RST writer: filter out empty inline containers (#4434).

There is nothing in RST that corresponds to e.g. `Emph []`, so we just filter out elements like this.

src/Text/Pandoc/Writers/RST.hs

@@ -378,9 +378,24 @@ blockListToRST :: PandocMonad m
 blockListToRST = blockListToRST' False
 
 transformInlines :: [Inline] -> [Inline]
-transformInlines =
-  removeLeadingTrailingSpace . removeSpaceAfterDisplayMath . insertBS
-  where -- remove spaces after displaymath, as they screw up indentation:
+transformInlines =  removeLeadingTrailingSpace . insertBS
+                    . filter hasContents . removeSpaceAfterDisplayMath
+  where -- empty inlines are not valid RST syntax
+        hasContents :: Inline -> Bool
+        hasContents (Str "")              = False
+        hasContents (Emph [])             = False
+        hasContents (Strong [])           = False
+        hasContents (Strikeout [])        = False
+        hasContents (Superscript [])      = False
+        hasContents (Subscript [])        = False
+        hasContents (SmallCaps [])        = False
+        hasContents (Quoted _ [])         = False
+        hasContents (Cite _ [])           = False
+        hasContents (Span _ [])           = False
+        hasContents (Link _ [] ("", ""))  = False
+        hasContents (Image _ [] ("", "")) = False
+        hasContents _                     = True
+        -- remove spaces after displaymath, as they screw up indentation:
         removeSpaceAfterDisplayMath (Math DisplayMath x : zs) =
               Math DisplayMath x : dropWhile (==Space) zs
         removeSpaceAfterDisplayMath (x:xs) = x : removeSpaceAfterDisplayMath xs

test/Tests/Writers/RST.hs

@@ -50,14 +50,18 @@ tests = [ testGroup "rubrics"
               , ""
               , "    quoted"]
           ]
-        , testGroup "spaces are stripped within inlines"
-          -- pandoc issue 4327 "The text within inline markup may not
-          -- begin or end with whitespace"
-          -- http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html#inline-markup
-          [ "multiple" =:
+        , testGroup "inlines"
+          [ "are removed when empty" =: -- #4434
+            plain (strong (str "")) =?> ""
+          , "do not cause the introduction of extra spaces when removed" =:
+            plain (strong (str "") <> emph (str "text")) =?> "*text*"
+          , "spaces are stripped at beginning and end" =:
+            -- pandoc issue 4327 "The text within inline markup may not
+            -- begin or end with whitespace"
+            -- http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html#inline-markup
             strong (space <> str "text" <> space <> space) =?> "**text**"
-          , "single" =:
-            strong (space) =?> "****"
+          , "single space stripped" =:
+            strong (space) =?> ""
           ]
         , testGroup "headings"
           [ "normal heading" =: