Remove decideKindGeneralisationPlan

TypeInType came with a new function: decideKindGeneralisationPlan.
This type-level counterpart to the term-level decideGeneralisationPlan
chose whether or not a kind should be generalized. The thinking was
that if `let` should not be generalized, then kinds shouldn't either
(under the same circumstances around -XMonoLocalBinds).

However, this is too conservative -- the situation described in the
motivation for "let should be be generalized" does not occur in types.

This commit thus removes decideKindGeneralisationPlan, always
generalizing.

One consequence is that tc_hs_sig_type_and_gen no longer calls
solveEqualities, which reports all unsolved constraints, instead
relying on the solveLocalEqualities in tcImplicitTKBndrs. An effect
of this is that reporing kind errors gets delayed more frequently.
This seems to be a net benefit in error reporting; often, alongside
a kind error, the type error is now reported (and users might find
type errors easier to understand).

Some of these errors ended up at the top level, where it was
discovered that the GlobalRdrEnv containing the definitions in the
local module was not in the TcGblEnv, and thus errors were reported
with qualified names unnecessarily. This commit rejiggers some of
the logic around captureTopConstraints accordingly.

One error message (typecheck/should_fail/T1633)
is a regression, mentioning the name of a default method. However,
that problem is already reported as #10087, its solution is far from
clear, and so I'm not addressing it here.

This commit fixes #15141. As it's an internal refactor, there is
no concrete test case for it.

Along the way, we no longer need the hsib_closed field of
HsImplicitBndrs (it was used only in decideKindGeneralisationPlan)
and so it's been removed, simplifying the datatype structure.

Along the way, I removed code in the validity checker that looks
at coercions. This isn't related to this patch, really (though
it was, at one point), but it's an improvement, so I kept it.

This updates the haddock submodule.

2 files changed, 7 insertions, 17 deletions

diff --git a/compiler/rename/RnSource.hs b/compiler/rename/RnSource.hs
index 33e7c7d2b6..beb7c5b705 100644
--- a/compiler/rename/RnSource.hs
+++ b/compiler/rename/RnSource.hs
@@ -765,8 +765,7 @@ rnFamInstEqn doc mb_cls rhs_kvars
              all_fvs  = fvs `addOneFV` unLoc tycon'
                         -- type instance => use, hence addOneFV
 
-       ; return (HsIB { hsib_ext = HsIBRn { hsib_vars = all_ibs
-                                          , hsib_closed = True }
+       ; return (HsIB { hsib_ext = all_ibs
                       , hsib_body
                           = FamEqn { feqn_ext    = noExt
                                    , feqn_tycon  = tycon'
@@ -1691,7 +1690,7 @@ rnLDerivStrategy doc mds thing_inside
         NewtypeStrategy  -> boring_case (L loc NewtypeStrategy)
         ViaStrategy via_ty ->
           do (via_ty', fvs1) <- rnHsSigType doc via_ty
-             let HsIB { hsib_ext  = HsIBRn { hsib_vars = via_imp_tvs }
+             let HsIB { hsib_ext  = via_imp_tvs
                       , hsib_body = via_body } = via_ty'
                  (via_exp_tv_bndrs, _, _) = splitLHsSigmaTy via_body
                  via_exp_tvs = map hsLTyVarName via_exp_tv_bndrs
diff --git a/compiler/rename/RnTypes.hs b/compiler/rename/RnTypes.hs
index a9e02dc5a5..e5f51660da 100644
--- a/compiler/rename/RnTypes.hs
+++ b/compiler/rename/RnTypes.hs
@@ -127,7 +127,8 @@ rn_hs_sig_wc_type always_bind_free_tvs ctxt
        ; rnImplicitBndrs bind_free_tvs tv_rdrs $ \ vars ->
     do { (wcs, hs_ty', fvs1) <- rnWcBody ctxt nwc_rdrs hs_ty
        ; let sig_ty' = HsWC { hswc_ext = wcs, hswc_body = ib_ty' }
-             ib_ty'  = mk_implicit_bndrs vars hs_ty' fvs1
+             ib_ty'  = HsIB { hsib_ext = vars
+                            , hsib_body = hs_ty' }
        ; (res, fvs2) <- thing_inside sig_ty'
        ; return (res, fvs1 `plusFV` fvs2) } }
 rn_hs_sig_wc_type _ _ (HsWC _ (XHsImplicitBndrs _)) _
@@ -300,7 +301,9 @@ rnHsSigType ctx (HsIB { hsib_body = hs_ty })
        ; vars <- extractFilteredRdrTyVarsDups hs_ty
        ; rnImplicitBndrs (not (isLHsForAllTy hs_ty)) vars $ \ vars ->
     do { (body', fvs) <- rnLHsType ctx hs_ty
-       ; return ( mk_implicit_bndrs vars body' fvs, fvs ) } }
+       ; return ( HsIB { hsib_ext = vars
+                       , hsib_body = body' }
+                , fvs ) } }
 rnHsSigType _ (XHsImplicitBndrs _) = panic "rnHsSigType"
 
 rnImplicitBndrs :: Bool    -- True <=> bring into scope any free type variables
@@ -367,18 +370,6 @@ rnLHsInstType :: SDoc -> LHsSigType GhcPs -> RnM (LHsSigType GhcRn, FreeVars)
 -- Do not try to decompose the inst_ty in case it is malformed
 rnLHsInstType doc inst_ty = rnHsSigType (GenericCtx doc) inst_ty
 
-mk_implicit_bndrs :: [Name]  -- implicitly bound
-                  -> a           -- payload
-                  -> FreeVars    -- FreeVars of payload
-                  -> HsImplicitBndrs GhcRn a
-mk_implicit_bndrs vars body fvs
-  = HsIB { hsib_ext = HsIBRn
-           { hsib_vars = vars
-           , hsib_closed = nameSetAll (not . isTyVarName) (vars `delFVs` fvs) }
-         , hsib_body = body }
-
-
-
 {- ******************************************************
 *                                                       *
            LHsType and HsType