Class: RuboCop::Cop::Lint::UselessAssignment

Inherits:
Base
  • Object
show all
Extended by:
AutoCorrector
Includes:
RangeHelp
Defined in:
lib/rubocop/cop/lint/useless_assignment.rb

Overview

Checks for every useless assignment to local variable in every scope. The basic idea for this cop was from the warning of ‘ruby -cw`:

source,console

assigned but unused variable - foo

Currently this cop has advanced logic that detects unreferenced reassignments and properly handles varied cases such as branch, loop, rescue, ensure, etc.

This cop’s autocorrection avoids cases like ‘a ||= 1` because removing assignment from operator assignment can cause `NameError` if this assignment has been used to declare a local variable. For example, replacing `a ||= 1` with `a || 1` may cause “undefined local variable or method `a’ for main:Object (NameError)”.

NOTE: Given the assignment ‘foo = 1, bar = 2`, removing unused variables can lead to a syntax error, so this case is not autocorrected.

Examples:


# bad
def some_method
  some_var = 1
  do_something
end

# good
def some_method
  some_var = 1
  do_something(some_var)
end

Constant Summary collapse

MSG = 
'Useless assignment to variable - `%<variable>s`.'

Constants included from RangeHelp

RangeHelp::BYTE_ORDER_MARK, RangeHelp::NOT_GIVEN

Constants inherited from Base

Base::RESTRICT_ON_SEND

Instance Attribute Summary

Attributes inherited from Base

#config, #processed_source

Class Method Summary collapse

Instance Method Summary collapse

Methods included from AutoCorrector

support_autocorrect?

Methods inherited from Base

#active_support_extensions_enabled?, #add_global_offense, #add_offense, #always_autocorrect?, autocorrect_incompatible_with, badge, #begin_investigation, #callbacks_needed, callbacks_needed, #config_to_allow_offenses, #config_to_allow_offenses=, #contextual_autocorrect?, #cop_config, cop_name, #cop_name, department, documentation_url, exclude_from_registry, #excluded_file?, #external_dependency_checksum, inherited, #initialize, #inspect, lint?, match?, #message, #offenses, #on_investigation_end, #on_new_investigation, #on_other_file, #parse, #parser_engine, #ready, #relevant_file?, requires_gem, #string_literals_frozen_by_default?, support_autocorrect?, support_multiple_source?, #target_gem_version, #target_rails_version, #target_ruby_version

Methods included from ExcludeLimit

#exclude_limit

Methods included from AutocorrectLogic

#autocorrect?, #autocorrect_enabled?, #autocorrect_requested?, #autocorrect_with_disable_uncorrectable?, #correctable?, #disable_uncorrectable?, #safe_autocorrect?

Methods included from IgnoredNode

#ignore_node, #ignored_node?, #part_of_ignored_node?

Methods included from Util

silence_warnings

Constructor Details

This class inherits a constructor from RuboCop::Cop::Base

Class Method Details

.joining_forces ⇒ Object 


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 47

def self.joining_forces
  VariableForce
end

Instance Method Details

#after_leaving_scope(scope, _variable_table) ⇒ Object 


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 51

def after_leaving_scope(scope, _variable_table)
  scope.variables.each_value { |variable| check_for_unused_assignments(variable) }
end

#autocorrect(corrector, assignment) ⇒ Object

rubocop:disable Metrics/AbcSize


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 165

def autocorrect(corrector, assignment)
  if assignment.exception_assignment?
    remove_exception_assignment_part(corrector, assignment.node)
  elsif assignment.multiple_assignment? || assignment.rest_assignment? ||
        assignment.for_assignment?
    rename_variable_with_underscore(corrector, assignment.node)
  elsif assignment.operator_assignment?
    remove_trailing_character_from_operator(corrector, assignment.node)
  elsif assignment.regexp_named_capture?
    replace_named_capture_group_with_non_capturing_group(corrector, assignment.node,
                                                         assignment.variable.name)
  else
    remove_local_variable_assignment_part(corrector, assignment.node)
  end
end

#chained_assignment?(node) ⇒ Boolean

Returns:

  • (Boolean) 

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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 106

def chained_assignment?(node)
  node.respond_to?(:expression) && node.expression&.lvasgn_type?
end

#check_for_unused_assignments(variable) ⇒ Object

rubocop:disable Metrics/AbcSize, Metrics/PerceivedComplexity, Metrics/CyclomaticComplexity


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 56

def check_for_unused_assignments(variable)
  return if variable.should_be_unused?

