Add To Record Codegen CLI shape to allow C# 9.0 with keyword interop

I propose adding to the F# Record CLI code gen, whatever is necessary to necessary to the F# Record CLI code gen to make it work the C# 9.0 with keyword.

Given in F#:

type Suit = Heart | Diamond | Club | Spade
type Card = {Rank:int; Suit:Suit}

it would be great if this would work in C#:

public const int Ace = 14;

var orig = new Card(Ace, Suit.Spade);
var lowAce = orig with { Rank = 1};

The existing way of approaching this interop works is as so:

var lowAce = new Card(1, orig.Suit);

Probably what is needed is a public clone method named <Clone>$ and adding the new .net 5.0 init only setters to the properties. I'm not sure what else is needed to make C# 9 think a class is a record, but it looks like the C#9 code gen for records is very limited when the record is a sealed class like they are in F#.

I was trying to test this out, using F#'s ability to name a method whatever I want, but it appears I could not create an object that matched the C# record shape in .net 5.0 preview 7, whether it was due to the preview release or due to limitation on CLI protection levels that get generated in F# (no private or protected), i am not sure.

Pros and Cons

The advantages of making this adjustment to F# are smoother interop with C#. Reduces the need in .net 5.0 for using CLIMutable on records since it would have init only setters.

The disadvantages are you'll have to coordinate with the C#/Rosyln Teams with what they are actually looking for to identify records, and keep it in sync, and this is still an unreleased feature of C# 9.0. Another disadvantage is init only properties only work on .net 5.0 runtime.

Extra information

Estimated cost: M

Here is where I attempted to extend the Shape of an F# record, but failed:
jbtule/ConsumeFSharp-FromCSharp#2

Here is a link to the C# 9.0 Record Spec Proposal:
https://github.com/dotnet/csharplang/blob/master/proposals/csharp-9.0/records.md

Affidavit (please submit!)

Please tick this by placing a cross in the box:

• This is not a question (e.g. like one you might ask on stackoverflow) and I have searched stackoverflow for discussions of this issue
• I have searched both open and closed suggestions on this site and believe this is not a duplicate
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Please tick all that apply:

• This is not a breaking change to the F# language design
• I or my company would be willing to help implement and/or test this

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