Add cop AssignmentInsteadOfComparison (#38)

config/default.yml

@@ -4,4 +4,8 @@ Overhaul/MutableReformDefaults:
   Enabled: true
   Include:
     app/concepts/**/*.rb
-  VersionAdded: '<<next>>'
+  VersionAdded: '0.0.1'
+Overhaul/AssignmentInsteadOfComparison:
+  Description: Detect enumerable comparison blocks returning an assignment.
+  Enabled: true
+  VersionAdded: '0.0.1'

lib/rubocop/cop/overhaul/assignment_instead_of_comparison.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Overhaul
+      # Checks whether the return value of an Enumerator block method (fetch,
+      # minmax, group, search and sort methods) isn't an assignment.
+      #
+      # @example
+      #   # bad
+      #   my_array.select { |x| x = 4 }
+      #
+      #   # good
+      #   my_array.select { |x| x == 4 }
+      #
+      class AssignmentInsteadOfComparison < RuboCop::Cop::Cop
+        MSG = "Do not return an assignment from the block of an Enumerable's search method."
+
+        RESTRICT_ENUM_METHODS = [
+          :take_while, :drop_while, # fetch
+          :min, :max, :min_by, :max_by, :minmax_by, # minmax
+          :group_by, :partition, :slice_after, :slice_before, :slice_when, :chunk, :chunk_while, # group
+          :find, :detect, :find_all, :filter, :select, :find_index, :reject, :uniq_by, # search
+          :sort, :sort_by # sort
+        ].freeze
+
+        # @!method search_method_calls?(node)
+        def_node_matcher :search_method_calls, <<~PATTERN
+          (block
+            (call _ /#{RESTRICT_ENUM_METHODS.join("|")}/)
+            (args ...)
+            $...
+          )
+        PATTERN
+
+        def on_block(node)
+          search_method_calls(node) do |value|
+            value = value.first
+            block_return_operation = value.begin_type? ? value.children.last : value
+            next unless block_return_operation.lvasgn_type?
+
+            add_offense(block_return_operation)
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/overhaul_cops.rb

@@ -1,3 +1,4 @@
 # frozen_string_literal: true
 
 require_relative "overhaul/mutable_reform_defaults"
+require_relative "overhaul/assignment_instead_of_comparison"

spec/rubocop/cop/overhaul/assignment_instead_of_comparison_spec.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Overhaul::AssignmentInsteadOfComparison, :config do
+  shared_examples "block returning comparison" do |block_statements|
+    it "allows #{block_statements}" do
+      source = "my_arr.#{method} { #{block_statements} }"
+      expect_no_offenses(source)
+    end
+  end
+
+  shared_examples "block returning assignment" do |block_statements|
+    it "registers an offense for #{block_statements}" do
+      *prefix, o = block_statements.split("; ")
+      prefix = prefix.join("; ")
+      prefix += "; " if prefix.length.positive?
+
+      expect_offense(<<~RUBY, o: o, block_statements: block_statements, prefix: prefix, method: method)
+        my_arr.%{method} { %{block_statements} }
+               _{method}   _{prefix}^{o} Do not return an assignment from the block of an Enumerable's search method.
+      RUBY
+    end
+  end
+
+  described_class::RESTRICT_ENUM_METHODS.each do |method|
+    context "when method is #{method}" do
+      let(:method) { method }
+
+      it_behaves_like "block returning assignment", "x = 3"
+      it_behaves_like "block returning assignment", "var = 10; x = 3"
+
+      it_behaves_like "block returning comparison", "x == 3"
+      it_behaves_like "block returning comparison", "x = 3; a"
+    end
+  end
+end