Rails 6 preserves status of #html_safe?

Before Rails 6

Before Rails 6, calling #html_safe? on a slice of an HTML safe string returns nil.

>> html_content = "<div>Hello, world!</div>".html_safe
# => "<div>Hello, world!</div>"
>> html_content.html_safe?
# => true
>> html_content[0..-1].html_safe?
# => nil

Also, before Rails 6, the ActiveSupport::SafeBuffer#* method does not preserve the HTML safe status as well.

>> line_break = "<br />".html_safe
# => "<br />"
>> line_break.html_safe?
# => true
>> two_line_breaks = (line_break * 2)
# => "<br /><br />"
>> two_line_breaks.html_safe?
# => nil

Rails 6 returns expected status of #html_safe?

In Rails 6, both of the above cases have been fixed properly.

Therefore, we will now get the status of #html_safe? as expected.

>> html_content = "<div>Hello, world!</div>".html_safe
# => "<div>Hello, world!</div>"
>> html_content.html_safe?
# => true
>> html_content[0..-1].html_safe?
# => true

>> line_break = "<br />".html_safe
# => "<br />"
>> line_break.html_safe?
# => true
>> two_line_breaks = (line_break * 2)
# => "<br /><br />"
>> two_line_breaks.html_safe?
# => true

Please check rails/rails#33808 and rails/rails#36012 for the relevant changes.