Guard CUSKs behind a language pragma

GHC Proposal #36 describes a transition plan away from CUSKs and to
top-level kind signatures:

1. Introduce a new extension, -XCUSKs, on by default, that detects CUSKs
   as they currently exist.
2. We turn off the -XCUSKs extension in a few releases and remove it
   sometime thereafter.

This patch implements phase 1 of this plan, introducing a new language
extension to control whether CUSKs are enabled. When top-level kind
signatures are implemented, we can transition to phase 2.

1 files changed, 20 insertions, 10 deletions

diff --git a/compiler/hsSyn/HsDecls.hs b/compiler/hsSyn/HsDecls.hs
index c194c2e21a..e328bf43c7 100644
--- a/compiler/hsSyn/HsDecls.hs
+++ b/compiler/hsSyn/HsDecls.hs
@@ -679,11 +679,15 @@ countTyClDecls decls
 
 -- | Does this declaration have a complete, user-supplied kind signature?
 -- See Note [CUSKs: complete user-supplied kind signatures]
-hsDeclHasCusk :: TyClDecl GhcRn -> Bool
-hsDeclHasCusk (FamDecl { tcdFam = fam_decl })
-  = famDeclHasCusk False fam_decl
+hsDeclHasCusk
+  :: Bool  -- True <=> the -XCUSKs extension is enabled
+  -> TyClDecl GhcRn
+  -> Bool
+hsDeclHasCusk _cusks_enabled@False _ = False
+hsDeclHasCusk cusks_enabled (FamDecl { tcdFam = fam_decl })
+  = famDeclHasCusk cusks_enabled False fam_decl
     -- False: this is not: an associated type of a class with no cusk
-hsDeclHasCusk (SynDecl { tcdTyVars = tyvars, tcdRhs = rhs })
+hsDeclHasCusk _cusks_enabled@True (SynDecl { tcdTyVars = tyvars, tcdRhs = rhs })
   -- NB: Keep this synchronized with 'getInitialKind'
   = hsTvbAllKinded tyvars && rhs_annotated rhs
   where
@@ -691,9 +695,9 @@ hsDeclHasCusk (SynDecl { tcdTyVars = tyvars, tcdRhs = rhs })
       HsParTy _ lty  -> rhs_annotated lty
       HsKindSig {}   -> True
       _              -> False
-hsDeclHasCusk (DataDecl { tcdDExt = DataDeclRn { tcdDataCusk = cusk }}) = cusk
-hsDeclHasCusk (ClassDecl { tcdTyVars = tyvars }) = hsTvbAllKinded tyvars
-hsDeclHasCusk (XTyClDecl _) = panic "hsDeclHasCusk"
+hsDeclHasCusk _cusks_enabled@True (DataDecl { tcdDExt = DataDeclRn { tcdDataCusk = cusk }}) = cusk
+hsDeclHasCusk _cusks_enabled@True (ClassDecl { tcdTyVars = tyvars }) = hsTvbAllKinded tyvars
+hsDeclHasCusk _ (XTyClDecl _) = panic "hsDeclHasCusk"
 
 -- Pretty-printing TyClDecl
 -- ~~~~~~~~~~~~~~~~~~~~~~~~
@@ -787,6 +791,10 @@ declaration before checking all of the others, supporting polymorphic recursion.
 See https://gitlab.haskell.org/ghc/ghc/wikis/ghc-kinds/kind-inference#proposed-new-strategy
 and #9200 for lots of discussion of how we got here.
 
+The detection of CUSKs is enabled by the -XCUSKs extension, switched on by default.
+Under -XNoCUSKs, all declarations are treated as if they have no CUSK.
+See https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0036-kind-signatures.rst
+
 PRINCIPLE:
   a type declaration has a CUSK iff we could produce a separate kind signature
   for it, just like a type signature for a function,
@@ -1080,11 +1088,13 @@ data FamilyInfo pass
 
 -- | Does this family declaration have a complete, user-supplied kind signature?
 -- See Note [CUSKs: complete user-supplied kind signatures]
-famDeclHasCusk :: Bool -- ^ True <=> this is an associated type family,
+famDeclHasCusk :: Bool -- ^ True <=> the -XCUSKs extension is enabled
+               -> Bool -- ^ True <=> this is an associated type family,
                        --            and the parent class has /no/ CUSK
                -> FamilyDecl pass
                -> Bool
-famDeclHasCusk assoc_with_no_cusk
+famDeclHasCusk _cusks_enabled@False _ _ = False
+famDeclHasCusk _cusks_enabled@True assoc_with_no_cusk
                (FamilyDecl { fdInfo      = fam_info
                            , fdTyVars    = tyvars
                            , fdResultSig = L _ resultSig })
@@ -1095,7 +1105,7 @@ famDeclHasCusk assoc_with_no_cusk
             -- Un-associated open type/data families have CUSKs
             -- Associated type families have CUSKs iff the parent class does
 
-famDeclHasCusk _ (XFamilyDecl {}) = panic "famDeclHasCusk"
+famDeclHasCusk _ _ (XFamilyDecl {}) = panic "famDeclHasCusk"
 
 -- | Does this family declaration have user-supplied return kind signature?
 hasReturnKindSignature :: FamilyResultSig a -> Bool