feat: erased vars

[kasiaMarek]

Adds erased vars. For an erased var all rhs's of an assignment must be pure and the assignements are erased.

How much have you relied on LLM-based tools in this contribution?

Moderately. First version (manually adjusted) of typedAssign was generated.

How was the solution tested?

The are new test cases for compilation

[bishabosha]

i am curious how it is useful to have this? can you ever inspect what value is stored there

[mbovel]

i am curious how it is useful to have this? can you ever inspect what value is stored there

No, not in Scala directly. You can reason about the value of the var in Stainless though, that's our use-case.

[vkuncak]

Actually, whenever you have a stateful object whose capabilities (but not identity) change over time, this would be necessary, right @odersky ?

[odersky]

The capability type checker is not flow sensitive, so var's would not help. We need to create objects with the union of all the capabilities they might hold during their lifetime.