trans::expr::trans_adt has unresolved questions

[catamorphism]

Two questions from a comment in this function: "is it sound to use the destination's repr on the base? would it ever be reasonable to be here with discr != 0?"

[catamorphism]

(Fixing this would involve creating test cases that answer these questions.)

[emberian]

Triage bump

[metajack]

Visiting for triage. Nothing to add.