Use the pure-Ruby generator on TruffleRuby as it is much faster

Rakefile

@@ -238,7 +238,11 @@ if defined?(RUBY_ENGINE) and RUBY_ENGINE == 'jruby'
   task :release => :build
 else
   desc "Compiling extension"
-  task :compile => [ EXT_PARSER_DL, EXT_GENERATOR_DL ]
+  if RUBY_ENGINE == 'truffleruby'
+    task :compile => [ EXT_PARSER_DL ]
+  else
+    task :compile => [ EXT_PARSER_DL, EXT_GENERATOR_DL ]
+  end
 
   file EXT_PARSER_DL => EXT_PARSER_SRC do
     cd EXT_PARSER_DIR do

ext/json/ext/generator/extconf.rb

@@ -1,4 +1,9 @@
 require 'mkmf'
 
-$defs << "-DJSON_GENERATOR"
-create_makefile 'json/ext/generator'
+if RUBY_ENGINE == 'truffleruby'
+  # The pure-Ruby generator is faster on TruffleRuby, so skip compiling the generator extension
+  File.write('Makefile', dummy_makefile("").join)
+else
+  $defs << "-DJSON_GENERATOR"
+  create_makefile 'json/ext/generator'
+end

lib/json/ext.rb

@@ -4,11 +4,19 @@ module JSON
   # This module holds all the modules/classes that implement JSON's
   # functionality as C extensions.
   module Ext
-    require 'json/ext/parser'
-    require 'json/ext/generator'
-    $DEBUG and warn "Using Ext extension for JSON."
-    JSON.parser = Parser
-    JSON.generator = Generator
+    if RUBY_ENGINE == 'truffleruby'
+      require 'json/ext/parser'
+      require 'json/pure'
+      $DEBUG and warn "Using Ext extension for JSON parser and Pure library for JSON generator."
+      JSON.parser = Parser
+      JSON.generator = JSON::Pure::Generator
+    else
+      require 'json/ext/parser'
+      require 'json/ext/generator'
+      $DEBUG and warn "Using Ext extension for JSON."
+      JSON.parser = Parser
+      JSON.generator = Generator
+    end
   end
 
   JSON_LOADED = true unless defined?(::JSON::JSON_LOADED)