Is there a way to prevent reboxing in W/W (due to OPAQUE pragma proposal)

[Christiaan Baaij]

Hi Ghc-Devs,

I believe I've mostly finished the implementation of GHC proposal 0415 [1],
the OPAQUE pragma, over at:
https://gitlab.haskell.org/ghc/ghc/-/merge_requests/5562

The only remaining issue is that I'm unsure whether there's a way to
prevent reboxing of worker arguments as witnessed here:
https://gitlab.haskell.org/ghc/ghc/-/merge_requests/5562/diffs#29ad02619a9f318d67c6cd19648917dbb17354e9_0_134

Note that the reboxing doesn't happen in the worker of the OPAQUE-annotated
bindings, because OPAQUE-annotated bindings aren't W/W-transformed, but in
a worker of a function that calls the OPAQUE-annotated binding.
This reboxing was the reason behind the descision to W/W transform
NOINLINE-annotated bindings in:
https://gitlab.haskell.org/ghc/ghc/-/commit/b572aadb20c2e41e2f6d7b48401bd0b4239ce9f8

But the whole idea behind OPAQUE is not to change calls to the annotated
binding f by some call of a name-mangled version of f; so W/W transforming
OPAQUE-annoted binders is not an option.

Any hints on how to avoid the reboxing (if possible) would be appreciated,
like:
* Do I need to change something in GHC.Core.Opt.WorkWrap? or
* Do I need to change the demand/strictness signature of OPAQUE-annoted
bindings?
* or something else?

Thanks,

Christiaan

[1]
https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0415-opaque-pragma.rst
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