swift-module-digester: diagnose adding variables with fixed binary order.

Removing these variables should be detected as API breakages already.

include/swift/AST/DiagnosticsModuleDiffer.def

@@ -54,6 +54,8 @@ ERROR(decl_new_attr,none,"%0 is now %1", (StringRef, StringRef))
 
 ERROR(decl_reorder,none,"%0 in a fixed layout type changes position from %1 to %2", (StringRef, unsigned, unsigned))
 
+ERROR(decl_added,none,"%0 is added to a non-resilient type", (StringRef))
+
 #ifndef DIAG_NO_UNDEF
 # if defined(DIAG)
 #  undef DIAG

test/api-digester/Inputs/cake2.swift

@@ -51,8 +51,12 @@ public extension P1 {
 @_fixed_layout
 public struct fixedLayoutStruct {
   public var a = 1
+  public func OKChange() {}
+  private static let constant = 0
   public var b = 2
   public func foo() {}
+  private var c = 3
+  private lazy var lazy_d = 4
 }
 
 @_frozen
@@ -60,4 +64,5 @@ public enum FrozenKind {
   case Unchanged
   case Rigid
   case Fixed
+  case AddedCase
 }

test/api-digester/Outputs/Cake-abi.txt

@@ -36,3 +36,6 @@ cake1: EnumElement FrozenKind.Fixed in a fixed layout type changes position from
 cake1: EnumElement FrozenKind.Rigid in a fixed layout type changes position from 2 to 1
 cake1: Var fixedLayoutStruct.a in a fixed layout type changes position from 1 to 0
 cake1: Var fixedLayoutStruct.b in a fixed layout type changes position from 0 to 1
+cake2: EnumElement FrozenKind.AddedCase is added to a non-resilient type
+cake2: Var fixedLayoutStruct.c is added to a non-resilient type
+cake2: Var fixedLayoutStruct.lazy_d.storage is added to a non-resilient type

tools/swift-api-digester/ModuleDiagsConsumer.cpp

@@ -47,6 +47,7 @@ static StringRef getCategoryName(uint32_t ID) {
     return "/* RawRepresentable Changes */";
   case LocalDiagID::generic_sig_change:
     return "/* Generic Signature Changes */";
+  case LocalDiagID::decl_added:
   case LocalDiagID::decl_reorder:
     return "/* Fixed-layout Type Changes */";
   default:

tools/swift-api-digester/swift-api-digester.cpp

@@ -762,6 +762,14 @@ public:
     case NodeMatchReason::Added:
       assert(!Left);
       Right->annotate(NodeAnnotation::Added);
+      if (Ctx.checkingABI()) {
+        if (auto *VAD = dyn_cast<SDKNodeDeclVar>(Right)) {
+          if (VAD->hasFixedBinaryOrder()) {
+            Ctx.getDiags().diagnose(SourceLoc(), diag::decl_added,
+                                    VAD->getScreenInfo());
+          }
+        }
+      }
       return;
     case NodeMatchReason::Removed:
       assert(!Right);