diff --git a/Gemfile b/Gemfile
index e810e0357e..2a42a4aea0 100644
--- a/Gemfile
+++ b/Gemfile
@@ -51,6 +51,9 @@ gem "dalli", ">= 2.2.1"
 gem "listen", ">= 3.0.5", "< 3.2", require: false
 gem "libxml-ruby", platforms: :ruby
 
+# Action View. For testing Erubis handler deprecation.
+gem "erubis", "~> 2.7.0", require: false
+
 # Active Job.
 group :job do
   gem "resque", github: "resque/resque", require: false
diff --git a/Gemfile.lock b/Gemfile.lock
index 67b5f8c240..0ad894a269 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -78,7 +78,7 @@ PATH
     actionview (5.1.0.alpha)
       activesupport (= 5.1.0.alpha)
       builder (~> 3.1)
-      erubis (~> 2.7.0)
+      erubi (~> 1.4)
       rails-dom-testing (~> 2.0)
       rails-html-sanitizer (~> 1.0, >= 1.0.2)
     activejob (5.1.0.alpha)
@@ -178,6 +178,7 @@ GEM
       http_parser.rb (>= 0.6.0)
     em-socksify (0.3.1)
       eventmachine (>= 1.0.0.beta.4)
+    erubi (1.4.0)
     erubis (2.7.0)
     event_emitter (0.2.5)
     eventmachine (1.2.1)
@@ -390,6 +391,7 @@ DEPENDENCIES
   delayed_job!
   delayed_job_active_record!
   em-hiredis
+  erubis (~> 2.7.0)
   hiredis
   jquery-rails
   json (>= 2.0.0)
@@ -432,4 +434,4 @@ DEPENDENCIES
   websocket-client-simple!
 
 BUNDLED WITH
-   1.13.7
+   1.14.3
diff --git a/actionpack/lib/abstract_controller/base.rb b/actionpack/lib/abstract_controller/base.rb
index 603c2e9ea7..e7cb6347a2 100644
--- a/actionpack/lib/abstract_controller/base.rb
+++ b/actionpack/lib/abstract_controller/base.rb
@@ -1,4 +1,3 @@
-require "erubis"
 require "abstract_controller/error"
 require "active_support/configurable"
 require "active_support/descendants_tracker"
@@ -22,7 +21,6 @@ module AbstractController
     include ActiveSupport::Configurable
     extend ActiveSupport::DescendantsTracker
 
-    undef_method :not_implemented
     class << self
       attr_reader :abstract
       alias_method :abstract?, :abstract
diff --git a/actionview/CHANGELOG.md b/actionview/CHANGELOG.md
index c12fb2e5ae..b071b260c9 100644
--- a/actionview/CHANGELOG.md
+++ b/actionview/CHANGELOG.md
@@ -1,3 +1,17 @@
+*   Change the ERB handler from Erubis to Erubi.
+
+    Erubi is an Erubis fork that's svelte, simple, and currently maintained.
+    Plus it supports `--enable-frozen-string-literal` in Ruby 2.3+.
+
+    Compatibility: Drops support for `<%===` tags for debug output.
+    These were an unused, undocumented side effect of the Erubis
+    implementation.
+
+    Deprecation: The Erubis handler will be removed in Rails 5.2, for the
+    handful of folks using it directly.
+
+    *Jeremy Evans*
+
 *   Allow render locals to be assigned to instance variables in a view.
 
     Fixes #27480.
diff --git a/actionview/actionview.gemspec b/actionview/actionview.gemspec
index 7bfdfbe29a..400bb156fa 100644
--- a/actionview/actionview.gemspec
+++ b/actionview/actionview.gemspec
@@ -22,7 +22,7 @@ Gem::Specification.new do |s|
   s.add_dependency "activesupport", version
 
   s.add_dependency "builder",       "~> 3.1"
-  s.add_dependency "erubis",        "~> 2.7.0"
+  s.add_dependency "erubi",         "~> 1.4"
   s.add_dependency "rails-html-sanitizer", "~> 1.0", ">= 1.0.2"
   s.add_dependency "rails-dom-testing", "~> 2.0"
 
