Revert "Remove regexp body mutation support"

This reverts commit 21d3fef.

This was not a clean revert. Note that:
- The version of `regexp_parser` was 1.3.0, now it is 1.8.2 to accomodate our current `rubocop` version and because there were some relevant bugfixes implemented between 1.3.x and 1.8.x. We should eventually move to 2.0 but it is currently incompatible with this integration. There are some issues with the frozen Regexp classes getting mutated so we may have to open an issue.
- Since "expected exception" support was removed from the specs, I have had to exclude two files entirely. This seems unfortunate as it reduces our overall coverage.
- Since unsupported nodes are no longer explicitly tracked, I removed the code that used to handle that for regular expressions. See: #1021
- I had to change the example case for where we are more permissive than `regexp_parser` because `regexp_parser` has decided to become more permissive and try to match Ruby's semantics. It was actually very hard to find a case that failed--I brute-forced 50 million regexp strings that had perfect parity of being accepted and then stumbled onto the single hex escape case by accident. See: ammar/regexp_parser#75
- Changed an access pattern for regexp mutations which became equivalent based on this: https://github.com/ammar/regexp_parser/blame/4ca7cec03b210e3e00473b7b1a7308f963190c1e/lib/regexp_parser/expression/subexpression.rb#L30-L33
- Some other minor conflicts and small spec assertion changes were resolved as well.

Changelog.md

@@ -1,3 +1,7 @@
+# Unreleased
+
+* Reintroduce regexp mutation support [#TODO]
+
 # v0.10.20 2020-12-16
 
   [#1159](https://github.com/mbj/mutant/pull/1159)

Gemfile.lock

@@ -14,6 +14,7 @@ PATH
       mprelude (~> 0.1.0)
       parser (~> 2.7.1)
       procto (~> 0.0.2)
+      regexp_parser (~> 1.8)
       unparser (~> 0.5.4)
       variable (~> 0.0.1)
 
@@ -51,7 +52,7 @@ GEM
       ast (~> 2.4.1)
     procto (0.0.3)
     rainbow (3.0.0)
-    regexp_parser (2.0.0)
+    regexp_parser (1.8.2)
     rexml (3.2.4)
     rspec (3.10.0)
       rspec-core (~> 3.10.0)

lib/mutant.rb

@@ -17,6 +17,7 @@
 require 'parser'
 require 'parser/current'
 require 'pathname'
+require 'regexp_parser'
 require 'set'
 require 'singleton'
 require 'stringio'
@@ -52,6 +53,15 @@ module Mutant
 require 'mutant/ast/named_children'
 require 'mutant/ast/node_predicates'
 require 'mutant/ast/find_metaclass_containing'
+require 'mutant/ast/regexp'
+require 'mutant/ast/regexp/transformer'
+require 'mutant/ast/regexp/transformer/direct'
+require 'mutant/ast/regexp/transformer/named_group'
+require 'mutant/ast/regexp/transformer/options_group'
+require 'mutant/ast/regexp/transformer/quantifier'
+require 'mutant/ast/regexp/transformer/recursive'
+require 'mutant/ast/regexp/transformer/root'
+require 'mutant/ast/regexp/transformer/text'
 require 'mutant/ast/meta'
 require 'mutant/ast/meta/send'
 require 'mutant/ast/meta/const'
@@ -74,6 +84,13 @@ module Mutant
 require 'mutant/mutator/util/symbol'
 require 'mutant/mutator/node'
 require 'mutant/mutator/node/generic'
+require 'mutant/mutator/node/regexp'
+require 'mutant/mutator/node/regexp/alternation_meta'
+require 'mutant/mutator/node/regexp/capture_group'
+require 'mutant/mutator/node/regexp/character_type'
+require 'mutant/mutator/node/regexp/end_of_line_anchor'
+require 'mutant/mutator/node/regexp/end_of_string_or_before_end_of_line_anchor'
+require 'mutant/mutator/node/regexp/greedy_zero_or_more'
 require 'mutant/mutator/node/literal'
 require 'mutant/mutator/node/literal/boolean'
 require 'mutant/mutator/node/literal/range'

lib/mutant/ast/regexp.rb

@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+module Mutant
+  module AST
+    # Regexp source mapper
+    module Regexp
+      # Parse regex string into expression
+      #
+      # @param regexp [String]
+      #
+      # @return [Regexp::Expression, nil]
+      #
+      # rubocop:disable Lint/HandleExceptions
+      def self.parse(regexp)
+        ::Regexp::Parser.parse(regexp)
+      # regexp_parser is more strict than MRI
+      rescue ::Regexp::Scanner::PrematureEndError
+      end
+      # rubocop:enable Lint/HandleExceptions
+
+      # Convert expression into ast node
+      #
+      # @param expression [Regexp::Expression]
+      #
+      # @return [Parser::AST::Node]
+      def self.to_ast(expression)
+        ast_type = :"regexp_#{expression.token}_#{expression.type}"
+
+        Transformer.lookup(ast_type).to_ast(expression)
+      end
+
+      # Convert node into expression
+      #
+      # @param node [Parser::AST::Node]
+      #
+      # @return [Regexp::Expression]
+      def self.to_expression(node)
+        Transformer.lookup(node.type).to_expression(node)
+      end
+    end # Regexp
+  end # AST
+end # Mutant

