CApiFFI: add ConstPtr for encoding const-qualified pointer return types (#22043)

Previously, when using `capi` calling convention in foreign declarations,
code generator failed to handle const-cualified pointer return types.
This resulted in CC toolchain throwing `-Wincompatible-pointer-types-discards-qualifiers`
warning.

`Foreign.C.Types.ConstPtr` newtype was introduced to handle these cases -
special treatment was put in place to generate appropritetly qualified C
wrapper that no longer triggers the above mentioned warning.

Fixes #22043

9 files changed, 62 insertions, 5 deletions

diff --git a/compiler/GHC/Builtin/Names.hs b/compiler/GHC/Builtin/Names.hs
index 07a3bd2f9d..a9802e3e95 100644
--- a/compiler/GHC/Builtin/Names.hs
+++ b/compiler/GHC/Builtin/Names.hs
@@ -350,7 +350,7 @@ basicKnownKeyNames
         zipName, foldrName, buildName, augmentName, appendName,
 
         -- FFI primitive types that are not wired-in.
-        stablePtrTyConName, ptrTyConName, funPtrTyConName,
+        stablePtrTyConName, ptrTyConName, funPtrTyConName, constPtrConName,
         int8TyConName, int16TyConName, int32TyConName, int64TyConName,
         word8TyConName, word16TyConName, word32TyConName, word64TyConName,
 
@@ -557,7 +557,7 @@ gHC_PRIM, gHC_PRIM_PANIC,
     aRROW, gHC_DESUGAR, rANDOM, gHC_EXTS, gHC_IS_LIST,
     cONTROL_EXCEPTION_BASE, gHC_TYPEERROR, gHC_TYPELITS, gHC_TYPELITS_INTERNAL,
     gHC_TYPENATS, gHC_TYPENATS_INTERNAL,
-    dATA_COERCE, dEBUG_TRACE, uNSAFE_COERCE :: Module
+    dATA_COERCE, dEBUG_TRACE, uNSAFE_COERCE, fOREIGN_C_TYPES :: Module
 
 gHC_PRIM        = mkPrimModule (fsLit "GHC.Prim")   -- Primitive types and values
 gHC_PRIM_PANIC  = mkPrimModule (fsLit "GHC.Prim.Panic")
@@ -627,6 +627,7 @@ gHC_TYPENATS_INTERNAL = mkBaseModule (fsLit "GHC.TypeNats.Internal")
 dATA_COERCE     = mkBaseModule (fsLit "Data.Coerce")
 dEBUG_TRACE     = mkBaseModule (fsLit "Debug.Trace")
 uNSAFE_COERCE   = mkBaseModule (fsLit "Unsafe.Coerce")
+fOREIGN_C_TYPES = mkBaseModule (fsLit "Foreign.C.Types")
 
 gHC_SRCLOC :: Module
 gHC_SRCLOC = mkBaseModule (fsLit "GHC.SrcLoc")
@@ -1665,6 +1666,10 @@ fingerprintDataConName :: Name
 fingerprintDataConName =
     dcQual gHC_FINGERPRINT_TYPE (fsLit "Fingerprint") fingerprintDataConKey
 
+constPtrConName :: Name
+constPtrConName =
+    tcQual fOREIGN_C_TYPES (fsLit "ConstPtr") constPtrTyConKey
+
 {-
 ************************************************************************
 *                                                                      *
@@ -1866,7 +1871,7 @@ statePrimTyConKey, stableNamePrimTyConKey, stableNameTyConKey,
     funPtrTyConKey, tVarPrimTyConKey, eqPrimTyConKey,
     eqReprPrimTyConKey, eqPhantPrimTyConKey,
     compactPrimTyConKey, stackSnapshotPrimTyConKey,
-    promptTagPrimTyConKey :: Unique
+    promptTagPrimTyConKey, constPtrTyConKey :: Unique
 statePrimTyConKey                       = mkPreludeTyConUnique 50
 stableNamePrimTyConKey                  = mkPreludeTyConUnique 51
 stableNameTyConKey                      = mkPreludeTyConUnique 52
@@ -2077,6 +2082,7 @@ typeConsSymbolTyFamNameKey = mkPreludeTyConUnique 413
 typeUnconsSymbolTyFamNameKey = mkPreludeTyConUnique 414
 typeCharToNatTyFamNameKey = mkPreludeTyConUnique 415
 typeNatToCharTyFamNameKey = mkPreludeTyConUnique 416
+constPtrTyConKey = mkPreludeTyConUnique 417
 
