Cache coercion roles in NthCowip/tdammers/D4394-squash

Most callers of mkNthCo know the role of the coercion they
are trying to make. So instead of calculating this role, we
pass it in, forcing the caller to calculate it when needed.

This introduces a performance regression in perf/compiler/T9872d.

1 files changed, 5 insertions, 3 deletions

diff --git a/compiler/coreSyn/CoreLint.hs b/compiler/coreSyn/CoreLint.hs
index 78902dfea4..af888b6fb0 100644
--- a/compiler/coreSyn/CoreLint.hs
+++ b/compiler/coreSyn/CoreLint.hs
@@ -1732,12 +1732,13 @@ lintCoercion co@(TransCo co1 co2)
        ; lintRole co r1 r2
        ; return (k1a, k2b, ty1a, ty2b, r1) }
 
-lintCoercion the_co@(NthCo n co)
+lintCoercion the_co@(NthCo r0 n co)
   = do { (_, _, s, t, r) <- lintCoercion co
        ; case (splitForAllTy_maybe s, splitForAllTy_maybe t) of
          { (Just (tv_s, _ty_s), Just (tv_t, _ty_t))
              |  n == 0
-             -> return (ks, kt, ts, tt, Nominal)
+             -> do { lintRole the_co Nominal r0
+                   ; return (ks, kt, ts, tt, r0) }
              where
                ts = tyVarKind tv_s
                tt = tyVarKind tv_t
@@ -1751,7 +1752,8 @@ lintCoercion the_co@(NthCo n co)
                  -- see Note [NthCo and newtypes] in TyCoRep
              , tys_s `equalLength` tys_t
              , tys_s `lengthExceeds` n
-             -> return (ks, kt, ts, tt, tr)
+             -> do { lintRole the_co tr r0
+                   ; return (ks, kt, ts, tt, r0) }
              where
                ts = getNth tys_s n
                tt = getNth tys_t n