[Fix rubocop#2464] Style/ParallelAssignment understands aref/attribut…

…e assignments

CHANGELOG.md

@@ -24,6 +24,7 @@
 * [#2471](https://github.com/bbatsov/rubocop/issues/2471): `Style/MethodName` doesn't choke on methods which are defined inside methods. ([@alexdowad][])
 * [#2449](https://github.com/bbatsov/rubocop/issues/2449): `Style/StabbyLambdaParentheses` only checks lambdas in the arrow form. ([@lumeet][])
 * [#2456](https://github.com/bbatsov/rubocop/issues/2456): `Lint/NestedMethodDefinition` doesn't register offenses for method definitions inside an eval block (either `instance_eval`, `class_eval`, or `module_eval`). ([@alexdowad][])
+* [#2464](https://github.com/bbatsov/rubocop/issues/2464): `Style/ParallelAssignment` understands aref and attribute assignments, and doesn't warn if they can't be correctly rearranged into a series of single assignments. ([@alexdowad][])
 
 ### Changes
 

lib/rubocop/cop/style/parallel_assignment.rb

@@ -99,6 +99,7 @@ class AssignmentSorter
 
           def_node_matcher :var_name, '{(casgn _ $_) (_ $_)}'
           def_node_search :uses_var?, '{({lvar ivar cvar gvar} %) (const _ %)}'
+          def_node_search :matching_calls, '(send %1 %2 $...)'
 
           def initialize(assignments)
             @assignments = assignments
@@ -115,7 +116,23 @@ def tsort_each_child(assignment)
 
             @assignments.each do |other|
               _other_lhs, other_rhs = *other
-              yield other if uses_var?(other_rhs, var_name(my_lhs))
+              if ((var = var_name(my_lhs)) && uses_var?(other_rhs, var)) ||
+                 (my_lhs.asgn_method_call? && accesses?(other_rhs, my_lhs))
+                yield other
+              end
+            end
+          end
+
+          # `lhs` is an assignment method call like `obj.attr=` or `ary[idx]=`.
+          # Does `rhs` access the same value which is assigned by `lhs`?
+          def accesses?(rhs, lhs)
+            if lhs.method_name == :[]=
+              matching_calls(rhs, lhs.receiver, :[]).any? do |args|
+                args == lhs.method_args
+              end
+            else
+              access_method = lhs.method_name.to_s.chop.to_sym
+              matching_calls(rhs, lhs.receiver, access_method).any?
             end
           end
         end

spec/rubocop/cop/style/parallel_assignment_spec.rb

@@ -46,10 +46,12 @@
   it_behaves_like('offenses', ['if true',
                                '  a, b = 1, 2',
                                'end'].join("\n"))
-  it_behaves_like('offenses',
-                  'a, b = Float::INFINITY, Float::INFINITY')
-  it_behaves_like('offenses',
-                  'Float::INFINITY, Float::INFINITY = 1, 2')
+  it_behaves_like('offenses', 'a, b = Float::INFINITY, Float::INFINITY')
+  it_behaves_like('offenses', 'Float::INFINITY, Float::INFINITY = 1, 2')
+  it_behaves_like('offenses', 'a[0], a[1] = a[1], a[2]')
+  it_behaves_like('offenses', 'obj.attr1, obj.attr2 = obj.attr3, obj.attr1')
+  it_behaves_like('offenses', 'obj.attr1, ary[0] = ary[1], obj.attr1')
+  it_behaves_like('offenses', 'a[0], a[1] = a[1], b[0]')
 
   shared_examples('allowed') do |source|
     it "allows assignment of: #{source}" do
@@ -85,8 +87,11 @@
                               'a, b, c = foo'].join("\n"))
   it_behaves_like('allowed', ['array = [1, 2, 3]',
                               'a, = array'].join("\n"))
-  it_behaves_like('allowed',
-                  'a, b = Float::INFINITY')
+  it_behaves_like('allowed', 'a, b = Float::INFINITY')
+  it_behaves_like('allowed', 'a[0], a[1] = a[1], a[0]')
+  it_behaves_like('allowed', 'obj.attr1, obj.attr2 = obj.attr2, obj.attr1')
+  it_behaves_like('allowed', 'obj.attr1, ary[0] = ary[0], obj.attr1')
+  it_behaves_like('allowed', 'ary[0], ary[1], ary[2] = ary[1], ary[2], ary[0]')
 
   it 'hightlights the entire expression' do
     inspect_source(cop, 'a, b = 1, 2')