Change to improved 0 test detected messaging

[Fix #123]

Changelog.md

@@ -12,6 +12,11 @@
 * [#1257](https://github.com/mbj/mutant/pull/1257)
   Fix crash on numblock mutations.
 
+* [#1258](https://github.com/mbj/mutant/pull/1258)
+  Add improved UI on detecting 0 tests.
+  This should be beneficial in onboarding scenarios or after manual
+  persistend rspec selections.
+
 # v0.10.33 2021-08-25
 
 * [#1249](https://github.com/mbj/mutant/pull/1249/files)

lib/mutant/cli/command/environment/run.rb

@@ -16,6 +16,20 @@ class Run < self
             See https://github.com/mbj/mutant#licensing
           MESSAGE
 
+          NO_TESTS_MESSAGE = <<~'MESSAGE'
+            ===============
+            Mutant found no tests. Mutation testing cannot be started.
+
+            This can have various reasons:
+
+            * You did not setup an integration, see:
+              https://github.com/mbj/mutant/blob/main/docs/configuration.md#integration
+            * You set environment variables like RSPEC_OPTS that filter out all tests.
+            * You set configuration optiosn like `config.filter_run :focus` which do
+              make rspec to not report any test.
+            ===============
+          MESSAGE
+
           # Test if command needs to be executed in zombie environment
           #
           # @return [Bool]
@@ -28,10 +42,19 @@ def zombie?
           def action
             soft_fail(License.call(world))
               .bind { bootstrap }
+              .bind(&method(:validate_tests))
               .bind(&Runner.public_method(:call))
               .bind(&method(:from_result))
           end
 
+          def validate_tests(environment)
+            if environment.integration.all_tests.length.zero?
+              Either::Left.new(NO_TESTS_MESSAGE)
+            else
+              Either::Right.new(environment)
+            end
+          end
+
           def from_result(result)
             if result.success?
               Either::Right.new(nil)

spec/unit/mutant/cli_spec.rb

@@ -748,28 +748,49 @@ def self.main_body
         end
 
         context 'on runner success with unsuccessful result' do
-          let(:expected_exit) { false             }
-          let(:runner_result) { right(env_result) }
+          context 'on alive mutations' do
+            let(:expected_exit) { false             }
+            let(:runner_result) { right(env_result) }
+
+            let(:env_result) do
+              instance_double(
+                Mutant::Result::Env,
+                success?: false
+              )
+            end
 
-          let(:env_result) do
-            instance_double(
-              Mutant::Result::Env,
-              success?: false
-            )
+            let(:expected_events) do
+              [
+                *super(),
+                [
+                  :stderr,
+                  :puts,
+                  'Uncovered mutations detected, exiting nonzero!'
+                ]
+              ]
+            end
+
+            include_examples 'CLI run'
           end
 
-          let(:expected_events) do
-            [
-              *super(),
+          context 'on not having found tests' do
+            let(:tests) { [] }
+
+            let(:expected_events) do
               [
-                :stderr,
-                :puts,
-                'Uncovered mutations detected, exiting nonzero!'
+                *super()[0..-2],
+                [
+                  :stderr,
+                  :puts,
+                  Mutant::CLI::Command::Environment::Run::NO_TESTS_MESSAGE
+                ]
               ]
-            ]
-          end
+            end
 
-          include_examples 'CLI run'
+            let(:expected_exit) { false }
+
+            include_examples 'CLI run'
+          end
         end
 
         context 'with valid subject expression' do