Assign fallback discriminators within top-level closures.

Fixes rdar://142425569.

lib/AST/Expr.cpp

@@ -1934,11 +1934,13 @@ unsigned AbstractClosureExpr::getDiscriminator() const {
   // If we don't have a discriminator, and either
   //   1. We have ill-formed code and we're able to assign a discriminator, or
   //   2. We are in a macro expansion buffer
+  //   3. We are within top-level code where there's nothing to anchor to
   //
   // then assign the next discriminator now.
   if (getRawDiscriminator() == InvalidDiscriminator &&
       (ctx.Diags.hadAnyError() ||
-       getParentSourceFile()->getFulfilledMacroRole() != std::nullopt)) {
+       getParentSourceFile()->getFulfilledMacroRole() != std::nullopt ||
+       getParent()->isModuleScopeContext())) {
     auto discriminator = ctx.getNextDiscriminator(getParent());
     ctx.setMaxAssignedDiscriminator(getParent(), discriminator + 1);
     const_cast<AbstractClosureExpr *>(this)->

test/Macros/fine_grained_dependencies.swift

@@ -0,0 +1,19 @@
+// REQUIRES: swift_swift_parser, executable_test
+
+// RUN: %empty-directory(%t)
+// RUN: %host-build-swift -swift-version 5 -emit-library -o %t/%target-library-name(MacroDefinition) -module-name=MacroDefinition %S/Inputs/syntax_macro_definitions.swift
+
+// RUN: %target-swift-frontend -typecheck -swift-version 5 -load-plugin-library %t/%target-library-name(MacroDefinition) -module-name MacroUser -parse-as-library -emit-reference-dependencies-path %t/MacroUser.swiftdeps -primary-file %s
+
+// Note: this test ensures that we don't crash when trying to mangle a symbol
+// within a closure passed to a macro.
+
+@freestanding(declaration)
+macro Empty<T>(_ x: T) = #externalMacro(module: "MacroDefinition", type: "EmptyDeclarationMacro")
+
+#Empty {
+    struct S {
+        static var foo: Int { 0 }
+    }
+    _ = S.foo
+}