Raise for unsupported fail_fast value

Changelog.md

@@ -11,6 +11,8 @@ Breaking Changes:
 * Remove `run_all_when_everything_filtered` configuration option. (Phil Pirozhkov, #2845)
 * Raise on unsupported hook scope usage. (Phil Pirozhkov, #????)
 * Raise on usage of metadata on suite-level scopes. (Phil Pirozhkov, #????)
+* Raise an error when `fail_fast` is configured with
+  an unsupported value. (Phil Pirozhkov, #????)
 
 Enhancements:
 

lib/rspec/core/configuration.rb

@@ -203,8 +203,7 @@ def fail_fast=(value)
           @fail_fast = value.to_i
 
           if value.to_i == 0
-            # TODO: in RSpec 4, consider raising an error here.
-            RSpec.warning "Cannot set `RSpec.configuration.fail_fast`" \
+            raise ArgumentError, "Cannot set `RSpec.configuration.fail_fast`" \
               " to `#{value.inspect}`. Only `true`, `false`, `nil` and integers" \
               " are valid values."
             @fail_fast = true

spec/rspec/core/configuration_spec.rb

@@ -110,14 +110,10 @@ module RSpec::Core
           allow(RSpec).to receive(:warning)
         end
 
-        it 'prints warning' do
-          config.fail_fast = 'yes'
-          expect(RSpec).to have_received(:warning).with(/Cannot set `RSpec.configuration.fail_fast`/i)
-        end
-
-        it 'is set to true' do
-          config.fail_fast = 'yes'
-          expect(config.fail_fast).to eq true
+        it 'raises an error' do
+          expect {
+            config.fail_fast = 'yes'
+          }.to raise_error(ArgumentError, /Cannot set `RSpec.configuration.fail_fast`/i)
         end
       end
     end