[Fix #11878] Add new Style/MapIntoArray cop

Fixes #11878.

This PR adds new `Style/MapIntoArray` cop.
This cop checks for usages of `each` with `<<`, `push`, or `append`
which can be replaced by `map`. It is unsafe, because it can produce
false positives if the receiver is not an `Enumerable`.

```ruby
# bad
dest = []
src.each { |e| dest << e * 2 }
dest

# good
dest = src.map { |e| e * 2 }

# good - contains another operation
dest = []
src.each { |e| dest << e * 2; puts e }
dest
```

changelog/new_add_new_style_map_into_array_cop.md

@@ -0,0 +1 @@
+* [#11878](https://github.com/rubocop/rubocop/issues/11878): Add new `Style/MapIntoArray` cop. ([@ymap][])

config/default.yml

@@ -4289,6 +4289,13 @@ Style/MapCompactWithConditionalBlock:
   Enabled: pending
   VersionAdded: '1.30'
 
+Style/MapIntoArray:
+  Description: 'Checks for usages of `each` with `<<`, `push`, or `append` which can be replaced by `map`.'
+  StyleGuide: '#functional-code'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+  Safe: false
+
 Style/MapToHash:
   Description: 'Prefer `to_h` with a block over `map.to_h`.'
   Enabled: pending

lib/rubocop.rb

@@ -557,6 +557,7 @@
 require_relative 'rubocop/cop/style/lambda_call'
 require_relative 'rubocop/cop/style/line_end_concatenation'
 require_relative 'rubocop/cop/style/magic_comment_format'
+require_relative 'rubocop/cop/style/map_into_array'
 require_relative 'rubocop/cop/style/map_to_hash'
 require_relative 'rubocop/cop/style/map_to_set'
 require_relative 'rubocop/cop/style/method_call_without_args_parentheses'

lib/rubocop/cop/style/map_into_array.rb

@@ -0,0 +1,175 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Style
+      # Checks for usages of `each` with `<<`, `push`, or `append` which
+      # can be replaced by `map`.
+      #
+      # If `PreferredMethods` is configured for `map` in `Style/CollectionMethods`,
+      # this cop uses the specified method for replacement.
+      #
+      # NOTE: The return value of `Enumerable#each` is `self`, whereas the
+      # return value of `Enumerable#map` is an `Array`. They are not autocorrected
+      # when a return value could be used because these types differ.
+      #
+      # NOTE: It only detects when the mapping destination is a local variable
+      # initialized as an empty array and referred to only by the pushing operation.
+      # This is because, if not, it's challenging to statically guarantee that the
+      # mapping destination variable remains an empty array:
+      #
+      # [source,ruby]
+      # ----
+      # @dest = []
+      # src.each { |e| @dest << e * 2 } # `src` method may mutate `@dest`
+      #
+      # dest = []
+      # src.each { |e| dest << transform(e, dest) } # `transform` method may mutate `dest`
+      # ----
+      #
+      # @safety
+      #   This cop is unsafe because not all objects that have an `each`
+      #   method also have a `map` method (e.g. `ENV`). Additionally, for calls
+      #   with a block, not all objects that have a `map` method return an array
+      #   (e.g. `Enumerator::Lazy`).
+      #
+      # @example
+      #   # bad
+      #   dest = []
+      #   src.each { |e| dest << e * 2 }
+      #   dest
+      #
+      #   # good
+      #   dest = src.map { |e| e * 2 }
+      #
+      #   # good - contains another operation
+      #   dest = []
+      #   src.each { |e| dest << e * 2; puts e }
+      #   dest
+      #
+      class MapIntoArray < Base
+        include RangeHelp
+        extend AutoCorrector
+
+        MSG = 'Use `%<new_method_name>s` instead of `each` to map elements into an array.'
+
+        # @!method each_block_with_push?(node)
+        def_node_matcher :each_block_with_push?, <<-PATTERN
+          [
+            ^({begin kwbegin} ...)
+            ({block numblock} (send _ :each) _
+              (send (lvar _) {:<< :push :append} _))
+          ]
+        PATTERN
+
+        # @!method empty_array_asgn?(node)
+        def_node_matcher :empty_array_asgn?, '(lvasgn _ (array))'
+
+        # @!method lvar_ref?(node, name)
+        def_node_matcher :lvar_ref?, '(lvar %1)'
+
+        def self.joining_forces
+          VariableForce
+        end
+
+        def after_leaving_scope(scope, _variable_table)
+          (@scopes ||= []) << scope
+        end
+
+        def on_block(node)
+          return unless each_block_with_push?(node)
+
+          dest_var = find_dest_var(node)
+          return unless (asgn = find_closest_assignment(node, dest_var))
+          return unless empty_array_asgn?(asgn)
+          return unless dest_used_only_for_mapping?(node, dest_var, asgn)
+
+          register_offense(node, dest_var, asgn)
+        end
+
+        alias on_numblock on_block
+
+        private
+
+        def find_dest_var(block)
+          node = block.body.receiver
+          name = node.children.first
+
+          candidates = @scopes.lazy.filter_map { |s| s.variables[name] }
+          candidates.find { |v| v.references.any? { |n| n.node.equal?(node) } }
+        end
+
+        def find_closest_assignment(block, dest_var)
+          dest_var.assignments.reverse_each.lazy.map(&:node).find do |node|
+            node.source_range.end_pos < block.source_range.begin_pos
+          end
+        end
+
+        def dest_used_only_for_mapping?(block, dest_var, asgn)
+          range = asgn.source_range.join(block.source_range)
+
+          asgn.parent.equal?(block.parent) &&
+            dest_var.references.one? { |r| range.contains?(r.node.source_range) } &&
+            dest_var.assignments.one? { |a| range.contains?(a.node.source_range) }
+        end
+
+        def register_offense(block, dest_var, asgn)
+          add_offense(block, message: format(MSG, new_method_name: new_method_name)) do |corrector|
+            next if return_value_used?(block)
+
+            corrector.replace(block.send_node.selector, new_method_name)
+            remove_assignment(corrector, asgn)
+            correct_push_node(corrector, block.body)
+            correct_return_value_handling(corrector, block, dest_var)
+          end
+        end
+
+        def new_method_name
+          default = 'map'
+          alternative = config.for_cop('Style/CollectionMethods').dig('PreferredMethods', default)
+          alternative || default
+        end
+
+        def return_value_used?(node)
+          parent = node.parent
+
+          case parent&.type
+          when nil
+            false
+          when :begin, :kwbegin
+            !node.right_sibling && return_value_used?(parent)
+          when :block, :numblock
+            !parent.void_context?
+          else
+            true
+          end
+        end
+
+        def remove_assignment(corrector, asgn)
+          range = range_with_surrounding_space(asgn.source_range, side: :right)
+          range = range_with_surrounding_space(range, side: :right, newlines: false)
+
+          corrector.remove(range)
+        end
+
+        def correct_push_node(corrector, push_node)
+          range = push_node.source_range
+          arg_range = push_node.first_argument.source_range
+
+          corrector.remove(range_between(range.begin_pos, arg_range.begin_pos))
+          corrector.remove(range_between(arg_range.end_pos, range.end_pos))
+        end
+
+        def correct_return_value_handling(corrector, block, dest_var)
+          next_node = block.right_sibling
+
+          if lvar_ref?(next_node, dest_var.name)
+            corrector.remove(range_with_surrounding_space(next_node.source_range, side: :left))
+          end
+
+          corrector.insert_before(block, "#{dest_var.name} = ")
+        end
+      end
+    end
+  end
+end