[Fix rubocop#3366] Make Style/MutableConstant cop aware of splat as…

…signments (rubocop#3367)

This cop would generate false positives on code like:

```
FOO = *(1...10)
```

and:

```
BAR = *[1...10].freeze
```

This change fixes that.

CHANGELOG.md

@@ -12,6 +12,7 @@
 * [#3351](https://github.com/bbatsov/rubocop/issues/3351): Fix bad auto-correct for `Performance/RedundantMatch` cop. ([@annaswims][])
 * [#3347](https://github.com/bbatsov/rubocop/issues/3347): Prevent infinite loop in `Style/TernaryParentheses` cop when used together with `Style/RedundantParentheses`. ([@drenmi][])
 * [#3209](https://github.com/bbatsov/rubocop/issues/3209): Remove faulty line length check from `Style/GuardClause` cop. ([@drenmi][])
+* [#3366](https://github.com/bbatsov/rubocop/issues/3366): Make `Style/MutableConstant` cop aware of splat assignments. ([@drenmi][])
 
 ### Changes
 

lib/rubocop/cop/style/mutable_constant.rb

@@ -14,10 +14,10 @@ module Style
       #   # good
       #   CONST = [1, 2, 3].freeze
       class MutableConstant < Cop
-        MSG = 'Freeze mutable objects assigned to constants.'.freeze
-
         include FrozenStringLiteral
 
+        MSG = 'Freeze mutable objects assigned to constants.'.freeze
+
         def on_casgn(node)
           _scope, _const_name, value = *node
           on_assignment(value)
@@ -31,6 +31,18 @@ def on_or_asgn(node)
           on_assignment(value)
         end
 
+        private
+
+        def on_assignment(value)
+          value = splat_value(value) if splat_value(value)
+
+          return unless value && value.mutable_literal?
+          return if FROZEN_STRING_LITERAL_TYPES.include?(value.type) &&
+                    frozen_string_literals_enabled?(processed_source)
+
+          add_offense(value, :expression)
+        end
+
         def autocorrect(node)
           expr = node.source_range
           lambda do |corrector|
@@ -43,15 +55,9 @@ def autocorrect(node)
           end
         end
 
-        private
-
-        def on_assignment(value)
-          return unless value && value.mutable_literal?
-          return if FROZEN_STRING_LITERAL_TYPES.include?(value.type) &&
-                    frozen_string_literals_enabled?(processed_source)
-
-          add_offense(value, :expression)
-        end
+        def_node_matcher :splat_value, <<-PATTERN
+          (array (splat $_))
+        PATTERN
       end
     end
   end

spec/rubocop/cop/style/mutable_constant_spec.rb

@@ -66,6 +66,29 @@
     expect(cop.offenses).to be_empty
   end
 
+  context 'when performing a splat assignment' do
+    it 'allows an immutable value' do
+      inspect_source(cop, 'FOO = *(1...10)')
+      expect(cop.offenses).to be_empty
+    end
+
+    it 'allows a frozen array value' do
+      inspect_source(cop, 'FOO = *[1...10].freeze')
+      expect(cop.offenses).to be_empty
+    end
+
+    it 'registers an offense for a mutable value' do
+      source = 'BAR = *[1, 2, 3]'
+
+      inspect_source(cop, source)
+      expect(cop.offenses.size).to eq(1)
+
+      corrected = autocorrect_source(cop, source)
+
+      expect(corrected).to eq('BAR = *[1, 2, 3].freeze')
+    end
+  end
+
   context 'when assigning an array without brackets' do
     it 'adds brackets when auto-correcting' do
       new_source = autocorrect_source(cop, 'XXX = YYY, ZZZ')