Lock down new `ImplicitRender` behavior for 5.0 RC

1. Conceptually revert #20276

   The feature was implemented for the `responders` gem. In the end,
   they did not need that feature, and have found a better fix (see
   plataformatec/responders#131).

   `ImplicitRender` is the place where Rails specifies our default
   policies for the case where the user did not explicitly tell us
   what to render, essentially describing a set of heuristics. If
   the gem (or the user) knows exactly what they want, they could
   just perform the correct `render` to avoid falling through to
   here, as `responders` did (the user called `respond_with`).

   Reverting the patch allows us to avoid exploding the complexity
   and defining “the fallback for a fallback” policies.

2. `respond_to` and templates are considered exhaustive enumerations

   If the user specified a list of formats/variants in a `respond_to`
   block, anything that is not explicitly included should result
   in an `UnknownFormat` error (which is then caught upstream to
   mean “406 Not Acceptable” by default). This is already how it
   works before this commit.

   Same goes for templates – if the user defined a set of templates
   (usually in the file system), that set is now considered exhaustive,
   which means that “missing” templates are considered `UnknownFormat`
   errors (406).

3. To keep API endpoints simple, the implicit render behavior for
   actions with no templates defined at all (regardless of formats,
   locales, variants, etc) are defaulted to “204 No Content”. This
   is a strictly narrower version of the feature landed in #19036 and
   #19377.

4. To avoid confusion when interacting in the browser, these actions
   will raise an `UnknownFormat` error for “interactive” requests
   instead. (The precise definition of “interactive” requests might
   change – the spirit here is to give helpful messages and avoid
   confusions.)

Closes #20666, #23062, #23077, #23564

[Godfrey Chan, Jon Moss, Kasper Timm Hansen, Mike Clark, Matthew Draper]

4 files changed, 39 insertions, 4 deletions

diff --git a/actionview/lib/action_view/lookup_context.rb b/actionview/lib/action_view/lookup_context.rb
index 4163e69a72..626c4b8f5e 100644
--- a/actionview/lib/action_view/lookup_context.rb
+++ b/actionview/lib/action_view/lookup_context.rb
@@ -136,6 +136,11 @@ module ActionView
       end
       alias :template_exists? :exists?
 
+      def any?(name, prefixes = [], partial = false)
+        @view_paths.exists?(*args_for_any(name, prefixes, partial))
+      end
+      alias :any_templates? :any?
+
       # Adds fallbacks to the view paths. Useful in cases when you are rendering
       # a :file.
       def with_fallbacks
@@ -172,6 +177,32 @@ module ActionView
         [user_details, details_key]
       end
 
+      def args_for_any(name, prefixes, partial) # :nodoc:
+        name, prefixes = normalize_name(name, prefixes)
+        details, details_key = detail_args_for_any
+        [name, prefixes, partial || false, details, details_key]
+      end
+
+      def detail_args_for_any # :nodoc:
+        @detail_args_for_any ||= begin
+          details = {}
+
+          registered_details.each do |k|
+            if k == :variants
+              details[k] = :any
+            else
+              details[k] = Accessors::DEFAULT_PROCS[k].call
+            end
+          end
+
+          if @cache
+            [details, DetailsKey.get(details)]
+          else
+            [details, nil]
+          end
+        end
+      end
+
       # Support legacy foo.erb names even though we now ignore .erb
       # as well as incorrectly putting part of the path in the template
       # name instead of the prefix.
diff --git a/actionview/lib/action_view/renderer/abstract_renderer.rb b/actionview/lib/action_view/renderer/abstract_renderer.rb
index 23e672a95f..1dddf53df0 100644
--- a/actionview/lib/action_view/renderer/abstract_renderer.rb
+++ b/actionview/lib/action_view/renderer/abstract_renderer.rb
@@ -15,7 +15,7 @@ module ActionView
   # that new object is called in turn. This abstracts the setup and rendering
   # into a separate classes for partials and templates.
   class AbstractRenderer #:nodoc:
-    delegate :find_template, :find_file, :template_exists?, :with_fallbacks, :with_layout_format, :formats, :to => :@lookup_context
+    delegate :find_template, :find_file, :template_exists?, :any_templates?, :with_fallbacks, :with_layout_format, :formats, :to => :@lookup_context
 
     def initialize(lookup_context)
       @lookup_context = lookup_context
diff --git a/actionview/lib/action_view/template/resolver.rb b/actionview/lib/action_view/template/resolver.rb
index 8a675cd521..b6de0b03bf 100644
--- a/actionview/lib/action_view/template/resolver.rb
+++ b/actionview/lib/action_view/template/resolver.rb
@@ -222,7 +222,7 @@ module ActionView
     end
 
     def find_template_paths(query)
-      Dir[query].reject do |filename|
+      Dir[query].uniq.reject do |filename|
         File.directory?(filename) ||
           # deals with case-insensitive file systems.
           !File.fnmatch(query, filename, File::FNM_EXTGLOB)
@@ -340,7 +340,11 @@ module ActionView
       query = escape_entry(File.join(@path, path))
 
       exts = EXTENSIONS.map do |ext, prefix|
-        "{#{details[ext].compact.uniq.map { |e| "#{prefix}#{e}," }.join}}"
+        if ext == :variants && details[ext] == :any
+          "{#{prefix}*,}"
+        else
+          "{#{details[ext].compact.uniq.map { |e| "#{prefix}#{e}," }.join}}"
+        end
       end.join
 
       query + exts
diff --git a/actionview/lib/action_view/view_paths.rb b/actionview/lib/action_view/view_paths.rb
index 37722013ce..b46fe06b01 100644
--- a/actionview/lib/action_view/view_paths.rb
+++ b/actionview/lib/action_view/view_paths.rb
@@ -10,7 +10,7 @@ module ActionView
       self._view_paths.freeze
     end
 
-    delegate :template_exists?, :view_paths, :formats, :formats=,
+    delegate :template_exists?, :any_templates?, :view_paths, :formats, :formats=,
              :locale, :locale=, :to => :lookup_context
 
     module ClassMethods