using UnityEngine;
using System.Collections;

public class Weapon : MonoBehaviour {

   void Update () {
    float dx = Input.mousePosition.x - Screen.width / 2.0f;
      float dy = Input.mousePosition.y - Screen.height / 2.0f;   
      float strawRadians = Mathf.Atan2(dx,dy);   
     float strawDigrees = 360.0f * strawRadians/(2.0f*Mathf.PI);
      transform.rotation.eulerAngles.z = -strawDigrees;
      
   }
}

Assets/Weapon.cs(11,36): error CS1612: Cannot modify a value type return value of `UnityEngine.Quaternion.eulerAngles'. Consider storing the value in a temporary variable
Assets/Weapon.cs(11,27): error CS1612: Cannot modify a value type return value of `UnityEngine.Transform.rotation'. Consider storing the value in a temporary variable