Allow `var` in refinements

[odersky]

Minimized example

  type A = Any { var x: Int }

Output

1 |  type A = Any { var x: Int }
  |                 ^^^^^^^^^^
  |                 refinement cannot be a mutable var.
  |                 You can use an explicit getter x and setter x_= instead

Expectation

Expand the refinement to

type A = Any { def x: Int, def x_=($x: Int): Unit }

instead.

That's what Scala 2 does. See also: https://contributors.scala-lang.org/t/why-is-there-a-restriction-on-refinements-not-having-vars/6498/6

To fix this, we need to touch the following pieces of code:

• Both versions of syntax.md, to allow the new syntax
• The parser, make sure the new syntax also shows up as a doc comment of the parsing method
• Code in Desugar that does the expansion.

[odersky]

This is a good Spree ticket, but if someone wants to pick it up before, please go ahead.

[hamzaremmal]

I will pick it up for the next Spree.

cc @mbovel

[mbovel]

This issue was picked for the Issue Spree of March 19th, 2024. @hamzaremmal, @aherlihy will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.