RST writer: fix backslash escaping after strings

The check whether a complex inline element following a string must be
escaped, now depends on the last character of the string instead of the
first.

Fixes: #5906

src/Text/Pandoc/Writers/RST.hs

@@ -462,7 +462,7 @@ transformInlines =  insertBS .
         okBeforeComplex Space = True
         okBeforeComplex SoftBreak = True
         okBeforeComplex LineBreak = True
-        okBeforeComplex (Str (T.uncons -> Just (c,_)))
+        okBeforeComplex (Str (T.unsnoc -> Just (_,c)))
                           = isSpace c || c `elemText` "-:/'\"<([{–—"
         okBeforeComplex _ = False
         isComplex :: Inline -> Bool
@@ -491,7 +491,7 @@ flatten outer
         combineAll = foldl combine []
 
         combine :: [Inline] -> Inline -> [Inline]
-        combine f i = 
+        combine f i =
           case (outer, i) of
           -- quotes are not rendered using RST inlines, so we can keep
           -- them and they will be readable and parsable

test/Tests/Writers/RST.hs

@@ -105,6 +105,9 @@ tests = [ testGroup "rubrics"
           , "keeps quotes" =:
             strong (str "f" <> doubleQuoted (str "d") <> str "l") =?>
             "**f“d”l**"
+          , "backslash inserted between str and code" =:
+            str "/api?query=" <> code "foo" =?>
+            "/api?query=\\ ``foo``"
           ]
         , testGroup "headings"
           [ "normal heading" =: