Unity Inspector dropdown for a single layer (as contrary to the LayerMask)

Example.cs

using UnityEngine;

namespace derHugo
{
    public class Example : MonoBehaviour
    {
        public LayerMask multiLayers;
        public Layer singleLayer;
    }
}

Layer.cs

using System;
using UnityEngine;

#if UNITY_EDITOR
using UnityEditor;
#endif

namespace derHugo
{
    [Serializable]
    public struct Layer
    {
        [SerializeField] private int m_Value;

        public SingleLayer(int value)
        {
            m_Value = value;
        }

        public static implicit operator int(SingleLayer layer)
        {
            return layer.m_Value;
        }

        public static implicit operator SingleLayer(int value)
        {
            return new SingleLayer(value);
        }

#if UNITY_EDITOR
        [CustomPropertyDrawer(typeof(Layer))]
        public class LayerDrawer : PropertyDrawer
        {
            public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
            {
                using (new EditorGUI.PropertyScope(position, label, property))
                {
                    position = EditorGUI.PrefixLabel(position, label);

                    var valueProperty = property.FindPropertyRelative(nameof(m_Value));

                    valueProperty.intValue = EditorGUI.LayerField(position, valueProperty.intValue);
                }
            }
        }
#endif
    }
}