diff --git a/actionview/lib/action_view/base.rb b/actionview/lib/action_view/base.rb
index b7c05fdb88..5387174467 100644
--- a/actionview/lib/action_view/base.rb
+++ b/actionview/lib/action_view/base.rb
@@ -11,7 +11,7 @@ module ActionView #:nodoc:
   # = Action View Base
   #
   # Action View templates can be written in several ways.
-  # If the template file has a <tt>.erb</tt> extension, then it uses the erubis[https://rubygems.org/gems/erubis]
+  # If the template file has a <tt>.erb</tt> extension, then it uses the erubi[https://rubygems.org/gems/erubi]
   # template system which can embed Ruby into an HTML document.
   # If the template file has a <tt>.builder</tt> extension, then Jim Weirich's Builder::XmlMarkup library is used.
   #
diff --git a/actionview/lib/action_view/template/handlers/erb.rb b/actionview/lib/action_view/template/handlers/erb.rb
index 5d047a6991..cee5408aa9 100644
--- a/actionview/lib/action_view/template/handlers/erb.rb
+++ b/actionview/lib/action_view/template/handlers/erb.rb
@@ -1,79 +1,12 @@
-require "erubis"
-
 module ActionView
   class Template
     module Handlers
-      class Erubis < ::Erubis::Eruby
-        def add_preamble(src)
-          @newline_pending = 0
-          src << "@output_buffer = output_buffer || ActionView::OutputBuffer.new;"
-        end
-
-        def add_text(src, text)
-          return if text.empty?
-
-          if text == "\n"
-            @newline_pending += 1
-          else
-            src << "@output_buffer.safe_append='"
-            src << "\n" * @newline_pending if @newline_pending > 0
-            src << escape_text(text)
-            src << "'.freeze;"
-
-            @newline_pending = 0
-          end
-        end
-
-        # Erubis toggles <%= and <%== behavior when escaping is enabled.
-        # We override to always treat <%== as escaped.
-        def add_expr(src, code, indicator)
-          case indicator
-          when "=="
-            add_expr_escaped(src, code)
-          else
-            super
-          end
-        end
-
-        BLOCK_EXPR = /\s*((\s+|\))do|\{)(\s*\|[^|]*\|)?\s*\Z/
-
-        def add_expr_literal(src, code)
-          flush_newline_if_pending(src)
-          if BLOCK_EXPR.match?(code)
-            src << "@output_buffer.append= " << code
-          else
-            src << "@output_buffer.append=(" << code << ");"
-          end
-        end
-
-        def add_expr_escaped(src, code)
-          flush_newline_if_pending(src)
-          if BLOCK_EXPR.match?(code)
-            src << "@output_buffer.safe_expr_append= " << code
-          else
-            src << "@output_buffer.safe_expr_append=(" << code << ");"
-          end
-        end
-
-        def add_stmt(src, code)
-          flush_newline_if_pending(src)
-          super
-        end
-
-        def add_postamble(src)
-          flush_newline_if_pending(src)
-          src << "@output_buffer.to_s"
-        end
-
-        def flush_newline_if_pending(src)
-          if @newline_pending > 0
-            src << "@output_buffer.safe_append='#{"\n" * @newline_pending}'.freeze;"
-            @newline_pending = 0
-          end
-        end
-      end
+      Erubis = ActiveSupport::Deprecation::DeprecatedConstantProxy.new("Erubis", "ActionView::Template::Handlers::ERB::Erubis", message: "ActionView::Template::Handlers::Erubis is deprecated and will be removed from Rails 5.2. Switch to ActionView::Template::Handlers::ERB::Erubi instead.")
 
       class ERB
+        autoload :Erubi, "action_view/template/handlers/erb/erubi"
+        autoload :Erubis, "action_view/template/handlers/erb/erubis"
+
         # Specify trim mode for the ERB compiler. Defaults to '-'.
         # See ERB documentation for suitable values.
         class_attribute :erb_trim_mode
@@ -81,7 +14,7 @@ module ActionView
 
         # Default implementation used.
         class_attribute :erb_implementation
-        self.erb_implementation = Erubis
+        self.erb_implementation = Erubi
 
