Add --scopes option to print out scoping information

src/Nix/Options.hs

@@ -8,6 +8,7 @@ data Options = Options
     , tracing      :: Bool
     , thunks       :: Bool
     , values       :: Bool
+    , scopes       :: Bool
     , reduce       :: Maybe FilePath
     , reduceSets   :: Bool
     , reduceLists  :: Bool
@@ -41,6 +42,7 @@ defaultOptions current = Options
     , tracing      = False
     , thunks       = False
     , values       = False
+    , scopes       = False
     , reduce       = Nothing
     , reduceSets   = False
     , reduceLists  = False

src/Nix/Options/Parser.hs

@@ -45,6 +45,9 @@ nixOptions current = Options
     <*> switch
         (   long "values"
          <> help "Enable reporting of value provenance in error messages")
+    <*> switch
+        (   long "scopes"
+         <> help "Enable reporting of scopes in evaluation traces")
     <*> optional (strOption
         (   long "reduce"
          <> help "When done evaluating, output the evaluated part of the expression to FILE"))

src/Nix/Render/Frame.hs

@@ -92,8 +92,10 @@ renderEvalFrame :: (MonadReader e m, Has e Options, MonadFile m)
 renderEvalFrame level f = do
     opts :: Options <- asks (view hasLens)
     case f of
-        EvaluatingExpr _scope e@(Fix (Compose (Ann ann _))) ->
-            fmap (:[]) $ renderLocation ann
+        EvaluatingExpr scope e@(Fix (Compose (Ann ann _))) -> do
+            let scopeInfo | scopes opts = [string (show scope)]
+                          | otherwise   = []
+            fmap (\x -> scopeInfo ++ [x]) $ renderLocation ann
                 =<< renderExpr level "While evaluating" "Expression" e
 
         ForcingExpr _scope e@(Fix (Compose (Ann ann _)))