Execute field_error_proc within view

Instead of treating it as an anonymous block, execute the
`ActionView::Base.field_error_proc` within the context of the
`ActionView::Base` instance.

This enables consumer applications to continue to override the proc as
they see fit, but frees them from declaring templating logic within a
`config/initializers/*.rb`, `config/environments/*.rb` or
`config/application.rb` file.

This makes it possible to replace something like:

```ruby
config.action_view.field_error_proc = proc do |html_tag, instance|
  <<~HTML.html_safe
    #{html_tag}
    <span class="errors">#{instance.error_message.to_sentence}</span>
  HTML
end
```

With inline calls to Action View helpers like:

```ruby
config.action_view.field_error_proc = proc do |html_tag, instance|
  safe_join [ html_tag, tag.span(instance.error_message.to_sentence, class: "errors") ]
end
```

Or with a view partial rendering, like:

```ruby
config.action_view.field_error_proc = proc do |html_tag, instance|
  render partial: "application/field_with_errors", locals: { html_tag: html_tag, instance: instance }
end
```

Then, elsewhere in `app/views/application/field_with_errors.html.erb`:

```erb
<%= html_tag %>
<span class="errors"><%= instance.error_message.to_sentence %></span>
```

Author: seanpdoyle

actionview/CHANGELOG.md

@@ -1,3 +1,12 @@
+*   Execute the `ActionView::Base.field_error_proc` within the context of the
+    `ActionView::Base` instance:
+
+    ```ruby
+    config.action_view.field_error_proc = proc { |html| content_tag(:div, html, class: "field_with_errors") }
+    ```
+
+    *Sean Doyle*
+
 *   Add `:day_format` option to `date_select`
 
         date_select("article", "written_on", day_format: ->(day) { day.ordinalize })

actionview/lib/action_view/base.rb

@@ -142,7 +142,7 @@ class Base
     include Helpers, ::ERB::Util, Context
 
     # Specify the proc used to decorate input tags that refer to attributes with errors.
-    cattr_accessor :field_error_proc, default: Proc.new { |html_tag, instance| "<div class=\"field_with_errors\">#{html_tag}</div>".html_safe }
+    cattr_accessor :field_error_proc, default: Proc.new { |html_tag, instance| content_tag :div, html_tag, class: "field_with_errors" }
 
     # How to complete the streaming when an exception occurs.
     # This is our best guess: first try to close the attribute, then the tag.

actionview/lib/action_view/helpers/active_model_helper.rb

@@ -27,7 +27,7 @@ def tag(type, options, *)
 
       def error_wrapping(html_tag)
         if object_has_errors?
-          Base.field_error_proc.call(html_tag, self)
+          @template_object.instance_exec(html_tag, self, &Base.field_error_proc)
         else
           html_tag
         end

actionview/test/actionpack/controller/render_test.rb

@@ -33,6 +33,12 @@ def nested_partial_with_form_builder
   end
 end
 
+class ValidatingPost < Post
+  include ActiveModel::Validations
+
+  validates :title, presence: true
+end
+
 class TestController < ActionController::Base
   protect_from_forgery
 
@@ -487,6 +493,14 @@ def partial_with_form_builder
     render partial: ActionView::Helpers::FormBuilder.new(:post, nil, view_context, {})
   end
 
+  def partial_with_form_builder_and_invalid_model
+    post = ValidatingPost.new
+
+    post.validate
+
+    render partial: ActionView::Helpers::FormBuilder.new(:post, post, view_context, {})
+  end
+
   def partial_with_form_builder_subclass
     render partial: LabellingFormBuilder.new(:post, nil, view_context, {})
   end
@@ -680,6 +694,7 @@ class RenderTest < ActionController::TestCase
     get :partial_only, to: "test#partial_only"
     get :partial_with_counter, to: "test#partial_with_counter"
     get :partial_with_form_builder, to: "test#partial_with_form_builder"
+    get :partial_with_form_builder_and_invalid_model, to: "test#partial_with_form_builder_and_invalid_model"
     get :partial_with_form_builder_subclass, to: "test#partial_with_form_builder_subclass"
     get :partial_with_hash_object, to: "test#partial_with_hash_object"
     get :partial_with_locals, to: "test#partial_with_locals"
@@ -1300,6 +1315,44 @@ def test_partial_with_form_builder
     assert_equal "<label for=\"post_title\">Title</label>\n", @response.body
   end
 
+  def test_partial_with_form_builder_and_invalid_model
+    get :partial_with_form_builder_and_invalid_model
+
+    assert_equal <<~HTML.strip, @response.body.strip
+      <div class="field_with_errors"><label for="post_title">Title</label></div>
+    HTML
+  end
+
+  def test_partial_with_form_builder_and_invalid_model_custom_field_error_proc
+    old_proc = ActionView::Base.field_error_proc
+    ActionView::Base.field_error_proc = proc { |html| tag.div html, class: "errors" }
+
+    get :partial_with_form_builder_and_invalid_model
+
+    assert_equal <<~HTML.strip, @response.body.strip
+      <div class="errors"><label for="post_title">Title</label></div>
+    HTML
+  ensure
+    ActionView::Base.field_error_proc = old_proc if old_proc
+  end
+
+  def test_partial_with_form_builder_and_invalid_model_custom_rendering_field_error_proc
+    old_proc = ActionView::Base.field_error_proc
+    ActionView::Base.field_error_proc = proc do |html_tag, instance|
+      render inline: <<~ERB, locals: { html_tag: html_tag, instance: instance }
+        <div class="field_with_errors"><%= html_tag %> <span class="error"><%= [instance.error_message].join(', ') %></span></div>
+      ERB
+    end
+
+    get :partial_with_form_builder_and_invalid_model
+
+    assert_equal <<~HTML.strip, @response.body.strip
+      <div class="field_with_errors"><label for="post_title">Title</label> <span class="error">can&#39;t be blank</span></div>
+    HTML
+  ensure
+    ActionView::Base.field_error_proc = old_proc if old_proc
+  end
+
   def test_partial_with_form_builder_subclass
     get :partial_with_form_builder_subclass
     assert_equal "<label for=\"post_title\">Title</label>\n", @response.body

guides/source/configuring.md

@@ -1083,6 +1083,14 @@ Controls whether or not templates should be reloaded on each request. Defaults t
 
 #### `config.action_view.field_error_proc`
 
+* `config.action_view.field_error_proc` provides an HTML generator for
+  displaying errors that come from Active Model. The block is evaluated within
+  the context of an Action View template. The default is
+
+    ```ruby
+    Proc.new { |html_tag, instance| content_tag :div, html_tag, class: "field_with_errors" }
+    ```
+
 Provides an HTML generator for displaying errors that come from Active Model. The default is
 
 ```ruby