avoid modify of frozen string in validatable.rb

[mameier]

frozen string was modified in assert_validations_api. Use multiline string instead.

The error is hard to reproduce but trivial:
As frozen_string_literal is set, the concatenation of the error message in line 50 is modifying a frozen string.
It's easy to use a multiline string instead.

[p8]

Hey @mameier, rhis does seem like a valid fix to me.
I think commit 2e521bf is unrelated and should be dropped from this PR.

Also I think we need to assert the error message here, to make sure it's a proper fix:
https://github.com/heartcombo/devise/blob/main/test/models/validatable_test.rb#L113-L115

[mameier]

Hey @p8 , I admit that commit 2e521bf is a different issue. I did'n mean to include it in this pull request.
I moved the commit to a new branch but don'n know how to remove it from this pull request on github.

[carlosantoniodasilva]

The behavior has changed slightly in Ruby 3 so that interpolated strings are no longer frozen:

% cat x.rb
# frozen_string_literal: true

blah = "BLAH"

raise "Could not use :validatable module since #{blah} does not respond " <<
      "to the following methods: LOL"

# on Ruby 2.7.7
% be ruby x.rb
Traceback (most recent call last):
x.rb:5:in `<main>': can't modify frozen String: "Could not use :validatable module since BLAH does not respond " (FrozenError)

# on Ruby 3.2.0
% be ruby x.rb
x.rb:5:in `<main>': Could not use :validatable module since BLAH does not respond to the following methods: LOL (RuntimeError)

See https://bugs.ruby-lang.org/issues/17104 and https://scriptday.com/blog/2020/09/16/ruby-3-0-interpolated-strings-are-no-longer-frozen for some references.

Funny enough, our tests were passing because FrozenError actually inherits from RuntimeError:

>> FrozenError.ancestors
=> [FrozenError, RuntimeError, StandardError, Exception, Object ...]

But if I change the test to check for the message, it passes on 3.2 but fails on Ruby 2.7 as expected:

Failure:
ValidatableTest#test_should_not_be_included_in_objects_with_invalid_API [test/models/validatable_test.rb:118]:
Expected /Could not use :validatable module since .* does not respond to the following methods: validates_presence_of/ to match # encoding: ASCII-8BIT
#    valid: true
"can't modify frozen String: \"Could not use :validatable module since #<Class:0x00000001315d98c8> does not respond \"".

In order to keep it compatible to the Ruby versions we currently support, I have cherry-picked the commit and expanded the tests in #5563. Thanks @mameier.