Add new cop Lint/OrAssignmentToConstant

This cop checks for unintended or-assignment to a constant.

Constants should always be assigned in the same location. And its value
should always be the same. If constants are assigned in multiple
locations, the result may vary depending on the order of `require`.

Also, if you already have such an implementation, auto-correction may
change the result.

```ruby
CONST ||= 1

CONST = 1
```

CHANGELOG.md

@@ -5366,3 +5366,4 @@
 [@ohbarye]: https://github.com/ohbarye
 [@magneland]: https://github.com/magneland
 [@k-karen]: https://github.com/k-karen
+[@uplus]: https://github.com/uplus

changelog/new_add_new_cop_lintorassignmenttoconstant.md

@@ -0,0 +1 @@
+* [#9363](https://github.com/rubocop-hq/rubocop/pull/9363): Add new cop `Lint/OrAssignmentToConstant`. ([@uplus][])

config/default.yml

@@ -1811,6 +1811,12 @@ Lint/NumberedParameterAssignment:
   Enabled: pending
   VersionAdded: '<<next>>'
 
+Lint/OrAssignmentToConstant:
+  Description: 'Checks unintended or-assignment to constant.'
+  Enabled: pending
+  Safe: false
+  VersionAdded: '<<next>>'
+
 Lint/OrderedMagicComments:
   Description: 'Checks the proper ordering of magic comments and whether a magic comment is not placed before a shebang.'
   Enabled: true

lib/rubocop.rb

@@ -313,6 +313,7 @@
 require_relative 'rubocop/cop/lint/non_local_exit_from_iterator'
 require_relative 'rubocop/cop/lint/number_conversion'
 require_relative 'rubocop/cop/lint/numbered_parameter_assignment'
+require_relative 'rubocop/cop/lint/or_assignment_to_constant'
 require_relative 'rubocop/cop/lint/ordered_magic_comments'
 require_relative 'rubocop/cop/lint/out_of_range_regexp_ref'
 require_relative 'rubocop/cop/lint/parentheses_as_grouped_expression'

lib/rubocop/cop/lint/or_assignment_to_constant.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Lint
+      # This cop checks for unintended or-assignment to a constant.
+      #
+      # Constants should always be assigned in the same location. And its value
+      # should always be the same. If constants are assigned in multiple
+      # locations, the result may vary depending on the order of `require`.
+      #
+      # Also, if you already have such an implementation, auto-correction may
+      # change the result.
+      #
+      # @example
+      #
+      #   # bad
+      #   CONST ||= 1
+      #
+      #   # good
+      #   CONST = 1
+      #
+      class OrAssignmentToConstant < Base
+        extend AutoCorrector
+
+        MSG = 'Avoid using or-assignment with constants.'
+
+        def on_or_asgn(node)
+          lhs, _rhs = *node
+          return unless lhs&.casgn_type?
+
+          add_offense(node.loc.operator) do |corrector|
+            corrector.replace(node.loc.operator, '=')
+          end
+        end
+      end
+    end
+  end
+end

spec/rubocop/cop/lint/or_assignment_to_constant_spec.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Lint::OrAssignmentToConstant, :config do
+  subject(:cop) { described_class.new(config) }
+
+  it 'registers an offense with or-assignment to a constant' do
+    expect_offense(<<~RUBY)
+      CONST ||= 1
+            ^^^ Avoid using or-assignment with constants.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      CONST = 1
+    RUBY
+  end
+
+  it 'does not register an offense with plain assignment to a constant' do
+    expect_no_offenses(<<~RUBY)
+      CONST = 1
+    RUBY
+  end
+
+  [
+    ['a local variable', 'var'],
+    ['an instance variable', '@var'],
+    ['a class variable', '@@var'],
+    ['a global variable', '$var'],
+    ['an attribute', 'self.var']
+  ].each do |type, var|
+    it "does not register an offense with or-assignment to #{type}" do
+      expect_no_offenses(<<~RUBY)
+        #{var} ||= 1
+      RUBY
+    end
+  end
+end