Load pp only when XPath debug tracing is enabled

lib/rexml/xpath_parser.rb

@@ -1,6 +1,5 @@
 # frozen_string_literal: false
 
-require "pp"
 require "set"
 
 require_relative 'namespace'
@@ -801,6 +800,11 @@ def evaluate_predicate(expression, nodesets)
     end
 
     def trace(*args)
+      # Loaded here rather than at the top of the file because this method is
+      # only reached when REXML_XPATH_PARSER_DEBUG is set. Requiring pp
+      # eagerly made every REXML user pay for a debugging aid.
+      require "pp"
+
       indent = "  " * @nest
       PP.pp(args, "").each_line do |line|
         puts("#{indent}#{line}")

test/test_require.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: false
+
+require "test/unit"
+
+module REXMLTests
+  # Guards the deferred `require "pp"` in REXML::XPathParser#trace. The checks
+  # run in a subprocess because this one has pp loaded already.
+  class TestRequire < Test::Unit::TestCase
+    PP_LOADED = '$LOADED_FEATURES.any? { |f| File.basename(f) == "pp.rb" }'
+
+    def subprocess(script)
+      lib = File.join(File.dirname(File.expand_path(__dir__)), "lib")
+      IO.popen([RbConfig.ruby, "-I", lib, "-e", script], &:read)
+    end
+
+    def test_requiring_document_does_not_load_pp
+      assert_equal("false", subprocess("require 'rexml/document'; print #{PP_LOADED}"))
+    end
+
+    def test_xpath_works_without_pp
+      script = "require 'rexml/document'; " \
+               "d = REXML::Document.new('<r><a>x</a><a>y</a></r>'); " \
+               "print REXML::XPath.match(d, '//a').map(&:text).join(',')"
+      assert_equal("x,y", subprocess(script))
+    end
+
+    def test_xpath_does_not_load_pp
+      script = "require 'rexml/document'; " \
+               "d = REXML::Document.new('<r><a>x</a></r>'); " \
+               "REXML::XPath.match(d, '//a'); " \
+               "print #{PP_LOADED}"
+      assert_equal("false", subprocess(script))
+    end
+  end
+end