lib/mutant/ast/regexp/transformer.rb

@@ -0,0 +1,187 @@
+# frozen_string_literal: true
+
+module Mutant
+  module AST
+    module Regexp
+      # Regexp bijective mapper
+      #
+      # Transforms parsed regular expression representation from
+      # `Regexp::Expression` instances (provided by `regexp_parser`) into
+      # equivalent representations using `Parser::AST::Node`
+      class Transformer
+        include AbstractType
+
+        REGISTRY = Registry.new
+
+        # Lookup transformer class for regular expression node type
+        #
+        # @param type [Symbol]
+        #
+        # @return [Class<Transformer>]
+        def self.lookup(type)
+          REGISTRY.lookup(type)
+        end
+
+        # Register transformer class as responsible for handling node type
+        #
+        # @param type [Symbol]
+        #
+        # @return [undefined]
+        def self.register(type)
+          REGISTRY.register(type, self)
+        end
+        private_class_method :register
+
+        # Transform expression
+        #
+        # @param expression [Regexp::Expression]
+        #
+        # @return [Parser::AST::Node]
+        def self.to_ast(expression)
+          self::ExpressionToAST.call(expression)
+        end
+
+        # Transform node
+        #
+        # @param node [Parser::AST::Node]
+        #
+        # @return [Regexp::Expression]
+        def self.to_expression(node)
+          self::ASTToExpression.call(node)
+        end
+
+        # Abstract expression transformer
+        class ExpressionToAST
+          PREFIX = :regexp
+
+          include Concord.new(:expression), Procto.call, AST::Sexp, AbstractType, Adamantium
+
+        private
+
+          # Node with provided children using node type constructed in `type`
+          #
+          # @param [Object,Parser::AST::Node] child of node
+          #
+          # @return [Parser::AST::Node]
+          def ast(*children)
+            s(type, *children)
+          end
+
+          # Wrap provided node in a quantifier
+          #
+          # @param node [Parser::AST::Node]
+          #
+          # @return [Parser::AST::Node]
+          #   quantifier node wrapping provided node if expression is quantified
+          #
+          # @return [Parser::AST::Node]
+          #   original node otherwise
+          def quantify(node)
+            return node unless expression.quantified?
+
+            Quantifier.to_ast(expression.quantifier).append(node)
+          end
+
+          # Transformed children of expression
+          #
+          # @return [Array<Parser::AST::Node>]
+          def children
+            expression.map(&Regexp.public_method(:to_ast))
+          end
+
+          # Node type constructed from token and type of `Regexp::Expression`
+          #
+          # @return [Symbol]
+          def type
+            :"#{PREFIX}_#{expression.token}_#{expression.type}"
+          end
+        end # ExpressionToAST
+
+        # Abstract node transformer
+        class ASTToExpression
+          include Concord.new(:node), Procto.call, AbstractType, Adamantium
+
+          # Call generic transform method and freeze result
+          #
+          # @return [Regexp::Expression]
+          def call
+            transform.freeze
+          end
+
+        private
+
+          # Transformation of ast into expression
+          #
+          # @return [Regexp::Expression]
+          abstract_method :transform
+
+          # Transformed children of node
+          #
+          # @return [Array<Regexp::Expression>]
+          def subexpressions
+            node.children.map(&Regexp.public_method(:to_expression))
+          end
+        end # ASTToExpression
+
+        # Mixin for node transformers
+        #
+        # Helps construct a mapping from Parser::AST::Node domain to
+        # Regexp::Expression domain
+        module LookupTable
+          Mapping = Class.new.include(Concord::Public.new(:token, :regexp_class))
+
+          # Table mapping ast types to object information for regexp domain
+          class Table
+
+            # Coerce array of mapping information into structured table
+            #
+            # @param [Array(Symbol, Array, Class<Regexp::Expression>)]
+            #
+            # @return [Table]
+            def self.create(*rows)
+              table = rows.map do |ast_type, token, klass|
+                [ast_type, Mapping.new(::Regexp::Token.new(*token), klass)]
+              end.to_h
+
+              new(table)
+            end
+
+            include Concord.new(:table), Adamantium
+
+            # Types defined by the table
+            #
+            # @return [Array<Symbol>]
+            def types
+              table.keys
+            end
+
+            # Lookup mapping information given an ast node type
+            #
+            # @param type [Symbol]
+            #
+            # @return [Mapping]
+            def lookup(type)
+              table.fetch(type)
+            end
+          end # Table
+
+        private
+
+          # Lookup expression token given node type
+          #
+          # @return [Regexp::Token]
+          def expression_token
+            self.class::TABLE.lookup(node.type).token
+          end
+
+          # Lookup regexp class given node type
+          #
+          # @return [Class<Regexp::Expression>]
+          def expression_class
+            self.class::TABLE.lookup(node.type).regexp_class
+          end
+        end # LookupTable
+      end # Transformer
+    end # Regexp
+  end # AST
+end # Mutant