trans::datum does something incorrect for structs with destructors

[catamorphism]

A comment in trans::datum::try_deref suggests that deref'ing a newtype struct with a destructor isn't being done correctly.

[catamorphism]

There's a similar comment in trans::expr::trans_def_dps_unadjusted.

[luqmana]

#6861 is what the comment in trans::expr::trans_def_dps_unadjusted would be talking about. Addressed by #6889.

[nikomatsakis]

This comment is just out of date. It was referring to the drop field that we add to structs with dtors; we used to insert this in the front, requiring that every index be offset by 1, but I changed it to insert in the back, avoiding that problem. I'll open a PR to remove the FIXME.

[bblum]

@nikomatsakis can this be closed?

[nikomatsakis]

The FIXME is still present, so to close we need to open a PR (and perhaps add a test -- though one probably exists), but yes I believe there is no actual bug here.

[jeremyletang]

@nikomatsakis trans::datum::try_deref and the FIXME doesn't exist anymore.
Maybe this issue can be closed?

[nikomatsakis]

Let's close.