Eval: addStackFrames: nodge GHC to optimize it

From my test, ~ a quarter of savings.

Author: Anton-Latukha

src/Nix/Eval.hs

@@ -509,9 +509,14 @@ addStackFrames
   :: forall v e m a
    . (Scoped v m, Framed e m, Typeable v, Typeable m)
   => Transform NExprLocF (m a)
-addStackFrames f v = do
-  scopes <- currentScopes :: m (Scopes m v)
-  withFrame Info (EvaluatingExpr scopes v) (f v)
+addStackFrames f v =
+  do
+    scopes <- currentScopes :: m (Scopes m v)
+
+    -- sectioning gives GHC optimization
+    (`withFrameInfo` f v) $ (`EvaluatingExpr` v) scopes
+ where
+  withFrameInfo = withFrame Info
 
 framedEvalExprLoc
   :: forall e v m

src/Nix/Utils.hs

@@ -125,7 +125,7 @@ adiM
 adiM f g = g ((f <=< traverse (adiM f g)) . unFix)
 
 class Has a b where
-    hasLens :: Lens' a b
+  hasLens :: Lens' a b
 
 instance Has a a where
   hasLens f = f