compiler: Name generated locals more descriptively

Previously `GHC.Types.Id.Make.newLocal` would name all locals `dt`,
making it unnecessarily difficult to determine their origin.
Noticed while looking at #19557.

4 files changed, 25 insertions, 17 deletions

diff --git a/compiler/GHC/Types/Id/Make.hs b/compiler/GHC/Types/Id/Make.hs
index 4c2f028360..fe672f6143 100644
--- a/compiler/GHC/Types/Id/Make.hs
+++ b/compiler/GHC/Types/Id/Make.hs
@@ -696,7 +696,7 @@ mkDataConRep dflags fam_envs wrap_name mb_bangs data_con
   = return NoDataConRep
 
   | otherwise
-  = do { wrap_args <- mapM newLocal wrap_arg_tys
+  = do { wrap_args <- mapM (newLocal (fsLit "conrep")) wrap_arg_tys
        ; wrap_body <- mk_rep_app (wrap_args `zip` dropList eq_spec unboxers)
                                  initial_wrap_app
 
@@ -986,10 +986,15 @@ case of a newtype constructor, we simply hardcode its dcr_bangs field to
 -}
 
 -------------------------
-newLocal :: Scaled Type -> UniqSM Var
-newLocal (Scaled w ty) = do { uniq <- getUniqueM
-                            ; return (mkSysLocalOrCoVar (fsLit "dt") uniq w ty) }
-                 -- We should not have "OrCoVar" here, this is a bug (#17545)
+
+-- | Conjure a fresh local binder.
+newLocal :: FastString   -- ^ a string which will form part of the 'Var'\'s name
+         -> Scaled Type  -- ^ the type of the 'Var'
+         -> UniqSM Var
+newLocal name_stem (Scaled w ty) =
+    do { uniq <- getUniqueM
+       ; return (mkSysLocalOrCoVar name_stem uniq w ty) }
+         -- We should not have "OrCoVar" here, this is a bug (#17545)
 
