Raise on legacy formatter usage

Author: pirj

Changelog.md

@@ -29,6 +29,8 @@ Breaking Changes:
 * Remove deprecated `PendingExampleFixedNotification` and
   `PendingExampleFailedAsExpectedNotification` classes. (Phil Pirozhkov, #2864)
 * Remove deprecated `rerun_argument` example method. (Phil Pirozhkov, #2864)
+* Raise on attempt to use a legacy formatter without `rspec-legacy_formatters`.
+  (Phil Pirozhkov, #2864)
 
 Enhancements:
 

lib/rspec/core/formatters.rb

@@ -160,19 +160,15 @@ def add(formatter_to_use, *paths)
         formatter = RSpec::LegacyFormatters.load_formatter formatter_class, *args
         register formatter, formatter.notifications
       else
-        call_site = "Formatter added at: #{::RSpec::CallerFilter.first_non_rspec_line}"
-
-        RSpec.warn_deprecation <<-WARNING.gsub(/\s*\|/, ' ')
+        raise ArgumentError, <<-ERROR.gsub(/\s*\|/, ' ')
           |The #{formatter_class} formatter uses the deprecated formatter
-          |interface not supported directly by RSpec 3.
+          |interface not supported directly by RSpec 4.
           |
           |To continue to use this formatter you must install the
           |`rspec-legacy_formatters` gem, which provides support
           |for legacy formatters or upgrade the formatter to a
           |compatible version.
-          |
-          |#{call_site}
-        WARNING
+        ERROR
       end
     end
 

spec/rspec/core/formatters_spec.rb

@@ -83,9 +83,9 @@ module RSpec::Core::Formatters
         end
 
         it "issues a deprecation" do
-          expect_warn_deprecation(
-            /The #{formatter_class} formatter uses the deprecated formatter interface.+#{__FILE__}:#{__LINE__ + 1}/)
-          loader.add formatter_class, output
+          expect {
+            loader.add formatter_class, output
+          }.to raise_error(ArgumentError, /The #{formatter_class} formatter uses the deprecated formatter interface not supported directly by RSpec 4/)
         end
       end