in_decl, none,
+      "%0 is supported only on a struct", (StringRef))      
 
 //------------------------------------------------------------------------------
 // MARK: bridged diagnostics

lib/Parse/ParseDecl.cpp

@@ -8131,9 +8131,8 @@ bool Parser::parseAccessorAfterIntroducer(
   }
 
   if (Kind == AccessorKind::Borrow || Kind == AccessorKind::Mutate) {
-    if (!Flags.contains(PD_InStruct) && !Flags.contains(PD_InEnum) &&
-        !Flags.contains(PD_InExtension)) {
-      diagnose(Tok, diag::accessor_not_supported_in_decl,
+    if (!Flags.contains(PD_InStruct) && !Flags.contains(PD_InExtension)) {
+      diagnose(Tok, diag::borrow_mutate_accessor_not_supported_in_decl,
                getAccessorNameForDiagnostic(Kind, /*article*/ true,
                                             /*underscored*/ false));
     }

lib/Sema/TypeCheckStorage.cpp

@@ -4324,16 +4324,22 @@ StorageImplInfoRequest::evaluate(Evaluator &evaluator,
   if (borrow || mutate) {
     if (auto *extDecl = dyn_cast<ExtensionDecl>(DC)) {
       auto extNominal = extDecl->getExtendedNominal();
-      if (!isa<StructDecl>(extNominal) && !isa<EnumDecl>(extNominal)) {
+      if (!isa<StructDecl>(extNominal)) {
         if (borrow) {
           storage->getASTContext().Diags.diagnose(
-              borrow->getLoc(), diag::accessor_not_supported_in_decl,
-              "a borrow accessor");
+              borrow->getLoc(),
+              diag::borrow_mutate_accessor_not_supported_in_decl,
+              getAccessorNameForDiagnostic(borrow->getAccessorKind(),
+                                           /*article*/ true,
+                                           /*underscored*/ false));
         }
         if (mutate) {
           storage->getASTContext().Diags.diagnose(
-              mutate->getLoc(), diag::accessor_not_supported_in_decl,
-              "a mutate accessor");
+              mutate->getLoc(),
+              diag::borrow_mutate_accessor_not_supported_in_decl,
+              getAccessorNameForDiagnostic(mutate->getAccessorKind(),
+                                           /*article*/ true,
+                                           /*underscored*/ false));
         }
       }
     }

test/Parse/borrow_and_mutate_accessors.swift

@@ -80,31 +80,14 @@ struct Wrapper {
 var i: Int
 
 var i_accessor: Int {
-  borrow { // expected-error{{a 'borrow' accessor is supported only on a struct or enum}}
+  borrow { // expected-error{{a 'borrow' accessor is supported only on a struct}}
     fatalError()
   }
-  mutate { // expected-error{{a 'mutate' accessor is supported only on a struct or enum}}
+  mutate { // expected-error{{a 'mutate' accessor is supported only on a struct}}
     return &i // expected-error{{'&' may only be used to pass an argument to inout parameter}}
   }
 }
 
-var _count: Int = 0
-
-enum Color {
-  case red
-  case green
-  case blue
-
-  var count: Int {
-    borrow {
-      return _count
-    }
-    mutate {
-      return &_count
-    }
-  }
-}
-
 class KlassWrapper {
   var _k: Klass
 
@@ -113,10 +96,10 @@ class KlassWrapper {
   }
 
   var k: Klass {
-    borrow {// expected-error{{a 'borrow' accessor is supported only on a struct or enum}}
+    borrow {// expected-error{{a 'borrow' accessor is supported only on a struct}}
       return _k
     }
-    mutate {// expected-error{{a 'mutate' accessor is supported only on a struct or enum}}
+    mutate {// expected-error{{a 'mutate' accessor is supported only on a struct}}
       return &_k
     }
   }

test/Sema/borrow_and_mutate_accessors.swift

@@ -41,10 +41,10 @@ struct Struct {
 
 extension Klass {
   var i: Int {
-    borrow { // expected-error{{a borrow accessor is supported only on a struct or enum}}
+    borrow { // expected-error{{a 'borrow' accessor is supported only on a struct}}
       return 0
     }
-    mutate { // expected-error{{a mutate accessor is supported only on a struct or enum}}
+    mutate { // expected-error{{a 'mutate' accessor is supported only on a struct}}
       return &_i
     }
   }
@@ -67,3 +67,19 @@ protocol P {
   var phone: String { mutate } // expected-error{{property in protocol must have explicit { get } or { get set } specifier}} // expected-error{{expected get, read, or set in a protocol property}}
 }
 
+enum OrderStatus: ~Copyable {
+  case processing(trackingNumber: String)
+  case cancelled(reason: String)
+  
+  var description: String {
+    borrow { // expected-error{{a 'borrow' accessor is supported only on a struct}}
+      switch self {
+        case .processing(let trackingNumber):
+          return trackingNumber
+        case .cancelled(let reason):
+          return reason
+      }
+    }
+  }
+}
+