Ban the use of accessor macros on bindings with multiple variables

As with property wrappers, we can't properly desugar the application of
accessor macros to bindings with multiple variables. Prohibit them up
front.

Fixes rdar://112783811.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -7236,6 +7236,9 @@ ERROR(macro_expand_circular_reference_unnamed, none,
 NOTE(macro_expand_circular_reference_unnamed_through, none,
      "circular reference expanding %0 macros", (StringRef))
 
+ERROR(accessor_macro_not_single_var, none,
+      "accessor macro %0 can only apply to a single variable", (DeclName))
+
 //------------------------------------------------------------------------------
 // MARK: Noncopyable Types Diagnostics
 //------------------------------------------------------------------------------

lib/Sema/TypeCheckMacros.cpp

@@ -1399,6 +1399,15 @@ bool swift::accessorMacroIntroducesInitAccessor(
 llvm::Optional<unsigned> swift::expandAccessors(AbstractStorageDecl *storage,
                                                 CustomAttr *attr,
                                                 MacroDecl *macro) {
+  if (auto var = dyn_cast<VarDecl>(storage)) {
+    // Check that the variable is part of a single-variable pattern.
+    auto binding = var->getParentPatternBinding();
+    if (binding && binding->getSingleVar() != var) {
+      var->diagnose(diag::accessor_macro_not_single_var, macro->getName());
+      return llvm::None;
+    }
+  }
+
   // Evaluate the macro.
   auto macroSourceFile =
       ::evaluateAttachedMacro(macro, storage, attr,

test/Macros/accessor_macros.swift

@@ -155,3 +155,11 @@ struct HasStoredTests {
   // expected-note@-3 2{{'z' declared here}}
 #endif
 }
+
+
+#if TEST_DIAGNOSTICS
+struct MultipleVars {
+  @AddWillSet var (x, y): (Int, Int) = (0, 0)
+  // expected-error@-1 2{{accessor macro 'AddWillSet()' can only apply to a single variable}}
+}
+#endif