 {-
 ************************************************************************
diff --git a/compiler/GHC/HsToCore/Foreign/C.hs b/compiler/GHC/HsToCore/Foreign/C.hs
index 160e9acc97..28ef42e2b9 100644
--- a/compiler/GHC/HsToCore/Foreign/C.hs
+++ b/compiler/GHC/HsToCore/Foreign/C.hs
@@ -246,10 +246,18 @@ dsFCall :: Id -> Coercion -> ForeignCall -> Maybe Header
         -> DsM ([(Id, Expr TyVar)], CHeader, CStub)
 dsFCall fn_id co fcall mDeclHeader = do
     let
-        ty                   = coercionLKind co
+        (ty,ty1)             = (coercionLKind co, coercionRKind co)
         (tv_bndrs, rho)      = tcSplitForAllTyVarBinders ty
         (arg_tys, io_res_ty) = tcSplitFunTys rho
 
+    let constQual -- provide 'const' qualifier (#22034)
+          | (_, res_ty1) <- tcSplitFunTys ty1
+          , newty <- maybe res_ty1 snd (tcSplitIOType_maybe res_ty1)
+          , Just (ptr, _) <- splitTyConApp_maybe newty
+          , tyConName ptr `elem` [constPtrConName]
+          = text "const"
+          | otherwise = empty
+
     args <- newSysLocalsDs arg_tys  -- no FFI representation polymorphism
     (val_args, arg_wrappers) <- mapAndUnzipM unboxArg (map Var args)
 
@@ -277,7 +285,7 @@ dsFCall fn_id co fcall mDeclHeader = do
                       includes = vcat [ text "#include \"" <> ftext h
                                         <> text "\""
                                       | Header _ h <- nub headers ]
-                      fun_proto = cResType <+> pprCconv <+> ppr wrapperName <> parens argTypes
+                      fun_proto = constQual <+> cResType <+> pprCconv <+> ppr wrapperName <> parens argTypes
                       cRet
                        | isVoidRes =                   cCall
                        | otherwise = text "return" <+> cCall
diff --git a/docs/users_guide/9.6.1-notes.rst b/docs/users_guide/9.6.1-notes.rst
index dfd800d11b..ef3078bbfa 100644
--- a/docs/users_guide/9.6.1-notes.rst
+++ b/docs/users_guide/9.6.1-notes.rst
@@ -157,6 +157,9 @@ Runtime system
 ``ghc`` library
 ~~~~~~~~~~~~~~~
 
+- Add `Foreign.C.Types.ConstPtr` was added to encode ``const``-qualified pointer return
+  types in foreign declarations when using ``CApiFFI`` extension.
+
 ``ghc-heap`` library
 ~~~~~~~~~~~~~~~~~~~~
 
diff --git a/docs/users_guide/exts/ffi.rst b/docs/users_guide/exts/ffi.rst
index b490ac662d..f23c223279 100644
--- a/docs/users_guide/exts/ffi.rst
+++ b/docs/users_guide/exts/ffi.rst
@@ -437,6 +437,18 @@ specified. The syntax looks like: ::
     data    {-# CTYPE "unistd.h" "useconds_t" #-} T = ...
     newtype {-# CTYPE            "useconds_t" #-} T = ...
 
