GodotComponentTest/utils/ExportFlagsEnumAttribute.cs

// Source: https://gist.github.com/kleonc/a2bab51686ac6f4d7cb28aec88efa5d9

using System;
using System.Collections.Generic;
using System.Linq;
using Godot;

namespace Namespace;

using UnderlyingType = UInt64;

[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field)]
public class ExportFlagsEnumAttribute : ExportAttribute {
    public ExportFlagsEnumAttribute(Type enumType)
        : base(PropertyHint.Flags, GetFlagsEnumHintString(enumType)) { }

    private static string GetFlagsEnumHintString(Type enumType) {
        Dictionary<UnderlyingType, List<string>> flagNamesByFlag = new();
        UnderlyingType flag = 1;
        foreach (string name in Enum.GetNames(enumType)) {
            UnderlyingType value =
                (UnderlyingType)Convert.ChangeType(Enum.Parse(enumType, name), typeof(UnderlyingType));
            while (value > flag) {
                if (!flagNamesByFlag.ContainsKey(flag)) {
                    flagNamesByFlag.Add(flag, new List<string>());
                }

                flag <<= 1;
            }

            if (value == flag) {
                if (!flagNamesByFlag.TryGetValue(flag, out List<string> names)) {
                    names = new List<string>();
                    flagNamesByFlag.Add(flag, names);
                }

                names.Add(name);
            }
        }

        return string.Join(", ", flagNamesByFlag.Values.Select(flagNames => string.Join(" / ", flagNames)));
    }
}