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

Example.cs

using System;
using UnityEngine;

public class Example : MonoBehaviour
{
    [SingleLayer] public int layer;
    
    [ContextMenu(nameof(Test))]
    private void Test()
    {
        Debug.Log(gameObject.layer == layer);
    }
}

SingleLayerAttribute.cs

using System;
using UnityEngine;

#if UNITY_EDITOR
using UnityEngine;
#endif

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

                if (property.propertyType != SerializedPropertyType.Integer)
                {
                    EditorGUI.HelpBox(position, $"{nameof(SingleLayerAttribute)} can only be used on fields of type int!", MessageType.Error);
                }
                else
                {
                    property.intValue = EditorGUI.LayerField(position, property.intValue);
                }
            }
        }
    }
#endif
}