Add a --fail-level check CLI option

lib/erb/formatter/command_line.rb

@@ -62,6 +62,10 @@ def initialize(argv, stdin: $stdin)
         $DEBUG = value
       end
 
+      parser.on("--fail-level LEVEL", "'check' exits(1) on any formatting changes)") do |value|
+        @fail_level = value
+      end
+
       parser.on("-h", "--help", "Prints this help") do
         puts parser
         exit
@@ -94,6 +98,8 @@ def run
       css_class_sorter = self.class.tailwindcss_class_sorter(@tailwind_output_path)
     end
 
+    files_changed = false
+
     files.each do |(filename, code)|
       if ignore_list.should_ignore_file? filename
         print code unless write
@@ -106,12 +112,15 @@ def run
           css_class_sorter: css_class_sorter
         )
 
+        files_changed = true if html.to_s != code
+
         if write
           File.write(filename, html)
         else
           puts html
         end
       end
     end
+    exit(1) if files_changed && @fail_level == "check"
   end
 end

test/erb/test_formatter.rb

@@ -20,6 +20,27 @@ def test_fixtures
     end
   end
 
+  def test_error_on_unformattable_file
+    cli = ERB::Formatter::CommandLine.new(["test/fixtures/unmatched_error.erb"])
+    error = assert_raises RuntimeError do |error|
+      cli.run
+    end
+    assert_match(/Unmatched close tag/, error.message)
+  end
+
+  def test_fail_level_flag_check_with_changes
+    cli = ERB::Formatter::CommandLine.new(["--fail-level", "check", "test/fixtures/attributes.html.erb"])
+    error = assert_raises SystemExit do
+      cli.run
+    end
+    assert_equal(1, error.status)
+  end
+
+  def test_fail_level_flag_check_without_changes
+    cli = ERB::Formatter::CommandLine.new(["--fail-level", "check", "test/fixtures/attributes.html.expected.erb"])
+    cli.run
+  end
+
   def test_format_text_with_extra_long_text
     text = <<~HTML * 30
       Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore

test/fixtures/unmatched_error.erb

@@ -0,0 +1,6 @@
+<% if true %>
+  <h1>
+    <%= link_to 'New Order', new_order_path, class: "btn btn-success" %>
+    <% end %>
+  </h1>
+

test/test_helper.rb

@@ -2,5 +2,6 @@
 
 $LOAD_PATH.unshift File.expand_path("../lib", __dir__)
 require "erb/formatter"
+require "erb/formatter/command_line"
 
 require "minitest/autorun"