Make sure HasField use counts for -Wunused-top-binds

This is a small fix that depends on the previous commit, because it
corrected the rnExpr free variable calculation for HsVars which refer
to ambiguous fields. Fixes #19213.

compiler/GHC/Tc/Gen/Head.hs

@@ -546,7 +546,9 @@ finish_ambiguous_selector lr@(L _ rdr) parent_type
            Nothing  -> failWithTc (fieldNotInType parent rdr) ;
            Just gre ->
 
+    -- See Note [Unused name reporting and HasField] in GHC.Tc.Instance.Class
     do { addUsedGRE True gre
+       ; keepAlive (greMangledName gre)
        ; return (greMangledName gre) } } } } }
 
 -- This field name really is ambiguous, so add a suitable "ambiguous

compiler/GHC/Tc/Instance/Class.hs

@@ -30,7 +30,7 @@ import GHC.Builtin.Types
 import GHC.Builtin.Types.Prim( eqPrimTyCon, eqReprPrimTyCon )
 import GHC.Builtin.Names
 
-import GHC.Types.Name.Reader( lookupGRE_FieldLabel )
+import GHC.Types.Name.Reader( lookupGRE_FieldLabel, greMangledName )
 import GHC.Types.SafeHaskell
 import GHC.Types.Name   ( Name, pprDefinedAt )
 import GHC.Types.Var.Env ( VarEnv )
@@ -672,6 +672,20 @@ may be solved by a user-supplied HasField instance.  Similarly, if we
 encounter a HasField constraint where the field is not a literal
 string, or does not belong to the type, then we fall back on the
 normal constraint solver behaviour.
+
+
+Note [Unused name reporting and HasField]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When a HasField constraint is solved by the type-checker, we must record a use
+of the corresponding field name, as otherwise it might be reported as unused.
+See #19213.  We need to call keepAlive to add the name to the tcg_keep set,
+which accumulates names used by the constraint solver, as described by
+Note [Tracking unused binding and imports] in GHC.Tc.Types.
+
+We need to call addUsedGRE as well because there may be a deprecation warning on
+the field, which will be reported by addUsedGRE.  But calling addUsedGRE without
+keepAlive is not enough, because the field might be defined locally, and
+addUsedGRE extends tcg_used_gres with imported GREs only.
 -}
 
 -- See Note [HasField instances]
@@ -721,7 +735,9 @@ matchHasField dflags short_cut clas tys
                      -- cannot have an existentially quantified type), and
                      -- it must not be higher-rank.
                    ; if not (isNaughtyRecordSelector sel_id) && isTauTy sel_ty
-                     then do { addUsedGRE True gre
+                     then do { -- See Note [Unused name reporting and HasField]
+                               addUsedGRE True gre
+                             ; keepAlive (greMangledName gre)
                              ; return OneInst { cir_new_theta = theta
                                               , cir_mk_ev     = mk_ev
                                               , cir_what      = BuiltinInstance } }

compiler/GHC/Tc/Types.hs

@@ -680,6 +680,9 @@ We gather three sorts of usage information
           Coercible solver updates tcg_keep's TcRef whenever it
           encounters a use of `coerce` that crosses newtype boundaries.
 
+      (e) Record fields that are used to solve HasField constraints
+          (see Note [Unused name reporting and HasField] in GHC.Tc.Instance.Class)
+
       The tcg_keep field is used in two distinct ways:
 
       * Desugar.addExportFlagsAndRules.  Where things like (a-c) are locally

testsuite/tests/overloadedrecflds/should_fail/DRFUnused.hs

+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE DuplicateRecordFields #-}
+{-# LANGUAGE TypeApplications #-}
+{-# OPTIONS_GHC -Werror=unused-top-binds #-}
+
+module DRFUnused (S(MkS), x, y) where
+
+import GHC.Records
+
+data S = MkS { foo :: Int }
+data T = MkT { foo :: Int }
+data U = MkU { foo :: Int }
+
+-- Should count as a use of the foo field belonging to T, but not the others.
+x = getField @"foo" (MkT 42)
+
+-- Should count as a use of the foo field belonging to U, but not the others.
+y = foo (MkU 42 :: U)

testsuite/tests/overloadedrecflds/should_fail/DRFUnused.stderr

+
+DRFUnused.hs:10:16: error: [-Wunused-top-binds (in -Wextra, -Wunused-binds), -Werror=unused-top-binds]
+    Defined but not used: ‘foo’

testsuite/tests/overloadedrecflds/should_fail/all.T

@@ -45,3 +45,4 @@ test('NFS9156', normal, compile_fail, [''])
 test('NFSDuplicate', normal, compile_fail, [''])
 test('NFSExport', normal, compile_fail, [''])
 test('T18999_NoDisambiguateRecordFields', normal, compile_fail, [''])
+test('DRFUnused', normal, compile_fail, [''])