  variable.assignments.reverse_each do |assignment|
    assignment_node = assignment.node
    next if assignment.used? || part_of_ignored_node?(assignment_node)

    message = message_for_useless_assignment(assignment)
    range = offense_range(assignment)

    add_offense(range, message: message) do |corrector|
      # In cases like `x = 1, y = 2`, where removing a variable would cause a syntax error,
      # and where changing `x ||= 1` to `x = 1` would cause `NameError`,
      # the autocorrect will be skipped, even if the variable is unused.
      if sequential_assignment?(assignment_node) || assignment_node.parent&.or_asgn_type?
        next
      end

      autocorrect(corrector, assignment)
    end

    ignore_node(assignment_node) if chained_assignment?(assignment_node)
  end
end

#collect_variable_like_names(scope) ⇒ Object 


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 149

def collect_variable_like_names(scope)
  names = scope.each_node.with_object(Set.new) do |node, set|
    set << node.method_name if variable_like_method_invocation?(node)
  end

  variable_names = scope.variables.each_value.map(&:name)
  names.merge(variable_names)
end

#message_for_useless_assignment(assignment) ⇒ Object

rubocop:enable Metrics/AbcSize, Metrics/PerceivedComplexity, Metrics/CyclomaticComplexity


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 82

def message_for_useless_assignment(assignment)
  variable = assignment.variable

  format(MSG, variable: variable.name) + message_specification(assignment, variable).to_s
end

#message_specification(assignment, variable) ⇒ Object 


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 110

def message_specification(assignment, variable)
  if assignment.multiple_assignment?
    multiple_assignment_message(variable.name)
  elsif assignment.operator_assignment?
    operator_assignment_message(variable.scope, assignment)
  else
    similar_name_message(variable)
  end
end

#multiple_assignment_message(variable_name) ⇒ Object 


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 120

def multiple_assignment_message(variable_name)
  " Use `_` or `_#{variable_name}` as a variable name to indicate " \
    "that it won't be used."
end

#offense_range(assignment) ⇒ Object 


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 88

def offense_range(assignment)
  if assignment.regexp_named_capture?
    assignment.node.children.first.source_range
  else
    assignment.node.loc.name
  end
end

#operator_assignment_message(scope, assignment) ⇒ Object 


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 125

def operator_assignment_message(scope, assignment)
  return_value_node = return_value_node_of_scope(scope)
  return unless assignment.meta_assignment_node.equal?(return_value_node)

  " Use `#{assignment.operator.delete_suffix('=')}` instead of `#{assignment.operator}`."
end

#remove_exception_assignment_part(corrector, node) ⇒ Object

rubocop:enable Metrics/AbcSize


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 182

def remove_exception_assignment_part(corrector, node)
  corrector.remove(
    range_between(
      (node.parent.children.first&.source_range || node.parent.location.keyword).end_pos,
      node.source_range.end_pos
    )
  )
end

#remove_local_variable_assignment_part(corrector, node) ⇒ Object 


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 206

def remove_local_variable_assignment_part(corrector, node)
  corrector.replace(node, node.expression.source)
end

#remove_trailing_character_from_operator(corrector, node) ⇒ Object 


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 195

def remove_trailing_character_from_operator(corrector, node)
  corrector.remove(node.parent.location.operator.end.adjust(begin_pos: -1))
end

#rename_variable_with_underscore(corrector, node) ⇒ Object 


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 191

def rename_variable_with_underscore(corrector, node)
  corrector.replace(node, '_')
end

#replace_named_capture_group_with_non_capturing_group(corrector, node, variable_name) ⇒ Object 


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 199

def replace_named_capture_group_with_non_capturing_group(corrector, node, variable_name)
  corrector.replace(
    node.children.first,
    node.children.first.source.sub(/\(\?<#{variable_name}>/, '(?:')
  )
end

#return_value_node_of_scope(scope) ⇒ Object

TODO: More precise handling (rescue, ensure, nested begin, etc.)


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 139

def return_value_node_of_scope(scope)
  body_node = scope.body_node

  if body_node.begin_type?
    body_node.children.last
  else
    body_node
  end
end

#sequential_assignment?(node) ⇒ Boolean

Returns:

  • (Boolean) 

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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 96

def sequential_assignment?(node)
  if node.lvasgn_type? && node.expression&.array_type? &&
     node.each_descendant.any?(&:assignment?)
    return true
  end
  return false unless node.parent

  sequential_assignment?(node.parent)
end

#similar_name_message(variable) ⇒ Object 


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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 132

def similar_name_message(variable)
  variable_like_names = collect_variable_like_names(variable.scope)
  similar_name = NameSimilarity.find_similar_name(variable.name, variable_like_names)
  " Did you mean `#{similar_name}`?" if similar_name
end

#variable_like_method_invocation?(node) ⇒ Boolean

Returns:

  • (Boolean) 

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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 158

def variable_like_method_invocation?(node)
  return false unless node.send_type?

  node.receiver.nil? && !node.arguments?
end