Implement NoFieldSelectors extension (ghc-proposals 160)

Fixes #5972. This adds an extension NoFieldSelectors to disable the generation
of selector functions corresponding to record fields.  When this extension is
enabled, record field selectors are not accessible as functions, but users are
still able to use them for record construction, pattern matching and updates.
See Note [NoFieldSelectors] in GHC.Rename.Env for details.

Defining the same field multiple times requires the DuplicateRecordFields
extension to be enabled, even when NoFieldSelectors is in use.

Along the way, this fixes the use of non-imported DuplicateRecordFields in GHCi
with -fimplicit-import-qualified (fixes #18729).

Moreover, it extends DisambiguateRecordFields to ignore non-fields when looking
up fields in record updates (fixes #18999), as described by
Note [DisambiguateRecordFields for updates].

Co-authored-by: Simon Hafner <hafnersimon@gmail.com>
Co-authored-by: Fumiaki Kinoshita <fumiexcel@gmail.com>

1 files changed, 31 insertions, 8 deletions

diff --git a/compiler/GHC/Types/Name/Reader.hs b/compiler/GHC/Types/Name/Reader.hs
index c40a7143ff..6eb81653a5 100644
--- a/compiler/GHC/Types/Name/Reader.hs
+++ b/compiler/GHC/Types/Name/Reader.hs
@@ -46,7 +46,7 @@ module GHC.Types.Name.Reader (
         GlobalRdrEnv, emptyGlobalRdrEnv, mkGlobalRdrEnv, plusGlobalRdrEnv,
         lookupGlobalRdrEnv, extendGlobalRdrEnv, greOccName, shadowNames,
         pprGlobalRdrEnv, globalRdrEnvElts,
-        lookupGRE_RdrName, lookupGRE_Name,
+        lookupGRE_RdrName, lookupGRE_RdrName', lookupGRE_Name,
         lookupGRE_GreName, lookupGRE_FieldLabel,
         lookupGRE_Name_OccName,
         getGRE_NameQualifier_maybes,
@@ -58,9 +58,11 @@ module GHC.Types.Name.Reader (
         gresToAvailInfo,
         greDefinitionModule, greDefinitionSrcSpan,
         greMangledName, grePrintableName,
+        greFieldLabel,
 
         -- ** Global 'RdrName' mapping elements: 'GlobalRdrElt', 'Provenance', 'ImportSpec'
-        GlobalRdrElt(..), isLocalGRE, isRecFldGRE, isOverloadedRecFldGRE, greFieldLabel,
+        GlobalRdrElt(..), isLocalGRE, isRecFldGRE,
+        isDuplicateRecFldGRE, isNoFieldSelectorGRE, isFieldSelectorGRE,
         unQualOK, qualSpecOK, unQualSpecOK,
         pprNameProvenance,
         GreName(..), greNameSrcSpan,
@@ -836,7 +838,15 @@ lookupGlobalRdrEnv env occ_name = case lookupOccEnv env occ_name of
                                   Just gres -> gres
 
 lookupGRE_RdrName :: RdrName -> GlobalRdrEnv -> [GlobalRdrElt]
-lookupGRE_RdrName rdr_name env
+-- ^ Look for this 'RdrName' in the global environment.  Omits record fields
+-- without selector functions (see Note [NoFieldSelectors] in GHC.Rename.Env).
+lookupGRE_RdrName rdr_name env =
+    filter (not . isNoFieldSelectorGRE) (lookupGRE_RdrName' rdr_name env)
+
+lookupGRE_RdrName' :: RdrName -> GlobalRdrEnv -> [GlobalRdrElt]
+-- ^ Look for this 'RdrName' in the global environment.  Includes record fields
+-- without selector functions (see Note [NoFieldSelectors] in GHC.Rename.Env).
+lookupGRE_RdrName' rdr_name env
   = case lookupOccEnv env (rdrNameOcc rdr_name) of
     Nothing   -> []
     Just gres -> pickGREs rdr_name gres
@@ -858,14 +868,14 @@ lookupGRE_GreName env gname
 lookupGRE_FieldLabel :: GlobalRdrEnv -> FieldLabel -> Maybe GlobalRdrElt
 -- ^ Look for a particular record field selector in the environment, where the
 -- selector name and field label may be different: the GlobalRdrEnv is keyed on
--- the label.  See Note [Parents for record fields] for why this happens.
+-- the label.  See Note [GreNames] for why this happens.
 lookupGRE_FieldLabel env fl
   = lookupGRE_Name_OccName env (flSelector fl) (mkVarOccFS (flLabel fl))
 
 lookupGRE_Name_OccName :: GlobalRdrEnv -> Name -> OccName -> Maybe GlobalRdrElt
 -- ^ Look for precisely this 'Name' in the environment, but with an 'OccName'
 -- that might differ from that of the 'Name'.  See 'lookupGRE_FieldLabel' and
--- Note [Parents for record fields].
+-- Note [GreNames].
 lookupGRE_Name_OccName env name occ
   = case [ gre | gre <- lookupGlobalRdrEnv env occ
                , greMangledName gre == name ] of
@@ -895,10 +905,23 @@ isLocalGRE (GRE {gre_lcl = lcl }) = lcl
 isRecFldGRE :: GlobalRdrElt -> Bool
 isRecFldGRE = isJust . greFieldLabel
 
-isOverloadedRecFldGRE :: GlobalRdrElt -> Bool
+isDuplicateRecFldGRE :: GlobalRdrElt -> Bool
 -- ^ Is this a record field defined with DuplicateRecordFields?
--- (See Note [Parents for record fields])
-isOverloadedRecFldGRE = maybe False flIsOverloaded . greFieldLabel
+-- (See Note [GreNames])
+isDuplicateRecFldGRE =
+    maybe False ((DuplicateRecordFields ==) . flHasDuplicateRecordFields) . greFieldLabel
+
+isNoFieldSelectorGRE :: GlobalRdrElt -> Bool
+-- ^ Is this a record field defined with NoFieldSelectors?
+-- (See Note [NoFieldSelectors] in GHC.Rename.Env)
+isNoFieldSelectorGRE =
+    maybe False ((NoFieldSelectors ==) . flHasFieldSelector) . greFieldLabel
+
+isFieldSelectorGRE :: GlobalRdrElt -> Bool
+-- ^ Is this a record field defined with FieldSelectors?
+-- (See Note [NoFieldSelectors] in GHC.Rename.Env)
+isFieldSelectorGRE =
+    maybe False ((FieldSelectors ==) . flHasFieldSelector) . greFieldLabel
 
 greFieldLabel :: GlobalRdrElt -> Maybe FieldLabel
 -- ^ Returns the field label of this GRE, if it has one