Class: Rubocop::Cop::Lint::UselessAssignment

Inherits:
Cop
  • Object
show all
Includes:
VariableInspector
Defined in:
lib/rubocop/cop/lint/useless_assignment.rb

Overview

This cop 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.

Constant Summary collapse

MSG = 
'Useless assignment to variable - %s'

Constants included from VariableInspector

VariableInspector::ARGUMENT_DECLARATION_TYPES, VariableInspector::BLOCK_ARGUMENT_DECLARATION_TYPE, VariableInspector::BLOCK_LOCAL_VARIABLE_DECLARATION_TYPE, VariableInspector::DECLARATION_TYPES, VariableInspector::LOGICAL_OPERATOR_ASSIGNMENT_TYPES, VariableInspector::LOOP_TYPES, VariableInspector::METHOD_ARGUMENT_DECLARATION_TYPES, VariableInspector::MULTIPLE_ASSIGNMENT_TYPE, VariableInspector::OPERATOR_ASSIGNMENT_TYPES, VariableInspector::POST_CONDITION_LOOP_TYPES, VariableInspector::REGEXP_NAMED_CAPTURE_TYPE, VariableInspector::RESCUE_TYPE, VariableInspector::SCOPE_TYPES, VariableInspector::TWISTED_SCOPE_TYPES, VariableInspector::VARIABLE_ASSIGNMENT_TYPE, VariableInspector::VARIABLE_ASSIGNMENT_TYPES, VariableInspector::VARIABLE_REFERENCE_TYPE, VariableInspector::ZERO_ARITY_SUPER_TYPE

Constants inherited from Cop

Cop::OPERATOR_METHODS

Instance Attribute Summary

Attributes inherited from Cop

#config, #corrections, #offences, #processed_source

Instance Method Summary collapse

Methods included from VariableInspector

#after_declaring_variable, #after_entering_scope, #before_declaring_variable, #before_entering_scope, #before_leaving_scope, #dispatch_node, #inspect_variables, #inspect_variables_in_scope, #mark_assignments_as_referenced_in_loop, #process_children, #process_loop, #process_node, #process_regexp_named_captures, #process_rescue, #process_scope, #process_variable_assignment, #process_variable_declaration, #process_variable_multiple_assignment, #process_variable_operator_assignment, #process_variable_referencing, #process_zero_arity_super, #scan, #scanned_node?, #scanned_nodes, #skip_children!, #variable_table

Methods inherited from Cop

#add_offence, all, #autocorrect?, #convention, #cop_config, cop_name, #cop_name, cop_type, #debug?, #ignore_node, inherited, #initialize, lint?, #message, non_rails, rails?, style?, #support_autocorrect?, #warning

Constructor Details

This class inherits a constructor from Rubocop::Cop::Cop

Instance Method Details

#after_leaving_scope(scope) ⇒ Object 



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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 24

def after_leaving_scope(scope)
  scope.variables.each_value do |variable|
    check_for_unused_assignments(variable)
    check_for_unused_block_local_variable(variable)
  end
end

#check_for_unused_assignments(variable) ⇒ Object 



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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 31

def check_for_unused_assignments(variable)
  return if variable.name.to_s.start_with?('_')

  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.loc.expression
               else
                 assignment.node.loc.name
               end

    warning(nil, location, message)
  end
end

#check_for_unused_block_local_variable(variable) ⇒ Object 



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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 79

def check_for_unused_block_local_variable(variable)
  return unless variable.block_local_variable?
  return unless variable.assignments.empty?
  message = sprintf(MSG, variable.name)
  warning(variable.declaration_node, :expression, message)
end

#investigate(processed_source) ⇒ Object 



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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 20

def investigate(processed_source)
  inspect_variables(processed_source.ast)
end

#message_for_useless_assignment(assignment) ⇒ Object 



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# File 'lib/rubocop/cop/lint/useless_assignment.rb', line 49

def message_for_useless_assignment(assignment)
  variable = assignment.variable

  message = sprintf(MSG, variable.name)

  if assignment.multiple_assignment?
    message << ". Use _ or _#{variable.name} as a variable name " +
               "to indicate that it won't be used."
  elsif assignment.operator_assignment?
    return_value_node = return_value_node_of_scope(variable.scope)
    if assignment.meta_assignment_node.equal?(return_value_node)
      non_assignment_operator = assignment.operator.sub(/=$/, '')
      message << ". Use just operator #{non_assignment_operator}."
    end
  end

  message
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 69

def return_value_node_of_scope(scope)
  body_node = scope.body_node

  if body_node.type == :begin
    body_node.children.last
  else
    body_node
  end
end