Load pp only when XPath debug tracing is enabled

[tas50]

Problem

lib/rexml/xpath_parser.rb requires pp at the top of the file. PP is used in exactly one place:

def trace(*args)
  indent = "  " * @nest
  PP.pp(args, "").each_line do |line|
    puts("#{indent}#{line}")
  end
end

trace is reached only through enter and leave, and every call site is guarded by if @debug (lines 225, 228, 246, 292, 373, 651, 690, 695, 769, 773, 781, 800). @debug comes from DEBUG, which is:

DEBUG = (ENV["REXML_XPATH_PARSER_DEBUG"] == "true")

So every REXML user loads pp and prettyprint for a debugging aid that is off by default.

Fix

Move the require into #trace, the only method that touches PP.

Measurements

Ruby 4.0.6 (arm64-darwin), best of seven runs:

	require "rexml/document"	files loaded
before	23.85 ms	38
after	21.72 ms	36
	−2.13 ms (9%)	−2

Small in absolute terms, but REXML sits underneath a good deal of the ecosystem, and the change carries no behavior risk.

Debug tracing is unaffected — running with REXML_XPATH_PARSER_DEBUG=true produces the same trace output:

[:enter,
 :expr,
 [:document, :descendant_or_self, :node, :child, :qname, "", "b"],
 [<UNDEFINED> ... </>]]
  [:while,

Tests

test/run.rb: 811 tests, 0 failures, unchanged (814 with the additions).

The three added tests cover that requiring rexml/document does not load pp, that XPath matching works without it, and that evaluating an XPath does not pull it in. Two of them fail against the previous code.

[kou]

Could you share what is your real world problem?