Fix clang-doc crash when comment contains invalid UTF-8 bytes (#210886)

When a source file comment contains non-UTF-8 bytes (e.g., byte 0x97),
clang-doc's JSONGenerator would crash with an assertion failure:

  Assertion `false && "Invalid UTF-8 in value used as JSON"' failed.

This happens because json::Value(StringRef) asserts on invalid UTF-8
input in debug builds. While the release-build path calls fixUTF8()
automatically, the assertion crash affects developers and fuzzer runs.

Fix by following the same pattern used elsewhere in LLVM (lldb, clangd):
check isUTF8() before constructing a json::Value, and call fixUTF8() to
sanitize the string if invalid bytes are detected. This avoids an
unnecessary copy for the common case of valid UTF-8.

The fix covers all paths in serializeComment() where user-written text
from source comments enters JSON objects:
  - CK_TextComment (I.Text)
  - CK_InlineCommandComment (I.Args)
  - CK_VerbatimBlockLineComment / CK_VerbatimLineComment (I.Text)
  - CK_Unknown (I.Text)

Also add a regression test with an actual invalid UTF-8 byte in a
comment.

AI assistance was used for code review analysis and local build
verification.

Fixes: https://github.com/llvm/llvm-project/issues/210675

---------

Co-authored-by: Chenguang Ding <dingchenguang@kylinos.cn>
Co-authored-by: Claude <noreply@anthropic.com>

clang-tools-extra/clang-doc/JSONGenerator.cpp

@@ -106,6 +106,12 @@ static void insertNonEmpty(StringRef Key, StringRef Value, Object &Obj) {
     Obj[Key] = Value;
 }
 
+static json::Value safeJSONString(StringRef S) {
+  if (LLVM_LIKELY(json::isUTF8(S)))
+    return S;
+  return json::fixUTF8(S);
+}
+
 static std::string infoTypeToString(InfoType IT) {
   switch (IT) {
   case InfoType::IT_default:
@@ -233,7 +239,7 @@ static Object serializeComment(const CommentInfo &I, Object &Description) {
   switch (I.Kind) {
   case CommentKind::CK_TextComment: {
     if (!I.Text.empty())
-      Obj.insert({commentKindToString(I.Kind), I.Text});
+      Obj.insert({commentKindToString(I.Kind), safeJSONString(I.Text)});
     return Obj;
   }
 
@@ -258,7 +264,7 @@ static Object serializeComment(const CommentInfo &I, Object &Description) {
     auto &ARef = *ArgsArr.getAsArray();
     ARef.reserve(I.Args.size());
     for (const auto &Arg : I.Args)
-      ARef.emplace_back(Arg);
+      ARef.emplace_back(safeJSONString(Arg));
     Child.insert({"Command", I.Name});
     Child.insert({"Args", ArgsArr});
     Child.insert({"Children", ChildArr});
@@ -292,7 +298,7 @@ static Object serializeComment(const CommentInfo &I, Object &Description) {
 
   case CommentKind::CK_VerbatimBlockLineComment:
   case CommentKind::CK_VerbatimLineComment: {
-    Child.insert({"Text", I.Text});
+    Child.insert({"Text", safeJSONString(I.Text)});
     Child.insert({"Children", ChildArr});
     Obj.insert({commentKindToString(I.Kind), ChildVal});
     return Obj;
@@ -333,7 +339,7 @@ static Object serializeComment(const CommentInfo &I, Object &Description) {
   }
 
   case CommentKind::CK_Unknown: {
-    Obj.insert({commentKindToString(I.Kind), I.Text});
+    Obj.insert({commentKindToString(I.Kind), safeJSONString(I.Text)});
     return Obj;
   }
   }

clang-tools-extra/test/clang-doc/Inputs/invalid-utf8-comment.cpp

@@ -0,0 +1,2 @@
+// non-UTF-8: <20>
+class A {};

clang-tools-extra/test/clang-doc/json/invalid-utf8-comment.cpp

@@ -0,0 +1,46 @@
+// RUN: rm -rf %t && mkdir -p %t
+// RUN: clang-doc --pretty-json --output=%t --format=json --executor=standalone %S/../Inputs/invalid-utf8-comment.cpp 2>&1
+// RUN: cat %t/json/GlobalNamespace/_ZTV1A.json | FileCheck %s
+
+// Regression test for https://github.com/llvm/llvm-project/issues/210675.
+// clang-doc should not crash when encountering comments with invalid UTF-8 bytes.
+// Instead, the invalid bytes should be replaced with valid UTF-8 replacement characters.
+
+// CHECK:       {
+// CHECK-NEXT:    "Contexts": [
+// CHECK-NEXT:      {
+// CHECK-NEXT:        "DocumentationFileName": "index",
+// CHECK-NEXT:        "End": true,
+// CHECK-NEXT:        "Name": "Global Namespace",
+// CHECK-NEXT:        "QualName": "GlobalNamespace",
+// CHECK-NEXT:        "RelativePath": "./",
+// CHECK-NEXT:        "USR": "0000000000000000000000000000000000000000"
+// CHECK-NEXT:      }
+// CHECK-NEXT:    ],
+// CHECK-NEXT:    "Description": {
+// CHECK-NEXT:      "HasParagraphComments": true,
+// CHECK-NEXT:      "ParagraphComments": [
+// CHECK-NEXT:        [
+// CHECK-NEXT:          {
+// CHECK:               "TextComment":
+// CHECK-NEXT:          }
+// CHECK-NEXT:        ]
+// CHECK-NEXT:      ]
+// CHECK-NEXT:    },
+// CHECK-NEXT:    "DocumentationFileName": "_ZTV1A",
+// CHECK-NEXT:    "HasContexts": true,
+// CHECK-NEXT:    "InfoType": "record",
+// CHECK-NEXT:    "IsTypedef": false,
+// CHECK-NEXT:    "Location": {
+// CHECK-NEXT:      "Filename": "{{.*}}invalid-utf8-comment.cpp",
+// CHECK-NEXT:      "LineNumber": 2
+// CHECK-NEXT:    },
+// CHECK-NEXT:    "MangledName": "_ZTV1A",
+// CHECK-NEXT:    "Name": "A",
+// CHECK-NEXT:    "Namespace": [
+// CHECK-NEXT:      "GlobalNamespace"
+// CHECK-NEXT:    ],
+// CHECK-NEXT:    "Path": "GlobalNamespace",
+// CHECK-NEXT:    "TagType": "class",
+// CHECK-NEXT:    "USR": "{{[0-9A-F]*}}"
+// CHECK-NEXT:  }