I think the test failures are mostly coming from Modis and will address its compatibility with Rails 7.1 first in rpush/modis#52.

This is actually breakage in Rails itself: rails/rails#52820

In rpush.gemspec, the line is currently: s.add_runtime_dependency 'activesupport', '>= 6.0', '< 7.1.0'.
Shouldn't it be: s.add_runtime_dependency 'activesupport', '>= 6.0', '< 7.1'?
I have Rails 7.1.3.4, and I can't add it.

Could not find compatible versions

Because rails < 7.1.4 depends on activesupport = 7.1.3.4
  and rails >= 7.1.4, < 7.2.0 depends on activesupport = 7.1.4,
  rails < 7.2.0 requires activesupport = 7.1.3.4 OR = 7.1.4.
And because rails >= 7.2.0, < 7.2.1 depends on activesupport = 7.2.0
  and rails >= 7.2.1 depends on activesupport = 7.2.1,
  activesupport = 7.1.3.4 OR = 7.1.4 OR = 7.2.0 OR = 7.2.1 is required.
So, because rpush >= 9.0.0 depends on activesupport >= 6.0, < 7.1.0
  and Gemfile depends on rpush ~> 9.0,
  version solving has failed.

In rpush.gemspec, the line is currently: s.add_runtime_dependency 'activesupport', '>= 6.0', '< 7.1.0'. Shouldn't it be: s.add_runtime_dependency 'activesupport', '>= 6.0', '< 7.1'? I have Rails 7.1.3.4, and I can't add it.

Could not find compatible versions

Because rails < 7.1.4 depends on activesupport = 7.1.3.4
  and rails >= 7.1.4, < 7.2.0 depends on activesupport = 7.1.4,
  rails < 7.2.0 requires activesupport = 7.1.3.4 OR = 7.1.4.
And because rails >= 7.2.0, < 7.2.1 depends on activesupport = 7.2.0
  and rails >= 7.2.1 depends on activesupport = 7.2.1,
  activesupport = 7.1.3.4 OR = 7.1.4 OR = 7.2.0 OR = 7.2.1 is required.
So, because rpush >= 9.0.0 depends on activesupport >= 6.0, < 7.1.0
  and Gemfile depends on rpush ~> 9.0,
  version solving has failed.

You're time traveling.

https://github.com/rpush/rpush/pull/675/files#diff-42f0537e2dac213adc4bf69f60cd7645eda3c5ef15d7021474470069cb370305R39

This is pending #668

@benlangfeld

If you want the tests to fail, adding this before spec/spec_helper#20 does the trick:

if client == :active_record
  require 'active_record'
  if ActiveRecord::Base.respond_to?(:default_column_serializer)
    # New default in Rails 7.1: https://github.com/rails/rails/pull/47422
    ActiveRecord::Base.default_column_serializer = nil
  end
end

(not the cleanest code)

The error seems to be thrown when eager loading rpush:

[...]
Failure/Error: serialize :registration_ids

ArgumentError:
  missing keyword: :coder If no default coder is configured, a coder must be provided to `serialize`.
[...]