RFC for shared self types

[asajeffrey]

An RFC about the treatment of self parameters in methods declared with function t:m(). Currently every method gets a separate type for self, but with this proposal they'd get the same type. This produces better type errors, and can drastically reduce the memory footprint of types.

Rendered: https://github.com/asajeffrey/luau/blob/rfc-shared-self-types/rfcs/shared-self-types.md

[vegorov-rbx]

Just to be precise, you mean if there is already a local named t in the scope that has unsealed type T?

[asajeffrey]

I don't think it needs to be unsealed (though it's not much use if it's sealed).

[vegorov-rbx]

If the user decides to add an explicit type annotation type t<As> = U, is it placed somewhere at the beginning (before method declarations), at the end (like developers often do right now with type t = typeof(...)) or is it independent of visit order?

[asajeffrey]

Related: https://devforum.roblox.com/t/go-to-declaration-for-metatables/2651946/2

[zeux]

This PR is closed as part of RFC migration; please see #1074 (comment) for context.

Additional notes: This represents a significant user need, so this feels like something we need to finish specifying and implement.