Commits on May 23, 2023

1. Fix scala#17549: Unify how Memoize and Constructors decide what field… …

   …s need storing.
   
   The Memoize and Constructors have to work together and agree on
   which `final val`s actually need storing in a field. Previously,
   they used slightly different criteria: one on the result type, and
   one on the rhs (with an additional Scala.js-specific eligibility
   condition). That discrepancy resulted in the crash/wrong codegen
   in the issue.
   
   We now unify both: we avoid a field if and only if all of the
   following apply:
   
   * it is a `final val`,
   * its result *type* is a `ConstantType`, and
   * it is eligible according to Scala.js semantics.
   
   In particular, there is no condition on the right-hand-side. We
   avoid a field even if the right-hand-side has side effects. The
   side effects are moved to the constructor regardless.
   
   ---
   
   This introduces both progressions and regressions in the amount of
   fields we generate. We can avoid fields even for side-effecting
   rhs'es, as long as the result type is constant. On the other hand,
   we now create a field for `final val`s with non-constant result
   type, even if the rhs is a literal.
   
   While the latter is a regression for Scala 3, it aligns with the
   behavior of Scala 2. It also has the nice benefit that whether or
   not a val has a field is now independent of its *implementation*,
   and only dependent on its *API*. Overall, I think this is a
   trade-off worth taking.
   
   We could reintroduce that optimization in the future (but in
   classes only; not in traits), if we really want to, although that
   would require dedicated code.

   

   sjrd committed May 23, 2023
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