[lib/util] Add debugging utilities to TextReader

lib/util/TextReader.v3

@@ -242,6 +242,38 @@ class TextReader {
 		}
 	}
 	private def tabColumn() {
-		column = 1 + (((column + 8) >>> 3) << 3);
+		column = nextTabStop(column);
+	}
+	def nextTabStop(column: int) -> int {
+		return 1 + (((column + 8) >>> 3) << 3);
+	}
+	def renderCurrentLineWithCaret(buf: StringBuilder, pos: int) -> StringBuilder {
+		var start = pos - 1;
+		while (start > 0) {
+			if (data[start] == '\n') {
+				start++;
+				break;
+			}
+			start--;
+		}
+		var end = start;
+		while (end < limit) {
+			if (data[end] == '\n') break;
+			end++;
+		}
+		buf.putr(data[start ... end]);
+		var column = 1;
+		for (i = start; i < end; i++) {
+			if (i == pos) break;
+			var ch = data[i];
+			if (ch == '\t') column = nextTabStop(column);
+			else column++;
+		}
+		if (start <= pos && pos < end) {
+			buf.ln();
+			for (i < column - 1) buf.sp();
+			buf.putc('^');
+		}
+		return buf;
 	}
 }