C# Compiler "Init Only Setter" not working in 2021.2?

I'm on version 2021.2.7, and in the documentation, I noticed that "Init Only Setter" was not listed as an unsupported C# feature:
https://docs.unity3d.com/2021.2/Documentation/Manual/CSharpCompiler.html

I tried using it on one property, and I was hit with CS0518:
"Predefined type 'System.Runtime.CompilerServices.IsExternalInit' is not defined or imported"

Maybe I missed related info somewhere else on the forum, but does this feature actually work in my Unity version?

 

I dug out this post: https://forum.unity.com/threads/unity-future-net-development-status.1092205/page-11#post-7698016

 

Code (CSharp):

1.  
2. // Add this magic code, you will be able to use record and init.
3. // I only tested in editor, not sure whether it would work on other platforms.
4. namespace System.Runtime.CompilerServices
5. {
6.     class IsExternalInit
7.     {
8.      
9.     }
10. }
11.  
12. public record TestRecord(string Name, int Age);
13.  
14. public class Persion
15. {
16.     public int Age
17.     {
18.         get;
19.         init;
20.     }
21. }
22.  
23.  

 

Thanks @icepower2012, it did work well for both Editor and Android build.