From f0b6dcf4bd31109c3632f9ed611395f1b08ae222 Mon Sep 17 00:00:00 2001
From: "Ethan D. Twardy" <ethan.twardy@gmail.com>
Date: Fri, 28 Nov 2025 18:26:16 -0600
Subject: [PATCH] kramdown: Optionally fall back to rouge

Due to the distributed development of tree-sitter parsers, it's not
always feasible to have a particular tree-sitter parser installed. In
some cases, we may choose to fall back to rouge if a tree-sitter parser
is not installed. This option defaults to false, but can be set via
syntax_highlighter_opts.
---
 .../syntax_highlighter/tree_sitter.rb         | 32 ++++++++++++++-----
 1 file changed, 24 insertions(+), 8 deletions(-)

diff --git a/lib/kramdown/converter/syntax_highlighter/tree_sitter.rb b/lib/kramdown/converter/syntax_highlighter/tree_sitter.rb
index 07c3ad7..423001d 100644
--- a/lib/kramdown/converter/syntax_highlighter/tree_sitter.rb
+++ b/lib/kramdown/converter/syntax_highlighter/tree_sitter.rb
@@ -2,6 +2,7 @@
 
 require 'kramdown'
 require 'tree_sitter_adapter'
+require 'rouge'
 
 require_relative '../../syntax_tree_sitter/languages'
 
@@ -18,15 +19,26 @@ module TreeSitter
         def self.call(converter, raw_text, language_alias, type, _)
           return nil unless language_alias
 
-          language_scope = LANGUAGE_SCOPES.fetch(language_alias, language_alias)
-          rendered_text = TreeSitterAdapter.highlight(
-            raw_text,
-            get_parsers_dir(converter),
-            language_scope,
-            get_use_css_classes(converter)
-          )
+          begin
+            language_scope = LANGUAGE_SCOPES.fetch(language_alias, language_alias)
+            rendered_text = TreeSitterAdapter.highlight(
+              raw_text,
+              get_parsers_dir(converter),
+              language_scope,
+              get_use_css_classes(converter)
+            )
+          rescue => e
+            if get_fallback_to_rouge(converter)
+              STDERR.puts "Falling back to rouge for syntax highlighting: #{e}"
+              rendered_text = Rouge.highlight(raw_text, language_alias, 'html')
+              attributes="class=\"highlight\""
+            else
+              raise
+            end
+          end
+
           # Code blocks are additionally wrapped in HTML code tags
-          type == :block ? "<pre><code>#{rendered_text}</code></pre>" : rendered_text
+          type == :block ? "<pre><code #{attributes}>#{rendered_text}</code></pre>" : rendered_text
         end
 
         def self.get_parsers_dir(converter)
@@ -39,6 +51,10 @@ def self.get_use_css_classes(converter)
           get_option(converter, :css_classes) || false
         end
 
+        def self.get_fallback_to_rouge(converter)
+          get_option(converter, :fallback_to_rouge) || false
+        end
+
         def self.get_option(converter, name)
           converter.options[:syntax_highlighter_opts][name]
         end