Don't mutate the text node while pretty printing

[youdie006]

Pretty#write_text (lib/rexml/formatters/pretty.rb:88-91) calls the destructive gsub! and squeeze! on what Text#to_s hands back, and Text#to_s returns the node's own string — @string when @raw, otherwise the memoized @normalized:

def to_s
  return @string if @raw
  @normalized ||= Text::normalize( @string, doctype, @entity_filter )
end

So pretty printing permanently rewrites the document it is printing. Through Document#write, the documented pretty-print entry point:

d = REXML::Document.new("<a><b>hello   world</b></a>")
d.write(out, 2)
d.to_s
# before: "<a><b>hello world</b></a>"    <- the source had three spaces
# after:  "<a><b>hello   world</b></a>"

It is not only spaces. Measured the same way:

source	to_s after write(out, 2) on master
<a><b>hello world</b></a>	<a><b>hello world</b></a>
<a><b>x &amp; y</b></a>	<a><b>x &amp; y</b></a>
<a><b>line1\nline2</b></a>	<a><b>line1 line2</b></a>

With a @raw text node, Text#value itself changes.

The other four call sites

grep -n "node\.to_s" lib/rexml/formatters/*.rb gives five results. Four of them — transitive.rb:54, default.rb:94, default.rb:99, default.rb:105 — are output << node.to_s, treating the return value as read-only. pretty.rb was the only one writing to it.

The change is to use the non-destructive gsub and squeeze. The rendered output is byte-for-byte the same.

Testing

New test/formatter/test_pretty.rb, following the test/formatter/test_default.rb convention, with four tests: two asserting the source is not modified, and two asserting the whitespace handling that gsub/squeeze perform.

• The two non-mutation tests fail on master (<"hello world"> expected but was <"hello world">) and pass here.
• Mutation-checked in three directions, each failing a disjoint test: reverting to gsub!/squeeze! fails the two non-mutation tests; dropping .squeeze fails only test_consecutive_spaces_are_squeezed; dropping .gsub fails only test_whitespace_is_replaced_with_space.

Worth stating plainly: with test/formatter/test_pretty.rb removed, the pre-existing 820-test suite passes under all three of those variants — including the bug itself. There was no coverage of pretty-print whitespace handling at all, which is why the two output assertions are in the patch; without them a behaviour-preserving change here is unobservable.

• ruby test/run.rb (what rake test shells out to): 824 tests, 2634 assertions, 0 failures, 0 errors.
• RUBYOPT="--enable-frozen-string-literal" ruby test/run.rb, matching the second CI job: same result. This one matters here, since the patch stops writing into a string that may be frozen.

I did not run the rake warning:error rdoc job — rdoc and bundle are not available in my environment. The patch adds no method and no doc comment, so I do not expect it to affect that job, but I have not executed it. I also only ran Ruby 3.2.3 on Linux, not the full 2.6+/JRuby/macOS/Windows matrix.

Noticed but not touched

Namespace::NAMESPLIT uses ^ rather than \A and has no trailing anchor, so "a:b:c" splits to prefix a, name b; Default#write_element_attributes sorts by name while Pretty and Transitive do not. Neither is verified and neither is in this patch — mentioning them only in case they are of interest.

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AI assistance disclosure: this patch was found and written with Claude Code. Every value above is verbatim from running it against master and against this branch.

[Copilot]

🟢 Approval recommended

The change addresses a clear correctness issue and adds focused regression tests, with only a minor allocation/performance nit noted.

Pull request overview

This PR fixes REXML::Formatters::Pretty#write_text so that pretty-printing no longer mutates the underlying REXML::Text node content, and adds targeted tests to prevent regressions.

Changes:

• Switch Pretty#write_text from destructive gsub!/squeeze! to non-destructive whitespace normalization to avoid rewriting the document during formatting.
• Add a new test/formatter/test_pretty.rb suite that asserts both non-mutation and expected whitespace normalization output.

File summaries

File	Description
lib/rexml/formatters/pretty.rb	Stops pretty-printing from mutating Text#to_s-backed internal strings while preserving output.
test/formatter/test_pretty.rb	Adds coverage for non-mutation and Pretty formatter whitespace behavior.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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