Add Lint/ConstantReassignment cop

changelog/new_lint_constant_reassignment_cop.md

@@ -0,0 +1 @@
+* [#13612](https://github.com/rubocop/rubocop/pull/13612): Create new cop `Lint/ConstantReassignment`. ([@lovro-bikic][])

config/default.yml

@@ -1665,6 +1665,11 @@ Lint/ConstantOverwrittenInRescue:
   Enabled: pending
   VersionAdded: '1.31'
 
+Lint/ConstantReassignment:
+  Description: 'Checks for constant reassignments.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+
 Lint/ConstantResolution:
   Description: 'Check that constants are fully qualified with `::`.'
   Enabled: false

lib/rubocop.rb

@@ -300,6 +300,7 @@
 require_relative 'rubocop/cop/lint/circular_argument_reference'
 require_relative 'rubocop/cop/lint/constant_definition_in_block'
 require_relative 'rubocop/cop/lint/constant_overwritten_in_rescue'
+require_relative 'rubocop/cop/lint/constant_reassignment'
 require_relative 'rubocop/cop/lint/constant_resolution'
 require_relative 'rubocop/cop/lint/debugger'
 require_relative 'rubocop/cop/lint/deprecated_class_methods'

lib/rubocop/cop/lint/constant_reassignment.rb

@@ -0,0 +1,124 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Lint
+      # Checks for constant reassignments.
+      #
+      # Emulates Ruby's runtime warning "already initialized constant X"
+      # when a constant is reassigned in the same file and namespace using the
+      # `NAME = value` syntax.
+      #
+      # The cop cannot catch all offenses, like, for example, when a constant
+      # is reassigned in another file, or when using metaprogramming (`Module#const_set`).
+      #
+      # The cop also tracks constant removal using `Module#remove_const` with a symbol
+      # argument.
+      #
+      # @example
+      #   # bad
+      #   X = :foo
+      #   X = :bar
+      #
+      #   # bad
+      #   class A
+      #     X = :foo
+      #     X = :bar
+      #   end
+      #
+      #   # bad
+      #   module A
+      #     X = :foo
+      #     X = :bar
+      #   end
+      #
+      #   # good - keep only one assignment
+      #   X = :bar
+      #
+      #   class A
+      #     X = :bar
+      #   end
+      #
+      #   module A
+      #     X = :bar
+      #   end
+      #
+      #   # good - use OR assignment
+      #   X = :foo
+      #   X ||= :bar
+      #
+      #   # good - remove the assigned constant first
+      #   class A
+      #     X = :foo
+      #     remove_const :X
+      #     X = :bar
+      #   end
+      #
+      class ConstantReassignment < Base
+        MSG = 'Constant `%<constant>s` is already assigned in this namespace.'
+
+        RESTRICT_ON_SEND = %i[remove_const].freeze
+
+        # @!method remove_sym_const(node)
+        def_node_matcher :remove_sym_const, <<~PATTERN
+          (send _ :remove_const
+            (sym $_))
+        PATTERN
+
+        def on_casgn(node)
+          return if no_reassignment?(node)
+          return if constant_names.add?(fully_qualified_constant_name(node))
+
+          add_offense(node, message: format(MSG, constant: node.name))
+        end
+
+        def on_send(node)
+          constant = remove_sym_const(node)
+
+          namespaces = ancestor_namespaces(node)
+
+          return if namespaces.none?
+
+          constant_names.delete(fully_qualified_name_for(namespaces, constant))
+        end
+
+        private
+
+        def no_reassignment?(node)
+          node.parent&.or_asgn_type?
+        end
+
+        def fully_qualified_constant_name(node)
+          if node.absolute?
+            namespace = node.namespace.const_type? ? node.namespace.source : nil
+
+            "#{namespace}::#{node.name}"
+          else
+            constant_namespaces = ancestor_namespaces(node) + constant_namespaces(node)
+
+            fully_qualified_name_for(constant_namespaces, node.name)
+          end
+        end
+
+        def fully_qualified_name_for(namespaces, constant)
+          ['', *namespaces, constant].join('::')
+        end
+
+        def constant_namespaces(node)
+          node.each_path.map(&:short_name)
+        end
+
+        def ancestor_namespaces(node)
+          node
+            .each_ancestor(:class, :module)
+            .map { |ancestor| ancestor.identifier.short_name }
+            .reverse
+        end
+
+        def constant_names
+          @constant_names ||= Set.new
+        end
+      end
+    end
+  end
+end