Bring [<Erased>] attribute to FSharp.Core and add simple features around it

[Thorium]

This spans from the discussion of fsharp/fslang-suggestions#538 "Erased type-tagged anonymous union types", which is from 2017 and not seeming to be implemented.

I actually like the simpler idea by @voronoipotato in his comment, because that sounds very implentable. And that's not a language feature, but just a minor addition to FSharp.Core and compiler.

1. Why not to add [<ErasedAttribute>] to F# core?

   • Definition: It's an instruction to the compiler that the type can be simplified by compile-time reduction.
   • What it actually does in practice depends on the compiler target (IL/JS/...)
   • Fable can replace its attribute with FSharp.Core version.

2. As the first traditional F# compiler, the feature would be: Remove tagged single-case DUs:

   • [<Erased>] type Phone = | Phone of string could compile into string
   • Benefit: Provide simpler C# interopability usage.
   • Benefit: Provide more performant runtime execution while still maintaining the type safety for the developer.
   • A compile-time thing, a little bit like ReflectedDefinitionAttribute. If a user breaks their own code with it (e.g. calling the type via another F# dll via ProjectReference instead of NuGet/dll-reference), it's their own problem. (Could it be erased in whole sln or is that too complex?)

[voronoipotato]

If there's a problem we can't solve, there's a simpler version of the problem we can solve :). Then when that succeeds we can use the insights gained to solve more complex problems. Thanks for making a feature request for this!

[voronoipotato]

If someone who has time/energy I wouldn't mind trying to chip away at this problem on a weekend if I had someone to bounce ideas off of. I would probably be pretty slow and sporadic in effort, but it doesn't seem like that would be an issue for this one.

[vzarytovskii]

I personally think this should go through its own suggestion, so it's explicitly outlined what's supported, what's not and what are the use-cases.

[T-Gro]

@voronoipotato :

What is the scope you would want to tackle in the first prototype, is it about erase single-case DUs ?
I think it is a good starting point, and the same Erased attribute could be later reused for more features.

It does make sense to open that specific piece (just single case DUs) as a new suggestions, to give a space where people can oppose it if they see fit.

I can be available to you for your bouncing ideas :))
In general, as a good starting point, https://deepwiki.com/search/there-is-a-language-suggestion_1dc96af3-d80c-4e23-b0b0-f00b8a8c2dc6 is quite good for bouncing off ideas (I typically prompt it to give me 3 different ideas on how to solve a problem, often at least one of them is a direction I would choose as well :-D)

[T-Gro]

TLDR:
👍 to start small
👍 to make a separate suggestion for it