<PackageReference Include="System.Collections.Immutable" Version="11.0.0-preview.5.26302.115" />

System.Runtime.CompilerServices.CollectionBuilderAttribute

namespace System.Runtime.CompilerServices { internal sealed class CollectionBuilderAttribute : Attribute { public Type BuilderType { get; } public string MethodName { get; } public CollectionBuilderAttribute(Type builderType, string methodName); } }