class RuboCop::Cop::Lint::UselessAssignment

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

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.

@safety

This cop's autocorrection is unsafe because removing assignment from
operator assignment can cause NameError if this assignment has been used to declare
local variable. For example, replacing `a ||= 1` to `a || 1` may cause
"undefined local variable or method `a' for main:Object (NameError)".

@example

# bad

def some_method
  some_var = 1
  do_something
end

@example

# good

def some_method
  some_var = 1
  do_something(some_var)
end

Constants

MSG

Public Class Methods

joining_forces() click to toggle source

# File lib/rubocop/cop/lint/useless_assignment.rb, line 46
def self.joining_forces
  VariableForce
end

Public Instance Methods

after_leaving_scope(scope, _variable_table) click to toggle source

# File lib/rubocop/cop/lint/useless_assignment.rb, line 50
def after_leaving_scope(scope, _variable_table)
  scope.variables.each_value { |variable| check_for_unused_assignments(variable) }
end

autocorrect(corrector, assignment) click to toggle source

rubocop:disable Metrics/AbcSize

# File lib/rubocop/cop/lint/useless_assignment.rb, line 136
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

check_for_unused_assignments(variable) click to toggle source

# File lib/rubocop/cop/lint/useless_assignment.rb, line 54
def check_for_unused_assignments(variable)
  return if variable.should_be_unused?

  variable.assignments.each do |assignment|
    next if assignment.used?

    message = message_for_useless_assignment(assignment)

    location = if assignment.regexp_named_capture?
                 assignment.node.children.first.source_range
               else
                 assignment.node.loc.name
               end

    add_offense(location, message: message) do |corrector|
      autocorrect(corrector, assignment)
    end
  end
end

collect_variable_like_names(scope) click to toggle source

# File lib/rubocop/cop/lint/useless_assignment.rb, line 120
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) click to toggle source

# File lib/rubocop/cop/lint/useless_assignment.rb, line 74
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) click to toggle source

# File lib/rubocop/cop/lint/useless_assignment.rb, line 80
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) click to toggle source

# File lib/rubocop/cop/lint/useless_assignment.rb, line 90
def multiple_assignment_message(variable_name)
  " Use `_` or `_#{variable_name}` as a variable name to indicate " \
    "that it won't be used."
end

operator_assignment_message(scope, assignment) click to toggle source

# File lib/rubocop/cop/lint/useless_assignment.rb, line 95
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.sub(/=$/, '')}` " \
    "instead of `#{assignment.operator}`."
end

remove_exception_assignment_part(corrector, node) click to toggle source

rubocop:enable Metrics/AbcSize

# File lib/rubocop/cop/lint/useless_assignment.rb, line 153
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) click to toggle source

# File lib/rubocop/cop/lint/useless_assignment.rb, line 177
def remove_local_variable_assignment_part(corrector, node)
  corrector.replace(node, node.expression.source)
end

remove_trailing_character_from_operator(corrector, node) click to toggle source

# File lib/rubocop/cop/lint/useless_assignment.rb, line 166
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) click to toggle source

# File lib/rubocop/cop/lint/useless_assignment.rb, line 162
def rename_variable_with_underscore(corrector, node)
  corrector.replace(node, '_')
end

replace_named_capture_group_with_non_capturing_group(corrector, node, variable_name) click to toggle source

# File lib/rubocop/cop/lint/useless_assignment.rb, line 170
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) click to toggle source

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

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

similar_name_message(variable) click to toggle source

# File lib/rubocop/cop/lint/useless_assignment.rb, line 103
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) click to toggle source

# File lib/rubocop/cop/lint/useless_assignment.rb, line 129
def variable_like_method_invocation?(node)
  return false unless node.send_type?

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