Add /\Astatic/ -> #start_with? mutations

- Adds the following mutations:
  * `a.match(/\Atext/)` -> `b.start_with?('text')`
  * `a.match?(/\Atext/)` -> `b.start_with?('text')`
  * `a =~ /\Atext/` -> `b.start_with?('text')`
- NOTE: Adds a `Mutant::AST::Regexp.expand_regexp_ast` to avoid repeating AST expansion logic in the `send` mutator. At that level, the node appears as a simple `parser` `s(:regexp ...)` node so I chose to fully parse the regexp rather than try to do a string pattern.
- Closes #169

Changelog.md

@@ -1,3 +1,12 @@
+# Unreleased
+
+  * [#1201](https://github.com/mbj/mutant/pull/1201)
+
+    * Add `/\Astatic/` -> `#start_with?` mutations:
+      * `a.match(/\Atext/)` -> `b.start_with?('text')`
+      * `a.match?(/\Atext/)` -> `b.start_with?('text')`
+      * `a =~ /\Atext/` -> `b.start_with?('text')`
+
 # v0.10.25 2021-01-02-03
 
 * [#1194](https://github.com/mbj/mutant/pull/1194)

lib/mutant/ast/regexp.rb

@@ -32,6 +32,23 @@ def self.to_ast(expression)
       def self.to_expression(node)
         Transformer.lookup(node.type).to_expression(node)
       end
+
+      # Convert's a `parser` `regexp` node into more fine-grained AST nodes.
+      #
+      # @param node [Parser::AST::Node]
+      #
+      # @return [Parser::AST::Node]
+      def self.expand_regexp_ast(node)
+        *body, _opts = node.children
+
+        # NOTE: We only mutate parts of regexp body if the body is composed of
+        # only strings. Regular expressions with interpolation are skipped
+        return unless body.all? { |node| node.type.equal?(:str) }
+
+        body_expression = parse(body.map(&:children).join)
+
+        to_ast(body_expression)
+      end
     end # Regexp
   end # AST
 end # Mutant

lib/mutant/mutator/node/literal/regex.rb

@@ -28,31 +28,18 @@ def dispatch
             emit_type(s(:str, NULL_REGEXP_SOURCE), options)
           end
 
-          # NOTE: will only mutate parts of regexp body if the
-          # body is composed of only strings. Regular expressions
-          # with interpolation are skipped
           def mutate_body
-            return unless body.all?(&method(:n_str?))
+            # NOTE: will only mutate parts of regexp body if the body is composed of only strings.
+            # Regular expressions with interpolation are skipped.
+            # require 'pry'; binding.pry
+            return unless (body_ast = AST::Regexp.expand_regexp_ast(input))
 
             Mutator.mutate(body_ast).each do |mutation|
               source = AST::Regexp.to_expression(mutation).to_s
               emit_type(s(:str, source), options)
             end
           end
 
-          def body_ast
-            AST::Regexp.to_ast(body_expression)
-          end
-
-          def body_expression
-            AST::Regexp.parse(body.map(&:children).join)
-          end
-          memoize :body_expression
-
-          def body
-            children.slice(0...-1)
-          end
-
         end # Regex
       end # Literal
     end # Node

lib/mutant/mutator/node/send.rb

@@ -32,8 +32,8 @@ class Send < self
           is_a?:         %i[instance_of?],
           kind_of?:      %i[instance_of?],
           map:           %i[each],
-          method:        %i[public_method],
           match:         %i[match?],
+          method:        %i[public_method],
           reverse_each:  %i[each],
           reverse_map:   %i[map each],
           reverse_merge: %i[merge],
@@ -51,6 +51,8 @@ class Send < self
           }
         )
 
+        REGEXP_MATCH_METHODS = %i[=~ match match?]
+
       private
 
         def dispatch
@@ -84,6 +86,7 @@ def normal_dispatch
         end
 
         def emit_selector_specific_mutations
+          emit_start_with_mutation
           emit_predicate_mutations
           emit_array_mutation
           emit_static_send
@@ -94,6 +97,23 @@ def emit_selector_specific_mutations
           emit_lambda_mutation
         end
 
