Reheat A Chilled String

In Ruby 3.4 and later, we no longer have to use the # frozen_string_literal: true magic comment at the beginning of all our files. That's because when that comment is missing, all string literals are treated as chilled. They aren't quite frozen, but now we will see warnings when code tries to mutate those chilled strings. This is an intermediate step towards strings being frozen by default.

Occasionally we want a string to be mutable so that the logic of our program can make changes to it. To avoid a warning or outright error, we need to reheat the string.

This can be done with the + unary operator on the String class.

Returns self if self is not frozen and can be mutated without warning issuance. Otherwise returns self.dup, which is not frozen.

Let's start an irb session (Ruby 4.0.9) with deprecation warnings on.

❯ irb -W:deprecated

⢀⡴⠊⢉⡟⢿  IRB v1.18.0 - Ruby 4.0.9
⣎⣀⣴⡋⡟⣻  "show_doc method" to view documentation
⣟⣼⣱⣽⣟⣾  ~/dev/ccsalespro/ccstorage

irb(main):001> str = ""
=> ""
irb(main):002> str.frozen?
=> false
irb(main):003> str << "are you chill?"
(irb):3: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
=> "are you chill?"
irb(main):004> str << "??"
=> "are you chill???"
irb(main):005> str2 = +""
=> ""
irb(main):006> str2 << "warmed up!"
=> "warmed up!"

In the above example, we see that a given string literal is not frozen, but it is chilled which means that when we first try to mutate it, we get a warning. Once it has been mutated, it is no longer chilled and we don't get subsequent warnings for that object.

I can avoid the warning all together (with str2) with the + unary operator.

Here is an error I kept seeing in my test suite related to this issue:

/Users/lastword/.local/share/mise/installs/ruby/4.0.9/lib/ruby/gems/4.0.0/gems/faraday-net_http-3.4.2/lib/faraday/adapter/net_http.rb:194:
warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)

This is because a test in the suite was declaring a string literal ("abc") that some under-the-hood Faraday code was mutating. The most straightforward fix was to reheat that string when it was declared (+"abc").