Always use rnImplicitBndrs to bring implicit tyvars into scope

This implements a first step towards #16762 by changing the renamer
to always use `rnImplicitBndrs` to bring implicitly bound type
variables into scope. The main change is in `rnFamInstEqn` and
`bindHsQTyVars`, which previously used _ad hoc_ methods of binding
their implicit tyvars.

There are a number of knock-on consequences:

* One of the reasons that `rnFamInstEqn` used an _ad hoc_ binding
  mechanism was to give more precise source locations in
  `-Wunused-type-patterns` warnings. (See
  https://gitlab.haskell.org/ghc/ghc/issues/16762#note_273343 for an
  example of this.) However, these warnings are actually a little
  _too_ precise, since implicitly bound type variables don't have
  exact binding sites like explicitly bound type variables do.
  A similar problem existed for
  "`Different names for the same type variable`" errors involving
  implicit tyvars bound by `bindHsQTyVars`.
  Therefore, we simply accept the less precise (but more accurate)
  source locations from `rnImplicitBndrs` in `rnFamInstEqn` and
  `bindHsQTyVars`. See
  `Note [Source locations for implicitly bound type variables]` in
  `GHC.Rename.HsType` for the full story.
* In order for `rnImplicitBndrs` to work in `rnFamInstEqn`, it needs
  to be able to look up names from the parent class (in the event
  that we are renaming an associated type family instance). As a
  result, `rnImplicitBndrs` now takes an argument of type
  `Maybe assoc`, which is `Just` in the event that a type family
  instance is associated with a class.
* Previously, GHC kept track of three type synonyms for free type
  variables in the renamer: `FreeKiTyVars`, `FreeKiTyVarsDups`
  (which are allowed to contain duplicates), and
  `FreeKiTyVarsNoDups` (which contain no duplicates). However, making
  is a distinction between `-Dups` and `-NoDups` is now pointless, as
  all code that returns `FreeKiTyVars{,Dups,NoDups}` will eventually
  end up being passed to `rnImplicitBndrs`, which removes duplicates.
  As a result, I decided to just get rid of `FreeKiTyVarsDups` and
  `FreeKiTyVarsNoDups`, leaving only `FreeKiTyVars`.
* The `bindLRdrNames` and `deleteBys` functions are now dead code, so
  I took the liberty of removing them.

1 files changed, 1 insertions, 6 deletions

diff --git a/compiler/GHC/Data/List/SetOps.hs b/compiler/GHC/Data/List/SetOps.hs
index 2d916e9dd5..15b370582c 100644
--- a/compiler/GHC/Data/List/SetOps.hs
+++ b/compiler/GHC/Data/List/SetOps.hs
@@ -10,7 +10,7 @@
 --
 -- Avoid using them as much as possible
 module GHC.Data.List.SetOps (
-        unionLists, minusList, deleteBys,
+        unionLists, minusList,
 
         -- Association lists
         Assoc, assoc, assocMaybe, assocUsing, assocDefault, assocDefaultUsing,
@@ -39,11 +39,6 @@ getNth :: Outputable a => [a] -> Int -> a
 getNth xs n = ASSERT2( xs `lengthExceeds` n, ppr n $$ ppr xs )
              xs !! n
 
-deleteBys :: (a -> a -> Bool) -> [a] -> [a] -> [a]
--- (deleteBys eq xs ys) returns xs-ys, using the given equality function
--- Just like 'Data.List.delete' but with an equality function
-deleteBys eq xs ys = foldl' (flip (L.deleteBy eq)) xs ys
-
 {-
 ************************************************************************
 *                                                                      *