+        def emit_start_with_mutation
+          return unless REGEXP_MATCH_METHODS.include?(selector)
+          return unless arguments.one?
+
+          argument = Mutant::Util.one(arguments)
+
+          return unless argument.type.equal?(:regexp) && (
+            regexp_ast = AST::Regexp.expand_regexp_ast(argument)
+          )
+
+          return unless regexp_ast.children.map(&:type).eql?(%i[regexp_bos_anchor regexp_literal_literal])
+
+          literal = Mutant::Util.one(regexp_ast.children.last.children)
+
+          emit(s(:send, receiver, :start_with?, s(:str, literal)))
+        end
+
         def emit_predicate_mutations
           return unless selector.match?(/\?\z/) && !selector.equal?(:defined?)
 

meta/send.rb

@@ -738,3 +738,76 @@
   mutation 'a === self'
   mutation 'a.is_a?(b)'
 end
+
+Mutant::Meta::Example.add :send do
+  source 'a.match?(/\Afoo/)'
+
+  singleton_mutations
+
+  mutation 'a'
+  mutation 'a.match?'
+  mutation '/\Afoo/'
+  mutation 'self.match?(/\Afoo/)'
+  mutation 'a.match?(//)'
+  mutation 'a.match?(/nomatch\A/)'
+  mutation "a.start_with?('foo')"
+  mutation 'false'
+  mutation 'true'
+end
+
+Mutant::Meta::Example.add :send do
+  source 'match(/\A\d/)'
+
+  singleton_mutations
+
+  mutation 'match'
+  mutation '/\A\d/'
+  mutation 'match?(/\A\d/)'
+  mutation 'match(/\A\D/)'
+  mutation 'match(//)'
+  mutation 'match(/nomatch\A/)'
+end
+
+Mutant::Meta::Example.add :send do
+  source 'a =~ /\Afoo/'
+
+  singleton_mutations
+
+  mutation 'a'
+  mutation 'nil =~ /\Afoo/'
+  mutation 'self =~ /\Afoo/'
+  mutation '/\Afoo/'
+  mutation 'a =~ //'
+  mutation 'a =~ /nomatch\A/'
+  mutation 'a.match?(/\Afoo/)'
+end
+
+Mutant::Meta::Example.add :send do
+  source 'match?(/\Afoo/, 1)'
+
+  singleton_mutations
+
+  mutation 'match?(/\Afoo/)'
+  mutation 'match?(1)'
+  mutation 'match?(/\Afoo/, nil)'
+  mutation 'match?(/\Afoo/, self)'
+  mutation 'match?(/\Afoo/, -1)'
+  mutation 'match?(/\Afoo/, 0)'
+  mutation 'match?(/\Afoo/, 2)'
+  mutation 'match?'
+  mutation 'match?(//, 1)'
+  mutation 'match?(/nomatch\A/, 1)'
+  mutation 'false'
+  mutation 'true'
+end
+
+Mutant::Meta::Example.add :send do
+  source 'foo(/\Abar/)'
+
+  singleton_mutations
+
+  mutation 'foo'
+  mutation '/\Abar/'
+  mutation 'foo(//)'
+  mutation 'foo(/nomatch\A/)'
+end

mutant.yml

@@ -12,7 +12,6 @@ matcher:
   ignore:
   - Mutant::Isolation::Fork::Parent#call
   - Mutant::Mutator::Node::Argument#skip?
-  - Mutant::Mutator::Node::Literal::Regex#body
   - Mutant::Mutator::Node::ProcargZero#dispatch
   - Mutant::Mutator::Node::When#mutate_conditions
   - Mutant::Zombifier#call

spec/unit/mutant/ast/regexp/expand_regexp_ast_spec.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+RSpec.describe Mutant::AST::Regexp, '.expand_regexp_ast' do
+  it 'returns the expanded AST' do
+    # /foo/
+    parser_ast = s(:regexp, s(:str, 'foo'), s(:regopt))
+
+    expect(described_class.expand_regexp_ast(parser_ast)).to eql(
+      s(:regexp_root_expression,
+        s(:regexp_literal_literal, "foo"))
+    )
+  end
+
+  it 'returns `nil` for complex regexps' do
+    # /foo#{bar}/
+    parser_ast =
+      s(:regexp,
+        s(:str, 'foo'),
+        s(:begin,
+          s(:send, nil, :bar)),
+          s(:regopt))
+
+    expect(described_class.expand_regexp_ast(parser_ast)).to be(nil)
+  end
+end