Add a debug flag to emit invalid swiftinterface files

Author: beccadax

include/swift/Frontend/ModuleInterfaceSupport.h

@@ -41,12 +41,15 @@ struct ModuleInterfaceOptions {
   /// back .swiftinterface and reconstructing .swiftmodule.
   std::string Flags;
 
-  // Print SPI decls and attributes.
+  /// Print SPI decls and attributes.
   bool PrintSPIs = false;
 
   /// Print imports with both @_implementationOnly and @_spi, only applies
   /// when PrintSPIs is true.
   bool ExperimentalSPIImports = false;
+
+  /// Intentionally print invalid syntax into the file.
+  bool DebugPrintInvalidSyntax = false;
 };
 
 extern version::Version InterfaceFormatVersion;

include/swift/Option/FrontendOptions.td

@@ -256,6 +256,9 @@ def debug_forbid_typecheck_prefix : Separate<["-"], "debug-forbid-typecheck-pref
   HelpText<"Triggers llvm fatal_error if typechecker tries to typecheck a decl "
            "with the provided prefix name">;
 
+def debug_emit_invalid_swiftinterface_syntax : Flag<["-"], "debug-emit-invalid-swiftinterface-syntax">,
+  HelpText<"Write an invalid declaration into swiftinterface files">;
+
 def debug_cycles : Flag<["-"], "debug-cycles">,
   HelpText<"Print out debug dumps when cycles are detected in evaluation">;
 def build_request_dependency_graph : Flag<["-"], "build-request-dependency-graph">,

lib/Frontend/CompilerInvocation.cpp

@@ -325,6 +325,8 @@ static void ParseModuleInterfaceArgs(ModuleInterfaceOptions &Opts,
     Args.hasArg(OPT_experimental_print_full_convention);
   Opts.ExperimentalSPIImports |=
     Args.hasArg(OPT_experimental_spi_imports);
+  Opts.DebugPrintInvalidSyntax |=
+    Args.hasArg(OPT_debug_emit_invalid_swiftinterface_syntax);
 }
 
 /// Save a copy of any flags marked as ModuleInterfaceOption, if running

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -551,5 +551,8 @@ bool swift::emitSwiftInterface(raw_ostream &out,
   if (needDummyProtocolDeclaration)
     InheritedProtocolCollector::printDummyProtocolDeclaration(out);
 
+  if (Opts.DebugPrintInvalidSyntax)
+    out << "#__debug_emit_invalid_swiftinterface_syntax__\n";
+
   return false;
 }