Use ruby file: ".ruby-version" for new apps

Previously, new apps would have a Ruby version set in both the Gemfile
and the .ruby-version file. This duplication makes it more difficult to
quickly change an application's ruby version as users must remember to
update multiple files.

This commit updates the app generator's Gemfile to read the Ruby version
from the .ruby-version file. Since this feature was introduced in the
latest version of Bundler, it will only be enabled if a supported version
of Bundler is used.

Alternatively, another solution mentioned on the original PR adding
.ruby-version was that the .ruby-version file could be removed once
rvm/rbenv support reading the Ruby version from the Gemfile. This has a
downside that many other tools like chruby do not have plans to support
reading a Ruby version from the Gemfile, and so users of those tools
would have a worse experience if the .ruby-version file is removed.

Co-authored-by: Takumi Shotoku <[email protected]>

Author: skipkayhil

railties/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Use `ruby file: ".ruby-version"` in generated Gemfile if supported.
+
+    *Hartley McGuire*
+
 *   `bin/rails test` will no longer load files named `*_test.rb` if they are located in the `fixtures` folder.
 
     *Edouard Chin*

railties/lib/rails/generators/app_base.rb

@@ -436,8 +436,20 @@ def to_s
         end
       end
 
+      def gem_ruby_entry
+        if Gem::Version.new(Bundler::VERSION) >= Gem::Version.new("2.4.20") # add file: option to #ruby
+          'ruby file: ".ruby-version"'
+        else
+          "ruby \"#{gem_ruby_version}\""
+        end
+      end
+
       def gem_ruby_version
-        Gem::Version.new(Gem::VERSION) >= Gem::Version.new("3.3.13") ? Gem.ruby_version : RUBY_VERSION
+        if Gem::Version.new(Gem::VERSION) >= Gem::Version.new("3.3.13") # patch level removed from Gem.ruby_version
+          Gem.ruby_version
+        else
+          RUBY_VERSION
+        end
       end
 
       def rails_prerelease?

railties/lib/rails/generators/rails/app/templates/Dockerfile.tt

@@ -42,7 +42,7 @@ RUN curl -fsSL https://bun.sh/install | bash -s -- "bun-v${BUN_VERSION}"
 
 <% end -%>
 # Install application gems
-COPY Gemfile Gemfile.lock ./
+COPY .ruby-version Gemfile Gemfile.lock ./
 RUN bundle install && \
     rm -rf ~/.bundle/ "${BUNDLE_PATH}"/ruby/*/cache "${BUNDLE_PATH}"/ruby/*/bundler/gems/*/.git<% if depend_on_bootsnap? -%> && \
     bundle exec bootsnap precompile --gemfile<% end %>

railties/lib/rails/generators/rails/app/templates/Gemfile.tt

@@ -1,6 +1,6 @@
 source "https://rubygems.org"
 
-ruby <%= "\"#{gem_ruby_version}\"" -%>
+<%= gem_ruby_entry %>
 
 <% gemfile_entries.each do |gemfile_entry| %>
 <%= gemfile_entry %>

railties/test/generators/app_generator_test.rb

@@ -1006,7 +1006,13 @@ def test_inclusion_of_ruby_version
     run_generator
 
     assert_file "Gemfile" do |content|
-      assert_match(/ruby "#{Gem::Version.new(Gem::VERSION) >= Gem::Version.new("3.3.13") ? Gem.ruby_version : RUBY_VERSION}"/, content)
+      if Gem::Version.new(Bundler::VERSION) >= Gem::Version.new("2.4.20") # add file: option to #ruby
+        assert_match('ruby file: ".ruby-version"', content)
+      elsif Gem::Version.new(Gem::VERSION) >= Gem::Version.new("3.3.13") # patch level removed from Gem.ruby_version
+        assert_match("ruby \"#{Gem.ruby_version}\"", content)
+      else
+        assert_match("ruby \"#{RUBY_VERSION}\"", content)
+      end
     end
     assert_file "Dockerfile" do |content|
       assert_match(/ARG RUBY_VERSION=#{Gem::Version.new(Gem::VERSION) >= Gem::Version.new("3.3.13") ? Gem.ruby_version : RUBY_VERSION}/, content)

railties/test/generators/generators_test_helper.rb

@@ -100,7 +100,7 @@ def evaluate_template_docker(file)
   private
     def gemfile_locals
       {
-        gem_ruby_version: RUBY_VERSION,
+        gem_ruby_entry: "ruby \"#{RUBY_VERSION}\"",
         rails_prerelease: false,
         skip_active_storage: true,
         depend_on_bootsnap: false,