         # Do not escape templates of these mime types.
         class_attribute :escape_whitelist
diff --git a/actionview/lib/action_view/template/handlers/erb/erubi.rb b/actionview/lib/action_view/template/handlers/erb/erubi.rb
new file mode 100644
index 0000000000..755cc84015
--- /dev/null
+++ b/actionview/lib/action_view/template/handlers/erb/erubi.rb
@@ -0,0 +1,81 @@
+require "erubi"
+
+module ActionView
+  class Template
+    module Handlers
+      class ERB
+        class Erubi < ::Erubi::Engine
+          # :nodoc: all
+          def initialize(input, properties = {})
+            @newline_pending = 0
+
+            # Dup properties so that we don't modify argument
+            properties = Hash[properties]
+            properties[:preamble]   = "@output_buffer = output_buffer || ActionView::OutputBuffer.new;"
+            properties[:postamble]  = "@output_buffer.to_s"
+            properties[:bufvar]     = "@output_buffer"
+            properties[:escapefunc] = ""
+
+            super
+          end
+
+          def evaluate(action_view_erb_handler_context)
+            pr = eval("proc { #{@src} }", binding, @filename || "(erubi)")
+            action_view_erb_handler_context.instance_eval(&pr)
+          end
+
+        private
+          def add_text(text)
+            return if text.empty?
+
+            if text == "\n"
+              @newline_pending += 1
+            else
+              src << "@output_buffer.safe_append='"
+              src << "\n" * @newline_pending if @newline_pending > 0
+              src << text.gsub(/['\\]/, '\\\\\&')
+              src << "'.freeze;"
+
+              @newline_pending = 0
+            end
+          end
+
+          BLOCK_EXPR = /\s*((\s+|\))do|\{)(\s*\|[^|]*\|)?\s*\Z/
+
+          def add_expression(indicator, code)
+            flush_newline_if_pending(src)
+
+            if (indicator == "==") || @escape
+              src << "@output_buffer.safe_expr_append="
+            else
+              src << "@output_buffer.append="
+            end
+
+            if BLOCK_EXPR.match?(code)
+              src << " " << code
+            else
+              src << "(" << code << ");"
+            end
+          end
+
+          def add_code(code)
+            flush_newline_if_pending(src)
+            super
+          end
+
+          def add_postamble(_)
+            flush_newline_if_pending(src)
+            super
+          end
+
+          def flush_newline_if_pending(src)
+            if @newline_pending > 0
+              src << "@output_buffer.safe_append='#{"\n" * @newline_pending}'.freeze;"
+              @newline_pending = 0
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/actionview/lib/action_view/template/handlers/erb/erubis.rb b/actionview/lib/action_view/template/handlers/erb/erubis.rb
new file mode 100644
index 0000000000..bf316ceec5
--- /dev/null
+++ b/actionview/lib/action_view/template/handlers/erb/erubis.rb
@@ -0,0 +1,80 @@
+require "erubis"
+
+module ActionView
+  class Template
+    module Handlers
+      class ERB
+        class Erubis < ::Erubis::Eruby
+          # :nodoc: all
+          def add_preamble(src)
+            @newline_pending = 0
+            src << "@output_buffer = output_buffer || ActionView::OutputBuffer.new;"
+          end
+
+          def add_text(src, text)
+            return if text.empty?
+
+            if text == "\n"
+              @newline_pending += 1
+            else
+              src << "@output_buffer.safe_append='"
+              src << "\n" * @newline_pending if @newline_pending > 0
+              src << escape_text(text)
+              src << "'.freeze;"
+
+              @newline_pending = 0
+            end
+          end
+
+          # Erubis toggles <%= and <%== behavior when escaping is enabled.
+          # We override to always treat <%== as escaped.
+          def add_expr(src, code, indicator)
+            case indicator
+            when "=="
+              add_expr_escaped(src, code)
+            else
+              super
+            end
+          end
+
+          BLOCK_EXPR = /\s*((\s+|\))do|\{)(\s*\|[^|]*\|)?\s*\Z/
+
+          def add_expr_literal(src, code)
+            flush_newline_if_pending(src)
+            if BLOCK_EXPR.match?(code)
+              src << "@output_buffer.append= " << code
+            else
+              src << "@output_buffer.append=(" << code << ");"
+            end
+          end
+
+          def add_expr_escaped(src, code)
+            flush_newline_if_pending(src)
+            if BLOCK_EXPR.match?(code)
+              src << "@output_buffer.safe_expr_append= " << code
+            else
+              src << "@output_buffer.safe_expr_append=(" << code << ");"
+            end
+          end
+
+          def add_stmt(src, code)
+            flush_newline_if_pending(src)
+            super
+          end
+
+          def add_postamble(src)
+            flush_newline_if_pending(src)
+            src << "@output_buffer.to_s"
+          end
+
+          def flush_newline_if_pending(src)
+            if @newline_pending > 0
+              src << "@output_buffer.safe_append='#{"\n" * @newline_pending}'.freeze;"
+              @newline_pending = 0
+            end
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/actionview/test/template/erb/deprecated_erubis_implementation_test.rb b/actionview/test/template/erb/deprecated_erubis_implementation_test.rb
new file mode 100644
index 0000000000..4a130e6c33
--- /dev/null
+++ b/actionview/test/template/erb/deprecated_erubis_implementation_test.rb
@@ -0,0 +1,11 @@
+require "abstract_unit"
+
+module ERBTest
+  class DeprecatedErubisImplementationTest < ActionView::TestCase
+    test "Erubis implementation is deprecated" do
+      assert_deprecated "ActionView::Template::Handlers::Erubis is deprecated and will be removed from Rails 5.2. Switch to ActionView::Template::Handlers::ERB::Erubi instead." do
+        assert_equal "ActionView::Template::Handlers::ERB::Erubis", ActionView::Template::Handlers::Erubis.to_s
+      end
+    end
+  end
+end
diff --git a/actionview/test/template/erb/helper.rb b/actionview/test/template/erb/helper.rb
index a1973068d5..bc1eedc15f 100644
--- a/actionview/test/template/erb/helper.rb
+++ b/actionview/test/template/erb/helper.rb
@@ -14,7 +14,7 @@ module ERBTest
   class BlockTestCase < ActiveSupport::TestCase
     def render_content(start, inside)
       template = block_helper(start, inside)
-      ActionView::Template::Handlers::Erubis.new(template).evaluate(ViewContext.new)
+      ActionView::Template::Handlers::ERB.erb_implementation.new(template).evaluate(ViewContext.new)
     end
 
