Parser error message reports both the line number and the cursor position. (#4954)

include/flatbuffers/idl.h

@@ -477,10 +477,28 @@ struct IDLOptions {
 
 // This encapsulates where the parser is in the current source file.
 struct ParserState {
-  ParserState() : cursor_(nullptr), line_(1), token_(-1) {}
+  ParserState()
+      : cursor_(nullptr), line_start_(nullptr), line_(0), token_(-1) {}
 
  protected:
+  void ResetState(const char *source) {
+    cursor_ = source;
+    line_ = 0;
+    MarkNewLine();
+  }
+
+  void MarkNewLine() {
+    line_start_ = cursor_;
+    line_ += 1;
+  }
+
+  int64_t CursorPosition() const {
+    FLATBUFFERS_ASSERT(cursor_ && line_start_ && cursor_ >= line_start_);
+    return static_cast<int64_t>(cursor_ - line_start_);
+  }
+
   const char *cursor_;
+  const char *line_start_;
   int line_;  // the current line being parsed
   int token_;
 

src/idl_parser.cpp

@@ -94,11 +94,12 @@ std::string MakeCamel(const std::string &in, bool first) {
 void Parser::Message(const std::string &msg) {
   error_ = file_being_parsed_.length() ? AbsolutePath(file_being_parsed_) : "";
   // clang-format off
-  #ifdef _WIN32
-    error_ += "(" + NumToString(line_) + ")";  // MSVC alike
-  #else
+  #ifdef _WIN32  // MSVC alike
+    error_ +=
+        "(" + NumToString(line_) + ", " + NumToString(CursorPosition()) + ")";
+  #else  // gcc alike
     if (file_being_parsed_.length()) error_ += ":";
-    error_ += NumToString(line_) + ":0";  // gcc alike
+    error_ += NumToString(line_) + ": " + NumToString(CursorPosition());
   #endif
   // clang-format on
   error_ += ": " + msg;
@@ -280,7 +281,7 @@ CheckedError Parser::Next() {
       case '\r':
       case '\t': break;
       case '\n':
-        line_++;
+        MarkNewLine();
         seen_newline = true;
         break;
       case '{':
@@ -420,7 +421,7 @@ CheckedError Parser::Next() {
           cursor_++;
           // TODO: make nested.
           while (*cursor_ != '*' || cursor_[1] != '/') {
-            if (*cursor_ == '\n') line_++;
+            if (*cursor_ == '\n') MarkNewLine();
             if (!*cursor_) return Error("end of file in comment");
             cursor_++;
           }
@@ -2259,8 +2260,8 @@ bool Parser::Parse(const char *source, const char **include_paths,
 CheckedError Parser::StartParseFile(const char *source,
                                     const char *source_filename) {
   file_being_parsed_ = source_filename ? source_filename : "";
-  source_ = cursor_ = source;
-  line_ = 1;
+  source_ = source;
+  ResetState(source_);
   error_.clear();
   ECHECK(SkipByteOrderMark());
   NEXT();