Add example verificatin to guard against emitting invalid AST

lib/mutant/meta/example/verification.rb

@@ -11,7 +11,7 @@ module Mutant
         #
         # @return [Boolean]
         def success?
-          [missing, unexpected, no_diffs].all?(&:empty?)
+          [missing, unexpected, no_diffs, invalid_syntax].all?(&:empty?)
         end
 
         # Error report
@@ -26,6 +26,7 @@ module Mutant
             'original_source' => example.source,
             'missing'         => format_mutations(missing),
             'unexpected'      => format_mutations(unexpected),
+            'invalid_syntax'  => format_mutations(invalid_syntax),
             'no_diff'         => no_diff_report
           )
         end
@@ -52,6 +53,17 @@ module Mutant
         end
         memoize :missing
 
+        # Mutations that generated invalid syntax
+        #
+        # @return [Enumerable<Mutation>]
+        def invalid_syntax
+          mutations.reject do |mutation|
+            ::Parser::CurrentRuby.parse(mutation.source)
+          rescue ::Parser::SyntaxError # rubocop:disable Lint/HandleExceptions
+          end
+        end
+        memoize :invalid_syntax
+
         # Mutations with no diff to original
         #
         # @return [Enumerable<Mutation>]

spec/unit/mutant/meta/example/verification_spec.rb

@@ -75,6 +75,7 @@ RSpec.describe Mutant::Meta::Example::Verification do
           - node: s(:nil)
             source: nil
           unexpected: []
+          invalid_syntax: []
           no_diff: []
         REPORT
       end
@@ -95,6 +96,7 @@ RSpec.describe Mutant::Meta::Example::Verification do
             source: 'false'
           - node: s(:nil)
             source: nil
+          invalid_syntax: []
           no_diff: []
         REPORT
       end
@@ -112,11 +114,41 @@ RSpec.describe Mutant::Meta::Example::Verification do
           original_source: 'true'
           missing: []
           unexpected: []
+          invalid_syntax: []
           no_diff:
           - node: s(:true)
             source: 'true'
         REPORT
       end
     end
+
+    context 'when the generated node is invalid syntax after unparsed' do
+      let(:invalid_node) do
+        s(:op_asgn, s(:send, s(:self), :at, s(:int, 1)), :+, s(:int, 1))
+      end
+
+      let(:expected_nodes)  { [invalid_node] }
+      let(:generated_nodes) { [invalid_node] }
+
+      specify do
+        should eql(<<~'REPORT')
+          ---
+          file: foo.rb
+          original_ast: s(:true)
+          original_source: 'true'
+          missing: []
+          unexpected: []
+          invalid_syntax:
+          - node: |-
+              s(:op_asgn,
+                s(:send,
+                  s(:self), :at,
+                  s(:int, 1)), :+,
+                s(:int, 1))
+            source: self.at(1) += 1
+          no_diff: []
+        REPORT
+      end
+    end
   end
 end