[Diagnostics] Add educational notes into serialized diagnostics

Use flag record (currently unused) to emit semi-colon separated
list of paths to educational notes associated with a diagnostic.

Resolves: rdar://118993780

Author: xedin

lib/Frontend/SerializedDiagnosticConsumer.cpp

@@ -87,11 +87,8 @@ struct SharedState : llvm::RefCountedBase<SharedState> {
   /// The collection of categories used.
   llvm::DenseMap<const char *, unsigned> Categories;
 
-  using DiagFlagsTy =
-      llvm::DenseMap<const void *, std::pair<unsigned, StringRef>>;
-
-  /// Map for uniquing strings.
-  DiagFlagsTy DiagFlags;
+  /// The collection of flags used.
+  llvm::StringMap<unsigned> Flags;
 
   /// Whether we have already started emission of any DIAG blocks. Once
   /// this becomes \c true, we never close a DIAG block until we know that we're
@@ -198,6 +195,14 @@ class SerializedDiagnosticConsumer : public DiagnosticConsumer {
   // Record identifier for the category.
   unsigned getEmitCategory(StringRef Category);
 
+  /// Emit a flag record that contains a semi-colon separated
+  /// list of all of the educational notes associated with the
+  /// diagnostic or `0` if there are no notes.
+  ///
+  /// \returns a flag record identifier that could be embedded in
+  /// other records.
+  unsigned emitEducationalNotes(const DiagnosticInfo &info);
+
   /// Add a source location to a record.
   void addLocToRecord(SourceLoc Loc,
                       SourceManager &SM,
@@ -319,6 +324,34 @@ unsigned SerializedDiagnosticConsumer::getEmitCategory(StringRef Category) {
   return entry;
 }
 
+unsigned
+SerializedDiagnosticConsumer::emitEducationalNotes(const DiagnosticInfo &Info) {
+  if (Info.EducationalNotePaths.empty())
+    return 0;
+
+  SmallString<32> scratch;
+  interleave(
+      Info.EducationalNotePaths,
+      [&scratch](const auto &notePath) { scratch += notePath; },
+      [&scratch] { scratch += ';'; });
+
+  StringRef paths = scratch.str();
+
+  unsigned &recordID = State->Flags[paths];
+  if (recordID)
+    return recordID;
+
+  recordID = State->Flags.size();
+
+  RecordData Record;
+  Record.push_back(RECORD_DIAG_FLAG);
+  Record.push_back(recordID);
+  Record.push_back(paths.size());
+  State->Stream.EmitRecordWithBlob(State->Abbrevs.get(RECORD_DIAG_FLAG), Record,
+                                   paths.data());
+  return recordID;
+}
+
 void SerializedDiagnosticConsumer::addLocToRecord(SourceLoc Loc,
                                                   SourceManager &SM,
                                                   StringRef Filename,
@@ -572,8 +605,10 @@ emitDiagnosticMessage(SourceManager &SM,
     Record.push_back(0);
   }
 
-  // FIXME: Swift diagnostics currently have no flags.
-  Record.push_back(0);
+  // Use "flags" slot to emit a semi-colon separated list of
+  // educational notes. If there are no notes associated with
+  // this diagnostic `0` placeholder would be emitted instead.
+  Record.push_back(emitEducationalNotes(Info));
 
   // Emit the message.
   Record.push_back(Text.size());

test/diagnostics/educational_notes_serialization.swift

@@ -0,0 +1,22 @@
+// not %target-swift-frontend -no-color-diagnostics -print-educational-notes -diagnostic-documentation-path %S/test-docs/ -typecheck %s
+
+// RUN: %empty-directory(%t)
+// RUN: not %target-swift-frontend -typecheck -no-color-diagnostics -print-educational-notes -diagnostic-documentation-path %S/test-docs/ -serialize-diagnostics-path %t/serialized.dia %s
+// RUN: c-index-test -read-diagnostics %t/serialized.dia > %t/serialized.txt 2>&1
+// RUN: %FileCheck %s < %t/serialized.txt
+
+typealias Fn = () -> ()
+extension Fn {}
+// CHECK: [[@LINE-1]]:1: error: non-nominal type 'Fn' (aka '() -> ()') cannot be extended [{{.*}}nominal-types.md] []
+
+
+// Shares the flag record with `Fn`
+typealias Dup = () -> ()
+extension Dup {}
+// CHECK: [[@LINE-1]]:1: error: non-nominal type 'Dup' (aka '() -> ()') cannot be extended [{{.*}}nominal-types.md] []
+
+do {
+  func noNote(_: Int) {}
+  noNote("Hello")
+  // CHECK: [[@LINE-1]]:10: error: cannot convert value of type 'String' to expected argument type 'Int' [] []
+}