Make sure that we don't perform in-place mutation on SafeBuffer string

This will make sure `render :inline` is working.

Closes #1633

2 files changed, 8 insertions, 0 deletions

diff --git a/actionpack/lib/action_view/template.rb b/actionpack/lib/action_view/template.rb
index 10797c010f..9c59d18310 100644
--- a/actionpack/lib/action_view/template.rb
+++ b/actionpack/lib/action_view/template.rb
@@ -218,6 +218,9 @@ module ActionView
         method_name = self.method_name
 
         if source.encoding_aware?
+          # Avoid performing in-place mutation for SafeBuffer
+          @source = source.to_str if source.html_safe?
+
           # Look for # encoding: *. If we find one, we'll encode the
           # String in that encoding, otherwise, we'll use the
           # default external encoding.
diff --git a/actionpack/test/template/template_test.rb b/actionpack/test/template/template_test.rb
index b0ca7de0b6..2beb168a9d 100644
--- a/actionpack/test/template/template_test.rb
+++ b/actionpack/test/template/template_test.rb
@@ -172,5 +172,10 @@ class TestERBTemplate < ActiveSupport::TestCase
     ensure
       silence_warnings { Encoding.default_external = old }
     end
+
+    def test_render_inline_safebuffer_should_not_raise_error
+      @template = new_template("Hello".html_safe)
+      render
+    end
   end
 end