[SR-2400] Add -continue-building-after-errors (#4437)

Currently the Swift driver stops invoking frontend commands as soon as one of
them reports an error. Add a flag to control this behavior, so that users can
choose whether to see all the errors at once or bail out early.

Author: modocache

include/swift/Option/Options.td

@@ -234,6 +234,10 @@ def warnings_as_errors : Flag<["-"], "warnings-as-errors">,
   Flags<[FrontendOption, DoesNotAffectIncrementalBuild]>,
   HelpText<"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">;
+
 // Platform options.
 def enable_app_extension : Flag<["-"], "application-extension">,
   Flags<[FrontendOption, NoInteractiveOption]>,

lib/Driver/Driver.cpp

@@ -367,6 +367,8 @@ std::unique_ptr<Compilation> Driver::buildCompilation(
     !ArgList->hasArg(options::OPT_embed_bitcode);
 
   bool SaveTemps = ArgList->hasArg(options::OPT_save_temps);
+  bool ContinueBuildingAfterErrors =
+    ArgList->hasArg(options::OPT_continue_building_after_errors);
 
   std::unique_ptr<DerivedArgList> TranslatedArgList(
     translateInputArgs(*ArgList));
@@ -506,10 +508,11 @@ std::unique_ptr<Compilation> Driver::buildCompilation(
   buildJobs(Actions, OI, OFM.get(), *TC, *C);
 
   // For updating code we need to go through all the files and pick up changes,
-  // even if they have compiler errors.
-  // Also for getting bulk fixits.
+  // even if they have compiler errors. Also for getting bulk fixits, or for when
+  // users explicitly request to continue building despite errors.
   if (OI.CompilerMode == OutputInfo::Mode::UpdateCode ||
-      OI.ShouldGenerateFixitEdits)
+      OI.ShouldGenerateFixitEdits ||
+      ContinueBuildingAfterErrors)
     C->setContinueBuildingAfterErrors();
 
   if (ShowIncrementalBuildDecisions)

test/Driver/Inputs/error.swift

@@ -0,0 +1,6 @@
+struct Foo {
+  let bar: Int
+  init() {
+    self.bar = self.bar
+  }
+}

test/Driver/continue-building-after-errors.swift

@@ -0,0 +1,13 @@
+// RUN: not %swiftc_driver %S/Inputs/error.swift %s 2>&1 | %FileCheck %s
+// RUN: not %swiftc_driver -continue-building-after-errors %S/Inputs/error.swift %s 2>&1 | %FileCheck -check-prefix=CHECK-CONTINUE %s
+
+// CHECK: self.bar = self.bar
+// CHECK-NOT: self.baz = self.baz
+// CHECK-CONTINUE: self.bar = self.bar
+// CHECK-CONTINUE: self.baz = self.baz
+struct Bar {
+  let baz: Int
+  init() {
+    self.baz = self.baz
+  }
+}