 
 -- | Unpack/Strictness decisions from source module.
@@ -1072,14 +1077,14 @@ wrapCo :: Coercion -> Type -> (Unboxer, Boxer) -> (Unboxer, Boxer)
 wrapCo co rep_ty (unbox_rep, box_rep)  -- co :: arg_ty ~ rep_ty
   = (unboxer, boxer)
   where
-    unboxer arg_id = do { rep_id <- newLocal (Scaled (idMult arg_id) rep_ty)
+    unboxer arg_id = do { rep_id <- newLocal (fsLit "cowrap_unbx") (Scaled (idMult arg_id) rep_ty)
                         ; (rep_ids, rep_fn) <- unbox_rep rep_id
                         ; let co_bind = NonRec rep_id (Var arg_id `Cast` co)
                         ; return (rep_ids, Let co_bind . rep_fn) }
     boxer = Boxer $ \ subst ->
             do { (rep_ids, rep_expr)
                     <- case box_rep of
-                         UnitBox -> do { rep_id <- newLocal (linear $ TcType.substTy subst rep_ty)
+                         UnitBox -> do { rep_id <- newLocal (fsLit "cowrap_bx") (linear $ TcType.substTy subst rep_ty)
                                        ; return ([rep_id], Var rep_id) }
                          Boxer boxer -> boxer subst
                ; let sco = substCoUnchecked subst co
@@ -1112,7 +1117,7 @@ dataConArgUnpack (Scaled arg_mult arg_ty)
       -- Note [Unpacking GADTs and existentials]
     ( rep_tys `zip` dataConRepStrictness con
     ,( \ arg_id ->
-       do { rep_ids <- mapM newLocal rep_tys
+       do { rep_ids <- mapM (newLocal (fsLit "unbx")) rep_tys
           ; let r_mult = idMult arg_id
           ; let rep_ids' = map (scaleIdBy r_mult) rep_ids
           ; let unbox_fn body
@@ -1120,7 +1125,7 @@ dataConArgUnpack (Scaled arg_mult arg_ty)
                              (DataAlt con) rep_ids' body
           ; return (rep_ids, unbox_fn) }
      , Boxer $ \ subst ->
-       do { rep_ids <- mapM (newLocal . TcType.substScaledTyUnchecked subst) rep_tys
+       do { rep_ids <- mapM (newLocal (fsLit "bx") . TcType.substScaledTyUnchecked subst) rep_tys
           ; return (rep_ids, Var (dataConWorkId con)
                              `mkTyApps` (substTysUnchecked subst tc_args)
                              `mkVarApps` rep_ids ) } ) )
diff --git a/testsuite/tests/simplCore/should_compile/T5366.stdout b/testsuite/tests/simplCore/should_compile/T5366.stdout
index 92fed9ddda..f2f58ea97f 100644
--- a/testsuite/tests/simplCore/should_compile/T5366.stdout
+++ b/testsuite/tests/simplCore/should_compile/T5366.stdout
@@ -1,2 +1,2 @@
-                 case ds of { Bar dt [Occ=Once1] _ [Occ=Dead] -> GHC.Types.I# dt }}]
-f = \ (ds :: Bar) -> case ds of { Bar dt dt1 -> GHC.Types.I# dt }
+                 case ds of { Bar bx [Occ=Once1] _ [Occ=Dead] -> GHC.Types.I# bx }}]
+f = \ (ds :: Bar) -> case ds of { Bar bx bx1 -> GHC.Types.I# bx }
diff --git a/testsuite/tests/simplCore/should_compile/T7360.stderr b/testsuite/tests/simplCore/should_compile/T7360.stderr
index 070d7ef7fe..6b6438bf14 100644
--- a/testsuite/tests/simplCore/should_compile/T7360.stderr
+++ b/testsuite/tests/simplCore/should_compile/T7360.stderr
@@ -13,11 +13,13 @@ T7360.$WFoo3 [InlPrag=INLINE[final] CONLIKE] :: Int %1 -> Foo
  Unf=Unf{Src=InlineStable, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True,
          Guidance=ALWAYS_IF(arity=1,unsat_ok=True,boring_ok=False)
-         Tmpl= \ (dt [Occ=Once1!] :: Int) ->
-                 case dt of { GHC.Types.I# dt [Occ=Once1] -> T7360.Foo3 dt }}]
+         Tmpl= \ (conrep [Occ=Once1!] :: Int) ->
+                 case conrep of { GHC.Types.I# unbx [Occ=Once1] ->
+                 T7360.Foo3 unbx
+                 }}]
 T7360.$WFoo3
-  = \ (dt [Occ=Once1!] :: Int) ->
-      case dt of { GHC.Types.I# dt [Occ=Once1] -> T7360.Foo3 dt }
+  = \ (conrep [Occ=Once1!] :: Int) ->
+      case conrep of { GHC.Types.I# unbx [Occ=Once1] -> T7360.Foo3 unbx }
 
 -- RHS size: {terms: 5, types: 2, coercions: 0, joins: 0/0}
 fun1 [InlPrag=NOINLINE] :: Foo -> ()
diff --git a/testsuite/tests/stranal/should_compile/T16029.stdout b/testsuite/tests/stranal/should_compile/T16029.stdout
index 76bf2617fb..20861eac28 100644
--- a/testsuite/tests/stranal/should_compile/T16029.stdout
+++ b/testsuite/tests/stranal/should_compile/T16029.stdout
@@ -1,6 +1,7 @@
   :: Int %1 -> Int %1 -> T
-         Tmpl= \ (dt [Occ=Once1!] :: Int) (dt [Occ=Once1!] :: Int) ->
-  = \ (dt [Occ=Once1!] :: Int) (dt [Occ=Once1!] :: Int) ->
+         Tmpl= \ (conrep [Occ=Once1!] :: Int)
+                 (conrep [Occ=Once1!] :: Int) ->
+  = \ (conrep [Occ=Once1!] :: Int) (conrep [Occ=Once1!] :: Int) ->
   :: GHC.Prim.Int# -> GHC.Prim.Int#
   = \ (ww :: GHC.Prim.Int#) ->
 g2 [InlPrag=[2]] :: T -> Int -> Int