Ruby 3.4 frozen string literal deprecation warnings

[PhilMeyr]

Description

When using combine_pdf with Ruby 3.4, deprecation warnings are emitted about frozen string literals.

Warning message

lib/combine_pdf/parser.rb:730: warning: string returned by :__WKANCHOR_2.to_s will be frozen in the future

Environment

• Ruby version: 3.4.x
• combine_pdf version: 1.0.31

Root cause

In Ruby 3.4, Symbol#to_s returns a frozen string. The code at line 730 of lib/combine_pdf/parser.rb likely modifies the result of to_s called on a symbol, which triggers this deprecation warning.

Suggested fix

When modifying a string returned by Symbol#to_s, create a mutable copy first:

# Instead of:
sym.to_s.prepend('_')

# Use:
"_#{sym}"
# or
sym.to_s.dup.prepend('_')

This is similar to the fix applied in other gems like marcel (rails/marcel#140).

[PhilMeyr]

Context: This appears to be related to a known Ruby issue.

See Ruby issue #21280 - these warnings were identified as false positives by Matz. The code should work correctly when strings are frozen by default.

As of Ruby 3.4.7, the warnings are still present in Ruby itself. Just sharing this context for reference.