[Serialization] Add whether allowing errors to the pretty stack output

It's currently not obvious in crash reports whether compiling with
errors is enabled or not. Since this option can make previously
"impossible" paths now possible, add a message to both the pretty stack
output and fatal deserialization diagnostics to point out that it is
enabled.

include/swift/AST/DiagnosticsSema.def

@@ -793,8 +793,9 @@ ERROR(serialization_fatal,Fatal,
       SWIFT_BUG_REPORT_MESSAGE,
       (StringRef))
 NOTE(serialization_misc_version,none,
-     "module '%0' full misc version is '%1'",
+     "module '%0' full misc version is '%1'"
-     (StringRef, StringRef))
+     "%select{ (built while allowing compiler errors)|}2",
+     (StringRef, StringRef, bool))
 NOTE(serialization_compatibility_version_mismatch,none,
      "compiling as Swift %0, with '%1' built as Swift %2 "
      "(this is supported but may expose additional compiler issues)",

lib/FrontendTool/FrontendTool.cpp

@@ -2046,6 +2046,11 @@ int swift::performFrontend(ArrayRef<const char *> Args,
     return finishDiagProcessing(1, /*verifierEnabled*/ false);
   }
 
+  Optional<llvm::PrettyStackTraceString> allowErrorsStackTrace;
+  if (Invocation.getFrontendOptions().AllowModuleWithCompilerErrors)
+    allowErrorsStackTrace.emplace("While allowing modules with compiler errors "
+                                  "enabled");
+
   // Make an array of PrettyStackTrace objects to dump the configuration files
   // we used to parse the arguments. These are RAII objects, so they and the
   // buffers they refer to must be kept alive in order to be useful. (That is,

lib/Serialization/Deserialization.cpp

@@ -169,8 +169,9 @@ static void skipRecord(llvm::BitstreamCursor &cursor, unsigned recordKind) {
 void ModuleFile::fatal(llvm::Error error) {
   if (FileContext) {
     getContext().Diags.diagnose(SourceLoc(), diag::serialization_fatal, Core->Name);
-    getContext().Diags.diagnose(SourceLoc(), diag::serialization_misc_version,
+    getContext().Diags.diagnose(
-      Core->Name, Core->MiscVersion);
+        SourceLoc(), diag::serialization_misc_version, Core->Name,
+        Core->MiscVersion, allowCompilerErrors());
 
     if (!Core->CompatibilityVersion.empty()) {
       if (getContext().LangOpts.EffectiveLanguageVersion
@@ -6706,3 +6707,9 @@ Optional<ForeignAsyncConvention> ModuleFile::maybeReadForeignAsyncConvention() {
       completionHandlerErrorFlagParamIndex,
       errorFlagPolarity);
 }
+
+void serialization::PrettyStackTraceModuleFile::outputModuleBuildInfo(
+    raw_ostream &os) const {
+  if (MF.compiledAllowingCompilerErrors())
+    os << " (built while allowing compiler errors)";
+}

lib/Serialization/DeserializationErrors.h

@@ -470,8 +470,12 @@ public:
       : PrettyStackTraceModuleFile("While reading from", module) {}
 
   void print(raw_ostream &os) const override {
-    os << Action << " \'" << getNameOfModule(&MF) << "'\n";
+    os << Action << " \'" << getNameOfModule(&MF) << "'";
+    outputModuleBuildInfo(os);
+    os << "\n";
   }
+
+  void outputModuleBuildInfo(raw_ostream &os) const;
 };
 
 class PrettyStackTraceModuleFileCore : public llvm::PrettyStackTraceEntry {

test/Frontend/crash.swift

@@ -10,6 +10,11 @@
 // CHECK-NEXT: test{{[\\/]}}Frontend{{[\\/]}}crash.swift{{$}}
 // CHECK-NEXT: ---
 
+// Check that a message when allowing errors is output
+// RUN: not --crash %target-swift-frontend -typecheck -debug-crash-after-parse -experimental-allow-module-with-compiler-errors %s 2>&1 | %FileCheck -check-prefix CHECK-ALLOW %s
+// CHECK-ALLOW-LABEL: Stack dump
+// CHECK-ALLOW: While allowing modules with compiler errors enabled
+
 func anchor() {}
 anchor()
 

test/Serialization/AllowErrors/crash-adds-message.swift

@@ -0,0 +1,16 @@
+// RUN: %empty-directory(%t)
+// RUN: touch %t/empty.swift
+
+// RUN: %target-swift-frontend -emit-module -o %t/errors.partial.swiftmodule -module-name errors -experimental-allow-module-with-compiler-errors %s
+// RUN: %target-swift-frontend -emit-module -o %t/errorsempty.partial.swiftmodule -module-name errors %t/empty.swift
+
+// Note - running the merge without allow errors to force a crash, we want
+// to check if there's a message about allowing compiler errors for the
+// deserialized module
+// RUN: not --crash %target-swift-frontend -module-name errors -emit-module -o %t/errors.swiftmodule %t/errors.partial.swiftmodule %t/errorsempty.partial.swiftmodule 2>&1 | %FileCheck %s
+
+// REQUIRES: asserts
+
+typealias AnAlias = SomeStruct
+
+// CHECK: While reading from 'errors' (built while allowing compiler errors)

test/Serialization/AllowErrors/invalid-alias.swift

@@ -0,0 +1,19 @@
+// RUN: %empty-directory(%t)
+
+// RUN: touch %t/empty.swift
+// RUN: %{python} %utils/split_file.py -o %t %s
+
+// RUN: %target-swift-frontend -verify -emit-module -o %t/errors.partial.swiftmodule -module-name errors -experimental-allow-module-with-compiler-errors %t/errors.swift
+// RUN: %target-swift-frontend -emit-module -o %t/errorsempty.partial.swiftmodule -module-name errors %t/empty.swift
+
+// RUN: %target-swift-frontend -module-name errors -emit-module -o %t/errors.swiftmodule -experimental-allow-module-with-compiler-errors %t/errors.partial.swiftmodule %t/errorsempty.partial.swiftmodule
+
+// RUN: %target-swift-frontend -emit-module -o %t/mods/uses.swiftmodule -experimental-allow-module-with-compiler-errors -I %t/mods %t/uses.swift 2>&1 | %FileCheck -check-prefix=CHECK-USES %s
+
+// BEGIN errors.swift
+typealias AnAlias = undefined // expected-error {{cannot find type 'undefined'}}
+
+// BEGIN uses.swift
+import errors
+func test(a: AnAlias) {}
+// CHECK-USES-NOT: cannot find type 'AnAlias' in scope