+In case foreign declarations contain ``const``-qualified pointer return
+type, `ConstPtr` from :base-ref:`Foreign.C.Types` may be used to
+encode this, e.g. ::
+
+    foreign import capi "header.h f" f :: CInt -> ConstPtr CInt
+
+which corresponds to
+
+.. code-block:: c
+
+    const *int f(int);
+
 ``hs_thread_done()``
 ~~~~~~~~~~~~~~~~~~~~
 
diff --git a/libraries/base/Foreign/C/Types.hs b/libraries/base/Foreign/C/Types.hs
index f2c83c4203..7f74a07bc8 100644
--- a/libraries/base/Foreign/C/Types.hs
+++ b/libraries/base/Foreign/C/Types.hs
@@ -86,8 +86,11 @@ module Foreign.C.Types
 
           -- Instances of: Eq and Storable
         , CFile,        CFpos,     CJmpBuf
+
+        , ConstPtr(..)
         ) where
 
+import Foreign.Ptr      ( Ptr )
 import Foreign.Storable
 import Data.Bits        ( Bits(..), FiniteBits(..) )
 import Data.Int         ( Int8,  Int16,  Int32,  Int64  )
@@ -223,6 +226,9 @@ INTEGRAL_TYPE(CUIntPtr,"uintptr_t",HTYPE_UINTPTR_T)
 INTEGRAL_TYPE(CIntMax,"intmax_t",HTYPE_INTMAX_T)
 INTEGRAL_TYPE(CUIntMax,"uintmax_t",HTYPE_UINTMAX_T)
 
+-- | Used to produce 'const' qualifier in C code generator
+newtype ConstPtr a = ConstPtr { unConstPtr :: Ptr a } deriving newtype (Show, Eq, Storable)
+
 -- C99 types which are still missing include:
 -- wint_t, wctrans_t, wctype_t
 
diff --git a/testsuite/tests/ffi/should_compile/T22034.h b/testsuite/tests/ffi/should_compile/T22034.h
new file mode 100644
index 0000000000..26c49d3a38
--- /dev/null
+++ b/testsuite/tests/ffi/should_compile/T22034.h
@@ -0,0 +1,2 @@
+const int *foo();
+const double *bar;
diff --git a/testsuite/tests/ffi/should_compile/T22034.hs b/testsuite/tests/ffi/should_compile/T22034.hs
new file mode 100644
index 0000000000..8c065c3ea1
--- /dev/null
+++ b/testsuite/tests/ffi/should_compile/T22034.hs
@@ -0,0 +1,10 @@
+{-# LANGUAGE CApiFFI #-}
+module T22034 where
+
+import Foreign.C.Types
+
+foreign import capi "T22034.h foo"
+    c_foo :: IO (ConstPtr CInt)
+
+foreign import capi "T22034.h value bar"
+    c_bar :: ConstPtr CDouble
diff --git a/testsuite/tests/ffi/should_compile/T22034_c.c b/testsuite/tests/ffi/should_compile/T22034_c.c
new file mode 100644
index 0000000000..e70b5a978f
--- /dev/null
+++ b/testsuite/tests/ffi/should_compile/T22034_c.c
@@ -0,0 +1,9 @@
+#include <stdlib.h>
+
+const int * foo() {
+    int *x = malloc(sizeof(int));
+    *x = 42;
+    return x;
+}
+
+const int *bar = 0;
diff --git a/testsuite/tests/ffi/should_compile/all.T b/testsuite/tests/ffi/should_compile/all.T
index d8afeb9f7b..532f5c3854 100644
--- a/testsuite/tests/ffi/should_compile/all.T
+++ b/testsuite/tests/ffi/should_compile/all.T
@@ -43,3 +43,4 @@ test(
     ],
 )
 test('T15531', normal, compile, ['-Wall'])
+test('T22034', [omit_ways(['ghci'])], compile, [''])