Dup formatters to prevent mutation

Changelog.md

@@ -64,6 +64,7 @@ Breaking Changes for 3.0.0:
 * Remove `backtrace_cleaner` as an alias of `backtrace_formatter`. (Jon Rowe)
 * Remove `filename_pattern` as an alias of `pattern`. (Jon Rowe)
 * Extract support for legacy formatters to `rspec-legacy_formatters`. (Jon Rowe)
+* `RSpec::Configuration#formatters` now returns a dup to prevent mutation. (Jon Rowe)
 
 Enhancements:
 

lib/rspec/core/configuration.rb

@@ -641,9 +641,14 @@ def default_formatter=(value)
         formatter_loader.default_formatter = value
       end
 
-      # @private
+      # Returns a duplicate of the formatters currently loaded in
+      # the `FormatterLoader` for introspection.
+      #
+      # Note as this is a duplicate, any mutations will be disregarded.
+      #
+      # @return [Array] the formatters currently loaded
       def formatters
-        formatter_loader.formatters
+        formatter_loader.formatters.dup
       end
 
       # @private

spec/rspec/core/configuration_spec.rb

@@ -853,6 +853,14 @@ def metadata_hash(*args)
       end
     end
 
+    describe "#formatters" do
+      it "returns a dup of the formatter_loader formatters" do
+        config.add_formatter 'doc'
+        config.formatters.clear
+        expect(config.formatters).to_not eq []
+      end
+    end
+
     describe "#default_formatter" do
       it 'defaults to `progress`' do
         expect(config.default_formatter).to eq('progress')