[Serialization] Intro -Rmodule-recovery to remark about silent errors

Deserialization recovery silently drops errors and the affected decls.
This can lead to surprises when a function from an imported module
simply disappears without an explanation.

This commit introduces the flag -Rmodule-recovery to report as remarks
some of these previously silently dropped issues. It can be used to
debug project configuration issues.

include/swift/Basic/LangOptions.h

@@ -238,6 +238,9 @@ namespace swift {
     /// Emit a remark after loading a module.
     bool EnableModuleLoadingRemarks = false;
 
+    /// Emit remarks about contextual inconsistencies in loaded modules.
+    bool EnableModuleRecoveryRemarks = false;
+
     /// Emit a remark when indexing a system module.
     bool EnableIndexingSystemModuleRemarks = false;
     

include/swift/Option/Options.td

@@ -388,7 +388,10 @@ def emit_cross_import_remarks : Flag<["-"], "Rcross-import">,
 
 def remark_loading_module : Flag<["-"], "Rmodule-loading">,
   Flags<[FrontendOption, DoesNotAffectIncrementalBuild]>,
-  HelpText<"Emit a remark and file path of each loaded module">;
+  HelpText<"Emit remarks about loaded module">;
+def remark_module_recovery : Flag<["-"], "Rmodule-recovery">,
+  Flags<[FrontendOption, DoesNotAffectIncrementalBuild]>,
+  HelpText<"Emit remarks about contextual inconsistencies in loaded modules">;
 
 def remark_indexing_system_module : Flag<["-"], "Rindexing-system-module">,
   Flags<[FrontendOption, DoesNotAffectIncrementalBuild]>,

lib/Frontend/CompilerInvocation.cpp

@@ -929,6 +929,7 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableCrossImportRemarks = Args.hasArg(OPT_emit_cross_import_remarks);
 
   Opts.EnableModuleLoadingRemarks = Args.hasArg(OPT_remark_loading_module);
+  Opts.EnableModuleRecoveryRemarks = Args.hasArg(OPT_remark_module_recovery);
 
   Opts.EnableIndexingSystemModuleRemarks = Args.hasArg(OPT_remark_indexing_system_module);
 

lib/Serialization/Deserialization.cpp

@@ -7384,6 +7384,14 @@ llvm::Error ModuleFile::consumeExpectedError(llvm::Error &&error) {
 }
 
 void ModuleFile::diagnoseAndConsumeError(llvm::Error error) const {
+  auto &ctx = getContext();
+  if (ctx.LangOpts.EnableModuleRecoveryRemarks) {
+    error = diagnoseModularizationError(std::move(error),
+                                        DiagnosticBehavior::Remark);
+    // If error was already diagnosed it was also consumed.
+    if (!error)
+      return;
+  }
 
   consumeError(std::move(error));
 }

test/Serialization/Recovery/module-recovery-remarks.swift

@@ -0,0 +1,49 @@
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/sdk)
+// RUN: split-file %s %t
+
+/// Compile two library modules A and A_related, and a middle library LibWithXRef with a reference to a type in A.
+// RUN: %target-swift-frontend %t/LibOriginal.swift -emit-module-path %t/A.swiftmodule -module-name A -I %t
+// RUN: %target-swift-frontend %t/Empty.swift -emit-module-path %t/A_related.swiftmodule -module-name A_related
+// RUN: %target-swift-frontend %t/LibWithXRef.swift -emit-module-path %t/sdk/LibWithXRef.swiftmodule -module-name LibWithXRef -I %t -swift-version 5 -enable-library-evolution
+
+/// Move MyType from A to A_related, triggering most notes.
+// RUN: %target-swift-frontend %t/EmptyOverlay.swift -emit-module-path %t/A.swiftmodule -module-name A -I %t
+// RUN: %target-swift-frontend %t/LibOriginal.swift -emit-module-path %t/A_related.swiftmodule -module-name A_related -I %t
+// RUN: not %target-swift-frontend -c -O %t/Client.swift -I %t -I %t/sdk -Rmodule-recovery -sdk %t/sdk -swift-version 4 2>&1 \
+// RUN:   | %FileCheck --check-prefixes CHECK-MOVED %s
+
+/// Main error downgraded to a remark.
+// CHECK-MOVED: LibWithXRef.swiftmodule:1:1: remark: reference to type 'MyType' broken by a context change; 'MyType' was expected to be in 'A', but now a candidate is found only in 'A_related'
+//--- module.modulemap
+module A {
+    header "A.h"
+}
+
+//--- A.h
+void foo() {}
+
+//--- Empty.swift
+
+//--- EmptyOverlay.swift
+@_exported import A
+
+//--- LibOriginal.swift
+@_exported import A
+
+public struct MyType {
+    public init() {}
+}
+
+//--- LibWithXRef.swift
+import A
+import A_related
+
+public func foo() -> MyType {
+    fatalError()
+}
+
+//--- Client.swift
+import LibWithXRef
+
+foo()