Markdown reader: fenced code block shortcuts with attributes (#8174)

This allows the combination of the fenced code block shortcut form with
attributes:

````
```haskell {.class #id}
```
````

The code syntax class will be combined with the attribute classes.

This syntax allows for more intuitive writing and for better compatibility
with other Markdown parsers such as GitHub or Codeberg.

Closes #8174.

1 files changed, 7 insertions, 3 deletions

diff --git a/src/Text/Pandoc/Readers/Markdown.hs b/src/Text/Pandoc/Readers/Markdown.hs
index 1e12a2314..3242122db 100644
--- a/src/Text/Pandoc/Readers/Markdown.hs
+++ b/src/Text/Pandoc/Readers/Markdown.hs
@@ -688,9 +688,13 @@ codeBlockFenced = try $ do
   rawattr <-
      (Left <$> (guardEnabled Ext_raw_attribute >> try rawAttribute))
     <|>
-     (Right <$> option ("",[],[])
-         ((guardEnabled Ext_fenced_code_attributes >> try attributes)
-          <|> ((\x -> ("",[toLanguageId x],[])) <$> many1Char nonspaceChar)))
+     (Right <$> (do
+           languageId <- option Nothing (Just . toLanguageId <$> try (many1Char $ satisfy (\x -> x `notElem` ['`', '{', '}'] && not (isSpace x))))
+           skipMany spaceChar
+           maybeAttr <- option Nothing (Just <$> (guardEnabled Ext_fenced_code_attributes >> try attributes))
+           return $ case maybeAttr of
+              Nothing -> ("", maybeToList languageId, [])
+              Just (elementId, classes, attrs) -> (elementId, maybe classes (: classes) languageId, attrs)))
   blankline
   contents <- T.intercalate "\n" <$>
                  manyTill (gobbleAtMostSpaces indentLevel >> anyLine)