Simplify bindLHsTyVarBndrs and bindHsQTyVarswip/simply-bind-tyvars

Both `bindLHsTyVarBndrs` and `bindHsQTyVars` take two separate
`Maybe` arguments, which I find terribly confusing. Thankfully, it's
possible to remove one `Maybe` argument from each of these functions,
which this patch accomplishes:

* `bindHsQTyVars` takes a `Maybe SDoc` argument, which is `Just` if
  GHC should warn about any of the quantified type variables going
  unused. However, every call site uses `Nothing` in practice. This
  makes sense, since it doesn't really make sense to warn about
  unused type variables bound by an `LHsQTyVars`. For instance, you
  wouldn't warn about the `a` in `data Proxy a = Proxy` going unused.

  As a result, I simply remove this `Maybe SDoc` argument altogether.
* `bindLHsTyVarBndrs` also takes a `Maybe SDoc` argument for the same
  reasons that `bindHsQTyVars` took one. To make things more
  confusing, however, `bindLHsTyVarBndrs` also takes a separate
  `HsDocContext` argument, which is pretty-printed (to an `SDoc`) in
  warnings and error messages.

  In practice, the `Maybe SDoc` and the `HsDocContext` often contain
  the same text. See the call sites for `bindLHsTyVarBndrs` in
  `rnFamInstEqn` and `rnConDecl`, for instance. There are only a
  handful of call sites where the text differs between the
  `Maybe SDoc` and `HsDocContext` arguments:

  * In `rnHsRuleDecl`, where the `Maybe SDoc` says "`In the rule`"
    and the `HsDocContext` says "`In the transformation rule`".
  * In `rnHsTyKi`/`rn_ty`, where the `Maybe SDoc` says
    "`In the type`" but the `HsDocContext` is inhereted from the
    surrounding context (e.g., if `rnHsTyKi` were called on a
    top-level type signature, the `HsDocContext` would be
    "`In the type signature`" instead)

  In both cases, warnings/error messages arguably _improve_ by
  unifying making the `Maybe SDoc`'s text match that of the
  `HsDocContext`. As a result, I decided to remove the `Maybe SDoc`
  argument to `bindLHsTyVarBndrs` entirely and simply reuse the text
  from the `HsDocContext`. (I decided to change the phrase
  "transformation rule" to "rewrite rule" while I was in the area.)

  The `Maybe SDoc` argument has one other purpose: signaling when to
  emit "`Unused quantified type variable`" warnings. To recover this
  functionality, I replaced the `Maybe SDoc` argument with a
  boolean-like `WarnUnusedForalls` argument. The only
  `bindLHsTyVarBndrs` call site that chooses _not_ to emit these
  warnings in `bindHsQTyVars`.

1 files changed, 1 insertions, 1 deletions

diff --git a/testsuite/tests/dependent/should_fail/T16326_Fail10.stderr b/testsuite/tests/dependent/should_fail/T16326_Fail10.stderr
index 4ef380e5ba..44a0d34a33 100644
--- a/testsuite/tests/dependent/should_fail/T16326_Fail10.stderr
+++ b/testsuite/tests/dependent/should_fail/T16326_Fail10.stderr
@@ -4,4 +4,4 @@ T16326_Fail10.hs:12:18: error:
         forall a -> a -> a
       (GHC does not yet support this)
     • In the type signature for ‘x’: forall a -> a -> a
-      When checking the transformation rule "flurmp"
+      When checking the rewrite rule "flurmp"