RST reader: add aligned environment when needed in math.

rst2latex.py uses an align* environment for math in
`.. math::` blocks, so this math may contain line breaks.
If it does, we put the math in an `aligned` environment
to simulate rst2latex.py's behavior.

Closes #4254.

src/Text/Pandoc/Readers/RST.hs

@@ -972,11 +972,16 @@ extractCaption = do
   legend <- optional blanklines >> (mconcat <$> many block)
   return (capt,legend)
 
--- divide string by blanklines
+-- divide string by blanklines, and surround with
+-- \begin{aligned}...\end{aligned} if needed.
 toChunks :: String -> [String]
 toChunks = dropWhile null
-           . map (trim . unlines)
+           . map (addAligned . trim . unlines)
            . splitBy (all (`elem` (" \t" :: String))) . lines
+  -- we put this in an aligned environment if it contains \\, see #4254
+  where addAligned s = if "\\\\" `isInfixOf` s
+                          then "\\begin{aligned}\n" ++ s ++ "\n\\end{aligned}"
+                          else s
 
 codeblock :: [String] -> Maybe String -> String -> String -> RSTParser m Blocks
 codeblock classes numberLines lang body =

test/command/4254.md

@@ -0,0 +1,12 @@
+```
+% pandoc -f rst -t latex
+.. math::
+
+   x &= y\\
+   y &= z
+^D
+\[\begin{aligned}
+x &= y\\
+y &= z
+\end{aligned}\]
+```

test/rst-reader.native

@@ -317,7 +317,7 @@ Pandoc (Meta {unMeta = fromList [("author",MetaList [MetaInlines [Str "John",Spa
 ,Para [Math DisplayMath "E=mc^2"]
 ,Para [Math DisplayMath "E = mc^2"]
 ,Para [Math DisplayMath "E = mc^2",Math DisplayMath "\\alpha = \\beta"]
-,Para [Math DisplayMath "E &= mc^2\\\\\nF &= \\pi E",Math DisplayMath "F &= \\gamma \\alpha^2"]
+,Para [Math DisplayMath "\\begin{aligned}\nE &= mc^2\\\\\nF &= \\pi E\n\\end{aligned}",Math DisplayMath "F &= \\gamma \\alpha^2"]
 ,Para [Str "All",Space,Str "done."]
 ,Header 1 ("default-role",[],[]) [Str "Default-Role"]
 ,Para [Str "Try",Space,Str "changing",Space,Str "the",Space,Str "default",Space,Str "role",Space,Str "to",Space,Str "a",Space,Str "few",Space,Str "different",Space,Str "things."]