Weird Haddock comments can't be formatted without `--unsafe`

[amesgen]

Describe the bug Since #722, certain weird Haddock comments can not be formatted without --unsafe.

To Reproduce Try to format BufferMode.hs from leksah, or this minimized snippet:

-- |
--
---
module Test where

Expected behavior Not yet decided, but it should not error.

[amesgen]

EDIT This analysis is incorrect, see comment below.

---

This appears to be a regression in ghc-lib-parser-9.0 and should be fixed in the upgrade to ghc-lib-parser-9.2 (which should be released in August according to the GHC status update :tada:).

---

The third line (the --- ) is actually not necessary for this issue. Right now, with --unsafe, the snippet

-- |
--
module Test where

is formatted into

-- |
module Test where

and again into

module Test where

(this is similiar to empty Haddock comments on function arguments not appearing in the AST since ghc-lib-parser-9.0).

Previously, in 0.1.4.1 with ghc-lib-parser-8.10, the first snippet would be formatted into the second one idempotently, and the AST did not differ as we drop trailing blank lines when printing/comparing Haddock comments. But now, with ghc-lib-parser-9.0, the parsed AST of the second snippet does no longer contain a Haddock comment, in contrast to the first snippet, so the AST differs.

[amesgen]

This issue is actually caused by invalid Haddock comments (cf. #803). Namely, this is a valid Haddock comment:

-- |
--
module Test where

which is formatted to

-- |
module Test where

as blank lines are collapsed. But in the second snippet, the Haddock comment is now invalid for some reason.

I think the best approach to fix this is to just drop (valid and invalid) Haddock comments if they only contain spaces.

---

For complete madness, consider how the GHC parser handles empty Haddock comments at different places:

-- |
module Test
  ( -- |
    test,
  )
where

-- |
test ::
  -- |
  test

Here, confusingly, the second and third Haddock comments are valid, but the first and second one are invalid. Hence, Ormolu formats this as

--
module Test
  ( -- |
    test,
  )
where

-- |
test ::
  --
  test

which seems very random. With the proposed change, it would be formatted as

module Test
  ( test,
  )
where

test ::
  test

which is a clear improvement IMO.