https://dartpad.dev/?id=298451842bcbb6cfa8b6da497f9cb6bf

drawRect.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('CustomPaint Example')),
        body: Center(
          child: CustomPaint(
            painter: MyCustomPainter(),
            child: Container(
              width: 400,
              height: 400,
            ),
          ),
        ),
      ),
    );
  }
}

class MyCustomPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final paint = Paint()
      ..color = Colors.black
      ..strokeWidth = 1.0;

    canvas.drawRect(Rect.fromPoints(Offset(0, 0), Offset(4, 4)), paint);

    //canvas.drawLine(Offset(0, 0), Offset(size.width, size.height), paint);
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
    return false;
  }
}