Use NonEmpty lists to represent lists of duplicate elements

Summary:
Three functions in `ListSetOps` which compute duplicate elements
represent lists of duplicates of `[a]`. This is a really bad way to go about
things, because these lists are guaranteed to always have at least one element
(the "representative" of the duplicates), and several places in the GHC API
call `head` (a partial function) on these lists of duplicates to retrieve the
representative.

This changes the representation of duplicates to `NonEmpty` lists instead,
which allow for many partial uses of `head` to be made total.

Fixes #13823.

Test Plan: ./validate

Reviewers: bgamari, austin, goldfire

Reviewed By: bgamari

Subscribers: goldfire, rwbarton, thomie

GHC Trac Issues: #13823

Differential Revision: https://phabricator.haskell.org/D3823

1 files changed, 6 insertions, 4 deletions

diff --git a/compiler/coreSyn/CoreLint.hs b/compiler/coreSyn/CoreLint.hs
index 8b6be2e661..390a3173d7 100644
--- a/compiler/coreSyn/CoreLint.hs
+++ b/compiler/coreSyn/CoreLint.hs
@@ -66,6 +66,8 @@ import DynFlags
 import Control.Monad
 import qualified Control.Monad.Fail as MonadFail
 import MonadUtils
+import Data.Foldable      ( toList )
+import Data.List.NonEmpty ( NonEmpty )
 import Data.Maybe
 import Pair
 import qualified GHC.LanguageExtensions as LangExt
@@ -2427,15 +2429,15 @@ pprLeftOrRight :: LeftOrRight -> MsgDoc
 pprLeftOrRight CLeft  = text "left"
 pprLeftOrRight CRight = text "right"
 
-dupVars :: [[Var]] -> MsgDoc
+dupVars :: [NonEmpty Var] -> MsgDoc
 dupVars vars
   = hang (text "Duplicate variables brought into scope")
-       2 (ppr vars)
+       2 (ppr (map toList vars))
 
-dupExtVars :: [[Name]] -> MsgDoc
+dupExtVars :: [NonEmpty Name] -> MsgDoc
 dupExtVars vars
   = hang (text "Duplicate top-level variables with the same qualified name")
-       2 (ppr vars)
+       2 (ppr (map toList vars))
 
 {-
 ************************************************************************