Work with ruby 2.3's --enable-frozen-string-literal - version 2

[larskanis]

Although bundler is fixed on master in respect to the frozen string literal, no version was released with this fixes, so far. So the travis-ci tests fail before the tests can run, currently.

However when rake test is started without bundler, all tests pass.

[flavorjones]

@larskanis Thanks for doing this work.

Are you sure that we want to test against 2.3.0 only with the frozen string option set?

I was thinking that perhaps we can refactor the travis matrix in a slightly different way: leave the existing matrix as it is, and then explicitly include two additional builds, which are linux/osx with the frozen string option set.

What do you think?

[flavorjones]

Also, it looks like the rubygems we're using doesn't support frozen strings yet? Let's hold off on merging until the full toolchain supports it.

[larskanis]

I usually prefer to name the tasks explicitly, that I distribute to someone else... However in this case I don't have a strong opinion. If you don't like it, I'll revert this change.

Are you sure that we want to test against 2.3.0 only with the frozen string option set?

I thought it wouldn't have a valuable benefit, to have two tests on 2.3.0. The frozen-string-literal is an additional restriction, that we should meet (someday). So why should we additionally test for not applying this restriction?

Apart from that, this PR is work in progress. I already fixed one issue in ruby and noticed some remaining issues in bundler. This seems to be enough for nokogiri, but in order to run the bundler tests, a lot of fixes are required on rspec.

[larskanis]

@flavorjones IMHO this is ready for merge now. Frozen strings are enabled for Ruby-2.4 only, because rubygems on Ruby-2.3 isn't compatible.

The test matrix for travis is re-arranged to explicit name the single test runs. At least for me this is much more readable than cross join the rvm and os entries and subtract the excludes from that matrix. In order to enable frozen strings per env variable this matrix would have to be extended by one dimension and all rubyies not supporting frozen strings would have to be added to the exclude list. So I think it's better to simply list all single test runs. Apart from that test runs are equally to the current on the master branch.

[flavorjones]

@larskanis I'm in the process of moving our CI to Concourse, where we'll have more control over the crazy flavors of tests that we want to run; so I'm not worried about the Travis bit.

I can walk you through what the Concourse pipelines look like; but when I merge this I'll likely set up a separate pipeline to test frozen strings.

(You can get a peek at the pipelines at https://ci.nokogiri.org/ and the /concourse/nokogiri.yml.erb file.)

[flavorjones]

Awesome! Merged, and I added the frozen string RUBYOPT options to the Concourse pipelines in 95684fd.

Thank you!