[ASTPrinter/Parse] Disambiguate accessor block in .swiftinterface

lib/AST/ASTPrinter.cpp

@@ -2716,6 +2716,32 @@ void PrintAST::printAccessors(const AbstractStorageDecl *ASD) {
     }
   }
 
+  Printer << " {";
+
+  // For var decls, if it has an initializer, the parser only expects observing
+  // accessors. But sometimes for example in '.swiftinterface', we want to print
+  // both the initializer and the accessors. So when we print the initializer
+  // *and* the accessor block not starting with willSet/didSet, print an
+  // attribute as a disambiguation marker.
+  bool needsDisambiguationAttr = false;
+  if (auto *VD = dyn_cast<VarDecl>(ASD)) {
+    if (auto *PBD = VD->getParentPatternBinding()) {
+      AccessorDecl *firstAccessor = *accessorsToPrint.begin();
+      if (!firstAccessor->isObservingAccessor()) {
+        const auto i = PBD->getPatternEntryIndexForVarDecl(VD);
+        if (Options.PrintExprs) {
+          needsDisambiguationAttr |= bool(PBD->getInit(i));
+        } else if (Options.VarInitializers) {
+          needsDisambiguationAttr |= bool(PBD->hasInitStringRepresentation(i) &&
+                                          VD->isInitExposedToClients());
+        }
+      }
+    }
+  }
+  if (needsDisambiguationAttr) {
+    Printer << " @_accessorBlock";
+  }
+
   // If we're not printing the accessor bodies and none of the accessors have
   // attributes then we can print in a shorter, compact form.
   bool PrintCompactAccessors =
@@ -2724,9 +2750,6 @@ void PrintAST::printAccessors(const AbstractStorageDecl *ASD) {
                   [](AccessorDecl *accessor) {
                     return accessor->getAttrs().isEmpty();
                   });
-
-  Printer << " {";
-
   if (!PrintCompactAccessors)
     Printer.printNewline();
 

lib/ASTGen/Sources/ASTGen/DeclAttrs.swift

@@ -367,6 +367,11 @@ extension ASTGenVisitor {
         break
 
       case .none:
+        // '@_accessorBlock' is a parser only disambiguation marker, ignore.
+        if attrName == "_accessorBlock" {
+          return
+        }
+
         // Fall back to CustomAttr.
         break
       }

lib/Parse/ParseDecl.cpp

@@ -8167,6 +8167,14 @@ ParserStatus Parser::parseGetSet(ParseDeclOptions Flags, ParameterList *Indices,
   bool IsFirstAccessor = true;
   bool hasEffectfulGet = false;
   accessors.LBLoc = consumeToken(tok::l_brace);
+
+  // Skip accessor-block disambiguation attribute if exist.
+  if (Tok.is(tok::at_sign) &&
+      peekToken().isContextualKeyword("_accessorBlock")) {
+    consumeToken(tok::at_sign);
+    consumeToken();
+  }
+
   while (!Tok.isAny(tok::r_brace, tok::eof)) {
     // Parse introducer if possible.
     DeclAttributes Attributes;

lib/Parse/ParseExpr.cpp

@@ -1127,6 +1127,10 @@ bool Parser::isStartOfGetSetAccessor() {
   // Eat the "{".
   consumeToken(tok::l_brace);
 
+  // '@_accessorBlock' is a builtin disambiguation marker.
+  if (peekToken().isContextualKeyword("_accessorBlock"))
+    return true;
+
   // Eat attributes, if present.
   while (consumeIf(tok::at_sign)) {
     if (!consumeIf(tok::identifier))

test/ModuleInterface/var-with-init-and-accessors.swiftinterface

@@ -0,0 +1,17 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -o %t/Test.swiftmodule -emit-module-interface-path %t/Test.swiftinterface -module-name Test %s
+// RUN: %target-swift-typecheck-module-from-interface(%t/Test.swiftinterface) -module-name Test
+
+// RUN: %FileCheck %s --check-prefix INTERFACE --input-file %t/Test.swiftinterface
+
+@frozen public struct Struct {
+    public var values: [Int] = .init() {
+        willSet {}
+    }
+}
+// INTERFACE: @frozen public struct Struct {
+// INTERFACE-LABEL: @_hasStorage public var values: [Swift.Int] = .init() { @_accessorBlock
+// INTERFACE-NEXT:     @_transparent get
+// INTERFACE-NEXT:     set
+// INTERFACE-NEXT:   }
+// INTERFACE: }