In most cases, when setting variant specific code, you're not sharing any code
within format.

Inline syntax can vastly simplify defining variants in those situations:

  respond_to do |format|
    format.js { render "trash" }
    format.html do |variant|
      variant.phone { redirect_to progress_path }
      variant.none  { render "trash" }
    end
  end

Becomes:

  respond_to do |format|
    format.js         { render "trash" }
    format.html.phone { redirect_to progress_path }
    format.html.none  { render "trash" }
  end

@responses hash needs to be initialized with mime types that we get from
Collector#collect_mimes_from_class_level. Mime::Type class as key and nil as
value. This need to happen before content negotiation. Before that, it was
looping though mime types and executing mime-type-generated method inside
collector (see
AbstractController::Collector#generate_method_for_mime). That approach resulted
in 2 unnecessary method calls for each mime type
collected by Collector#collect_mimes_from_class_level.

Now hash is initialized in place, without usage of Collector#custom method.

This reverts commit 18616114, reversing
changes made to cad9eb17.

In most cases, when setting variant specific code, you're not sharing any code
within format.

Inline syntax can vastly simplify defining variants in those sitiations:

  respond_to do |format|
    format.js { render "trash" }
    format.html do |variant|
      variant.phone { redirect_to progress_path }
      variant.none  { render "trash" }
    end
  end
`
Becomes:

  respond_to do |format|
    format.js         { render "trash" }
    format.html.phone { redirect_to progress_path }
    format.html.none  { render "trash" }
  end

@responses hash needs to be initialized with mime types that we get from
Collector#collect_mimes_from_class_level. Mime::Type class as key and nil as
value. This need to happen before content negotiation. Before that, it was
looping though mime types and executing mime-type-generated method inside
collector (see
AbstractController::Collector#generate_method_for_mime). That approach resulted
in 2 unnecessary method calls for each mime type
collected by Collector#collect_mimes_from_class_level.

Now hash is initialized in place, without usage of Collector#custom method.

Avoid one-liner conditionals when they are too big. Avoid concatenating
strings to build error messages. Improve messages a bit.

By default, variants in the templates will be picked up if a variant is set
and there's a match. The format will be:

  app/views/projects/show.html.erb
  app/views/projects/show.html+tablet.erb
  app/views/projects/show.html+phone.erb

If request.variant = :tablet is set, we'll automatically be rendering the
html+tablet template.

In the controller, we can also tailer to the variants with this syntax:

  class ProjectsController < ActionController::Base
    def show
      respond_to do |format|
        format.html do |html|
          @stars = @project.stars

          html.tablet { @notifications = @project.notifications }
          html.phone  { @chat_heads    = @project.chat_heads }
        end

        format.js
        format.atom
      end
    end
  end

The variant itself is nil by default, but can be set in before filters, like
so:

  class ApplicationController < ActionController::Base
    before_action do
      if request.user_agent =~ /iPad/
        request.variant = :tablet
      end
    end
  end

This is modeled loosely on custom mime types, but it's specifically not
intended to be used together. If you're going to make a custom mime type,
you don't need a variant. Variants are for variations on a single mime
types.

Revert "Used Yield instead of block.call" -- this causes all of atom_feed_helper_test.rb to fail with "SystemStackError: stack level too deep".

This reverts commit d3a1ce1c.

Ruby 1.8 legacy. Since 1.9 hash preserves insertion order. No need for additional array to achieve this

This fixes an issue where the respond_with worked directly with the given
options hash, so that if a user relied on it after calling respond_with,
the hash wouldn't be the same.

Fixes #12029

This reverts commit 90c887fa.

Selecting which key extensions to include in active_support/rails
made apparent the systematic usage of Object#in? in the code base.
After some discussion in

    https://github.com/rails/rails/commit/5ea6b0df9a36d033f21b52049426257a4637028d

we decided to remove it and use plain Ruby, which seems enough
for this particular idiom.

In this commit the refactor has been made case by case. Sometimes
include? is the natural alternative, others a simple || is the
way you actually spell the condition in your head, others a case
statement seems more appropriate. I have chosen the one I liked
the most in each case.

- #respond_to's documentation refer to .respond_to, but it was
  written as just <respond_to>. Added class name for clarification.
 

This reverts commit fa6d921e.

Reason: Not a fan of such massive changes. We usually close such changes
if made to Rails master as a pull request. Following the same principle
here and reverting.

[ci skip]

For future reference, this is the regex I used: ^\s*#\s*\n(?!\s*#). Replace
with the first match, and voilà! Note that the regex matches a little bit too
much, so you probably want to `git add -i .` and go through every single diff
to check if it actually should be changed.

Raise a rescuable exception when Rails doesn't know what to do with the format, rather than responding with a head :not_acceptable (406)