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, 12 insertions, 10 deletions

diff --git a/compiler/GHC/Rename/Expr.hs b/compiler/GHC/Rename/Expr.hs
index ab5330cce6..3b362d0729 100644
--- a/compiler/GHC/Rename/Expr.hs
+++ b/compiler/GHC/Rename/Expr.hs
@@ -47,6 +47,7 @@ import GHC.Rename.Pat
 import GHC.Driver.Session
 import GHC.Builtin.Names
 
+import GHC.Types.FieldLabel
 import GHC.Types.Fixity
 import GHC.Types.Name
 import GHC.Types.Name.Set
@@ -120,12 +121,13 @@ rnUnboundVar v =
           ; return (HsVar noExtField (noLoc n), emptyFVs) }
 
 rnExpr (HsVar _ (L l v))
-  = do { opt_DuplicateRecordFields <- xoptM LangExt.DuplicateRecordFields
-       ; mb_name <- lookupOccRn_overloaded opt_DuplicateRecordFields v
-       ; dflags <- getDynFlags
+  = do { dflags <- getDynFlags
+       ; let dup_fields_ok = xopt_DuplicateRecordFields dflags
+       ; mb_name <- lookupExprOccRn dup_fields_ok v
+
        ; case mb_name of {
            Nothing -> rnUnboundVar v ;
-           Just (Left name)
+           Just (UnambiguousGre (NormalGreName name))
               | name == nilDataConName -- Treat [] as an ExplicitList, so that
                                        -- OverloadedLists works correctly
                                        -- Note [Empty lists] in GHC.Hs.Expr
@@ -134,12 +136,12 @@ rnExpr (HsVar _ (L l v))
 
               | otherwise
               -> finishHsVar (L l name) ;
-            Just (Right [s]) ->
-              return ( HsRecFld noExtField (Unambiguous s (L l v) ), unitFV s) ;
-           Just (Right fs@(_:_:_)) ->
-              return ( HsRecFld noExtField (Ambiguous noExtField (L l v))
-                     , mkFVs fs);
-           Just (Right [])         -> panic "runExpr/HsVar" } }
+            Just (UnambiguousGre (FieldGreName fl)) ->
+              let sel_name = flSelector fl in
+              return ( HsRecFld noExtField (Unambiguous sel_name (L l v) ), unitFV sel_name) ;
+            Just AmbiguousFields ->
+              return ( HsRecFld noExtField (Ambiguous noExtField (L l v) ), emptyFVs) } }
+
 
 rnExpr (HsIPVar x v)
   = return (HsIPVar x v, emptyFVs)