Improve promoted type applications in GHC < 9.

This is https://github.com/lspitzner/brittany/pull/376 backported so we
can use it on GHC 8.10.7.

Backporting the tests would have been a hassle so I didn't bother.

src/Language/Haskell/Brittany/Internal/Layouters/Type.hs

@@ -50,8 +50,7 @@ layoutType ltype@(L _ typ) = docWrapNode ltype $ case typ of
 #else /* ghc-8.6 */
       Promoted -> docSeq
 #endif
-        [ docSeparator
+        [ docTick
-        , docTick
         , docWrapNode name $ docLit t
         ]
       NotPromoted -> docWrapNode name $ docLit t
@@ -562,14 +561,25 @@ layoutType ltype@(L _ typ) = docWrapNode ltype $ case typ of
   HsRecTy{} -> -- TODO
     briDocByExactInlineOnly "HsRecTy{}" ltype
   HsExplicitListTy _ _ typs -> do
+    -- `'['Foo]` isn't valid because it parses as a character. So if the list
+    -- starts with a promoted type var, we swap to `'[ 'Foo ]`.
+    let
+      sepIfHeadPromoted = case typs of
+        (L _ t) : _ | startsWithTick t -> docSeparator
+        _ -> docEmpty
+
+    -- When rendering on multiple lines, this causes commas to line up with the
+    -- opening bracket. Unfortunately it also adds unnecessary space when
+    -- rendering on a single line.
+    let specialCommaSep = appSep $ docLit $ Text.pack " ,"
+
     typDocs <- docSharedWrapper layoutType `mapM` typs
     hasComments <- hasAnyCommentsBelow ltype
-    let specialCommaSep = appSep $ docLit $ Text.pack " ,"
     docAlt
       [ docSeq
-      $  [docLit $ Text.pack "'["]
+      $  [docLit $ Text.pack "'[", sepIfHeadPromoted]
       ++ List.intersperse specialCommaSep (docForceSingleline <$> typDocs)
-      ++ [docLit $ Text.pack "]"]
+      ++ [sepIfHeadPromoted, docLit $ Text.pack "]"]
       , case splitFirstLast typDocs of
         FirstLastEmpty -> docSeq
           [ docLit $ Text.pack "'["
@@ -578,7 +588,9 @@ layoutType ltype@(L _ typ) = docWrapNode ltype $ case typ of
         FirstLastSingleton e -> docAlt
           [ docSeq
             [ docLit $ Text.pack "'["
+            , sepIfHeadPromoted
             , docNodeAnnKW ltype (Just AnnOpenS) $ docForceSingleline e
+            , sepIfHeadPromoted
             , docLit $ Text.pack "]"
             ]
           , docSetBaseY $ docLines
@@ -593,7 +605,7 @@ layoutType ltype@(L _ typ) = docWrapNode ltype $ case typ of
         FirstLast e1 ems eN -> runFilteredAlternative $ do
           addAlternativeCond (not hasComments)
             $  docSeq
-            $  [docLit $ Text.pack "'["]
+            $  [docLit $ Text.pack "'[", sepIfHeadPromoted]
             ++ List.intersperse specialCommaSep (docForceSingleline <$> (e1:ems ++ [docNodeAnnKW ltype (Just AnnOpenS) eN]))
             ++ [docLit $ Text.pack " ]"]
           addAlternative $
@@ -663,3 +675,10 @@ processTyVarBndrsSingleline bndrDocs = bndrDocs >>= \case
     , docForceSingleline $ doc
     , docLit $ Text.pack ")"
     ]
+
+-- | Determine if the type starts with a tick mark (single quote) when rendered.
+startsWithTick :: HsType pass -> Bool
+startsWithTick = \case
+  HsTyVar _ IsPromoted _ -> True
+  HsAppTy _ (L _ t) _ -> startsWithTick t
+  _ -> False