<PackageReference Include="System.Collections.Immutable" Version="11.0.0-rc.1.26425.128" />

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); } }