Add compiler option to *disable* warnings-as-errors

`-no-warnings-as-errors`

This functionality is required for build systems to be able to overload/disable a given Swift project's preference of treating warnings as errors.

Resolves rdar://problem/35699776

Author: artemcm

include/swift/Option/Options.td

@@ -528,6 +528,10 @@ def warnings_as_errors : Flag<["-"], "warnings-as-errors">,
   Flags<[FrontendOption]>,
   HelpText<"Treat warnings as errors">;
 
+def no_warnings_as_errors : Flag<["-"], "no-warnings-as-errors">,
+  Flags<[FrontendOption]>,
+  HelpText<"Don't treat warnings as errors">;
+
 def continue_building_after_errors : Flag<["-"], "continue-building-after-errors">,
   Flags<[FrontendOption, DoesNotAffectIncrementalBuild]>,
   HelpText<"Continue building, even after errors are encountered">;

lib/Driver/Driver.cpp

@@ -140,7 +140,8 @@ static void validateBridgingHeaderArgs(DiagnosticEngine &diags,
 static void validateWarningControlArgs(DiagnosticEngine &diags,
                                        const ArgList &args) {
   if (args.hasArg(options::OPT_suppress_warnings) &&
-      args.hasArg(options::OPT_warnings_as_errors)) {
+      args.hasFlag(options::OPT_warnings_as_errors,
+                   options::OPT_no_warnings_as_errors, false)) {
     diags.diagnose(SourceLoc(), diag::error_conflicting_options,
                    "-warnings-as-errors", "-suppress-warnings");
   }

lib/Driver/ToolChains.cpp

@@ -216,7 +216,8 @@ void ToolChain::addCommonFrontendArgs(const OutputInfo &OI,
   inputArgs.AddLastArg(arguments, options::OPT_profile_generate);
   inputArgs.AddLastArg(arguments, options::OPT_profile_use);
   inputArgs.AddLastArg(arguments, options::OPT_profile_coverage_mapping);
-  inputArgs.AddLastArg(arguments, options::OPT_warnings_as_errors);
+  inputArgs.AddAllArgs(arguments, options::OPT_warnings_as_errors,
+                       options::OPT_no_warnings_as_errors);
   inputArgs.AddLastArg(arguments, options::OPT_sanitize_EQ);
   inputArgs.AddLastArg(arguments, options::OPT_sanitize_recover_EQ);
   inputArgs.AddLastArg(arguments, options::OPT_sanitize_coverage_EQ);

lib/Frontend/CompilerInvocation.cpp

@@ -760,7 +760,8 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts,
     Opts.PCHDisableValidation |= Args.hasArg(OPT_pch_disable_validation);
   }
 
-  if (Args.hasArg(OPT_warnings_as_errors))
+  if (Args.hasFlag(options::OPT_warnings_as_errors,
+                   options::OPT_no_warnings_as_errors, false))
     Opts.ExtraArgs.push_back("-Werror");
 
   Opts.DebuggerSupport |= Args.hasArg(OPT_debugger_support);
@@ -845,7 +846,9 @@ static bool ParseDiagnosticArgs(DiagnosticOptions &Opts, ArgList &Args,
                    /*Default=*/llvm::sys::Process::StandardErrHasColors());
   Opts.FixitCodeForAllDiagnostics |= Args.hasArg(OPT_fixit_all);
   Opts.SuppressWarnings |= Args.hasArg(OPT_suppress_warnings);
-  Opts.WarningsAsErrors |= Args.hasArg(OPT_warnings_as_errors);
+  Opts.WarningsAsErrors = Args.hasFlag(options::OPT_warnings_as_errors,
+                                       options::OPT_no_warnings_as_errors,
+                                       false);
   Opts.PrintDiagnosticNames |= Args.hasArg(OPT_debug_diagnostic_names);
   Opts.PrintEducationalNotes |= Args.hasArg(OPT_print_educational_notes);
   Opts.EnableExperimentalFormatting |=

test/diagnostics/no_warnings_as_errors.swift

@@ -0,0 +1,12 @@
+// RUN: %target-swift-frontend -typecheck -warnings-as-errors -no-warnings-as-errors %s 2>&1 | %FileCheck %s --check-prefix=CHECK-WARNING
+// RUN: not %target-swift-frontend -typecheck -no-warnings-as-errors -warnings-as-errors  %s 2>&1 | %FileCheck %s --check-prefix=CHECK-ERROR
+
+
+// This test verifies that the -no-warnings-as-errors option nullifies the effect of the -warnings-as-errors option
+// CHECK-WARNING-NOT: error: initialization of immutable value 'c' was never used;
+// CHECK-WARNING: warning: initialization of immutable value 'c' was never used;
+// CHECK-ERROR-NOT: warning: initialization of immutable value 'c' was never used;
+// CHECK-ERROR: error: initialization of immutable value 'c' was never used;
+func b() {
+  let c = 2
+}