Let apply run endomorphisms (#1505)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: isovector

plugins/hls-tactics-plugin/src/Ide/Plugin/Tactic/Tactics.hs

@@ -167,10 +167,7 @@ apply hi = requireConcreteHole $ tracing ("apply' " <> show (hi_name hi)) $ do
       func = hi_name hi
   ty' <- freshTyvars ty
   let (_, _, args, ret) = tacticsSplitFunTy ty'
-  -- TODO(sandy): Bug here! Prevents us from doing mono-map like things
-  -- Don't require new holes for locally bound vars; only respect linearity
-  -- see https://github.com/haskell/haskell-language-server/issues/1447
-  requireNewHoles $ rule $ \jdg -> do
+  rule $ \jdg -> do
     unify g (CType ret)
     ext
         <- fmap unzipTrace

plugins/hls-tactics-plugin/test/CodeAction/AutoSpec.hs

@@ -45,6 +45,7 @@ spec = do
     autoTest  2 14 "FmapJoin.hs"
     autoTest  2  9 "Fgmap.hs"
     autoTest  4 19 "FmapJoinInLet.hs"
+    autoTest  9 12 "AutoEndo.hs"
 
     failing "flaky in CI" $
       autoTest 2 11 "GoldenApplicativeThen.hs"

plugins/hls-tactics-plugin/test/golden/AutoEndo.hs

@@ -0,0 +1,10 @@
+data Synthesized b a = Synthesized
+  { syn_trace :: b
+  , syn_val   :: a
+  }
+  deriving (Eq, Show)
+
+
+mapTrace :: (b -> b) -> Synthesized b a -> Synthesized b a
+mapTrace = _
+

plugins/hls-tactics-plugin/test/golden/AutoEndo.hs.expected

@@ -0,0 +1,11 @@
+data Synthesized b a = Synthesized
+  { syn_trace :: b
+  , syn_val   :: a
+  }
+  deriving (Eq, Show)
+
+
+mapTrace :: (b -> b) -> Synthesized b a -> Synthesized b a
+mapTrace fbb (Synthesized b a)
+  = Synthesized {syn_trace = fbb b, syn_val = a}
+