     def block_helper(str, rest)
diff --git a/activesupport/lib/active_support/deprecation/proxy_wrappers.rb b/activesupport/lib/active_support/deprecation/proxy_wrappers.rb
index 1c6618b19a..ce39e9a232 100644
--- a/activesupport/lib/active_support/deprecation/proxy_wrappers.rb
+++ b/activesupport/lib/active_support/deprecation/proxy_wrappers.rb
@@ -122,10 +122,11 @@ module ActiveSupport
     #        (Backtrace information…)
     #        ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
     class DeprecatedConstantProxy < DeprecationProxy
-      def initialize(old_const, new_const, deprecator = ActiveSupport::Deprecation.instance)
+      def initialize(old_const, new_const, deprecator = ActiveSupport::Deprecation.instance, message: "#{old_const} is deprecated! Use #{new_const} instead.")
         @old_const = old_const
         @new_const = new_const
         @deprecator = deprecator
+        @message = message
       end
 
       # Returns the class of the new constant.
@@ -143,7 +144,7 @@ module ActiveSupport
         end
 
         def warn(callstack, called, args)
-          @deprecator.warn("#{@old_const} is deprecated! Use #{@new_const} instead.", callstack)
+          @deprecator.warn(@message, callstack)
         end
     end
   end
diff --git a/activesupport/test/deprecation_test.rb b/activesupport/test/deprecation_test.rb
index 5be93f3a1a..5f72fbf662 100644
--- a/activesupport/test/deprecation_test.rb
+++ b/activesupport/test/deprecation_test.rb
@@ -285,6 +285,16 @@ class DeprecationTest < ActiveSupport::TestCase
     end
   end
 
+  def test_deprecated_constant_with_custom_message
+    deprecator = deprecator_with_messages
+
+    klass = Class.new
+    klass.const_set(:OLD, ActiveSupport::Deprecation::DeprecatedConstantProxy.new("klass::OLD", "Object", deprecator, message: "foo"))
+
+    klass::OLD.to_s
+    assert_match "foo", deprecator.messages.last
+  end
+
   def test_deprecated_instance_variable_with_instance_deprecator
     deprecator = deprecator_with_messages