Remove unused `DepthFirst` visitor

We only use `ToSQL` visitors in the our codebase, do not use
`DepthFirst` and `Dot` visitors.

The `DepthFirst` visitor (which was introduced at c86c37e5f) is used to
traverse an Arel (partial) ast with depth first.

Is there any worth to keep that undocumented feature with much code and
test cases.

This removes that unused `DepthFirst` code and test cases.

3 files changed, 0 insertions, 212 deletions

diff --git a/activerecord/lib/arel/nodes/node.rb b/activerecord/lib/arel/nodes/node.rb
index 8086102bde..0416ff58de 100644
--- a/activerecord/lib/arel/nodes/node.rb
+++ b/activerecord/lib/arel/nodes/node.rb
@@ -6,7 +6,6 @@ module Arel # :nodoc: all
     # Abstract base class for all AST nodes
     class Node
       include Arel::FactoryMethods
-      include Enumerable
 
       ###
       # Factory method to create a Nodes::Not node that has the recipient of
@@ -38,13 +37,6 @@ module Arel # :nodoc: all
         collector = engine.connection.visitor.accept self, collector
         collector.value
       end
-
-      # Iterate through AST, nodes will be yielded depth-first
-      def each(&block)
-        return enum_for(:each) unless block_given?
-
-        ::Arel::Visitors::DepthFirst.new(block).accept self
-      end
     end
   end
 end
diff --git a/activerecord/lib/arel/visitors.rb b/activerecord/lib/arel/visitors.rb
index e350f52e65..a1097f6750 100644
--- a/activerecord/lib/arel/visitors.rb
+++ b/activerecord/lib/arel/visitors.rb
@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require "arel/visitors/visitor"
-require "arel/visitors/depth_first"
 require "arel/visitors/to_sql"
 require "arel/visitors/sqlite"
 require "arel/visitors/postgresql"
diff --git a/activerecord/lib/arel/visitors/depth_first.rb b/activerecord/lib/arel/visitors/depth_first.rb
deleted file mode 100644
index 98c3f92cf1..0000000000
--- a/activerecord/lib/arel/visitors/depth_first.rb
+++ /dev/null
@@ -1,203 +0,0 @@
-# frozen_string_literal: true
-
-module Arel # :nodoc: all
-  module Visitors
-    class DepthFirst < Arel::Visitors::Visitor
-      def initialize(block = nil)
-        @block = block || Proc.new
-        super()
-      end
-
-      private
-        def visit(o, _ = nil)
-          super
-          @block.call o
-        end
-
-        def unary(o)
-          visit o.expr
-        end
-        alias :visit_Arel_Nodes_Else              :unary
-        alias :visit_Arel_Nodes_Group             :unary
-        alias :visit_Arel_Nodes_Cube              :unary
-        alias :visit_Arel_Nodes_RollUp            :unary
-        alias :visit_Arel_Nodes_GroupingSet       :unary
-        alias :visit_Arel_Nodes_GroupingElement   :unary
-        alias :visit_Arel_Nodes_Grouping          :unary
-        alias :visit_Arel_Nodes_Having            :unary
-        alias :visit_Arel_Nodes_Lateral           :unary
-        alias :visit_Arel_Nodes_Limit             :unary
-        alias :visit_Arel_Nodes_Not               :unary
-        alias :visit_Arel_Nodes_Offset            :unary
-        alias :visit_Arel_Nodes_On                :unary
-        alias :visit_Arel_Nodes_Ordering          :unary
-        alias :visit_Arel_Nodes_Ascending         :unary
-        alias :visit_Arel_Nodes_Descending        :unary
-        alias :visit_Arel_Nodes_UnqualifiedColumn :unary
-        alias :visit_Arel_Nodes_OptimizerHints    :unary
-        alias :visit_Arel_Nodes_ValuesList        :unary
-
-        def function(o)
-          visit o.expressions
-          visit o.alias
-          visit o.distinct
-        end
-        alias :visit_Arel_Nodes_Avg    :function
-        alias :visit_Arel_Nodes_Exists :function
-        alias :visit_Arel_Nodes_Max    :function
-        alias :visit_Arel_Nodes_Min    :function
-        alias :visit_Arel_Nodes_Sum    :function
-
-        def visit_Arel_Nodes_NamedFunction(o)
-          visit o.name
-          visit o.expressions
-          visit o.distinct
-          visit o.alias
-        end
-
-        def visit_Arel_Nodes_Count(o)
-          visit o.expressions
-          visit o.alias
-          visit o.distinct
-        end
-
-        def visit_Arel_Nodes_Case(o)
-          visit o.case
-          visit o.conditions
-          visit o.default
-        end
-
-        def nary(o)
-          o.children.each { |child| visit child }
-        end
-        alias :visit_Arel_Nodes_And :nary
-
-        def binary(o)
-          visit o.left
-          visit o.right
-        end
-        alias :visit_Arel_Nodes_As                 :binary
-        alias :visit_Arel_Nodes_Assignment         :binary
-        alias :visit_Arel_Nodes_Between            :binary
-        alias :visit_Arel_Nodes_Concat             :binary
-        alias :visit_Arel_Nodes_DeleteStatement    :binary
-        alias :visit_Arel_Nodes_DoesNotMatch       :binary
-        alias :visit_Arel_Nodes_Equality           :binary
-        alias :visit_Arel_Nodes_FullOuterJoin      :binary
-        alias :visit_Arel_Nodes_GreaterThan        :binary
-        alias :visit_Arel_Nodes_GreaterThanOrEqual :binary
-        alias :visit_Arel_Nodes_In                 :binary
-        alias :visit_Arel_Nodes_InfixOperation     :binary
-        alias :visit_Arel_Nodes_JoinSource         :binary
-        alias :visit_Arel_Nodes_InnerJoin          :binary
-        alias :visit_Arel_Nodes_LessThan           :binary
-        alias :visit_Arel_Nodes_LessThanOrEqual    :binary
-        alias :visit_Arel_Nodes_Matches            :binary
-        alias :visit_Arel_Nodes_NotEqual           :binary
-        alias :visit_Arel_Nodes_NotIn              :binary
-        alias :visit_Arel_Nodes_NotRegexp          :binary
-        alias :visit_Arel_Nodes_IsNotDistinctFrom  :binary
-        alias :visit_Arel_Nodes_IsDistinctFrom     :binary
-        alias :visit_Arel_Nodes_Or                 :binary
-        alias :visit_Arel_Nodes_OuterJoin          :binary
-        alias :visit_Arel_Nodes_Regexp             :binary
-        alias :visit_Arel_Nodes_RightOuterJoin     :binary
-        alias :visit_Arel_Nodes_TableAlias         :binary
-        alias :visit_Arel_Nodes_When               :binary
-
-        def visit_Arel_Nodes_StringJoin(o)
-          visit o.left
-        end
-
-        def visit_Arel_Attribute(o)
-          visit o.relation
-          visit o.name
-        end
-        alias :visit_Arel_Attributes_Integer :visit_Arel_Attribute
-        alias :visit_Arel_Attributes_Float :visit_Arel_Attribute
-        alias :visit_Arel_Attributes_String :visit_Arel_Attribute
-        alias :visit_Arel_Attributes_Time :visit_Arel_Attribute
-        alias :visit_Arel_Attributes_Boolean :visit_Arel_Attribute
-        alias :visit_Arel_Attributes_Attribute :visit_Arel_Attribute
-        alias :visit_Arel_Attributes_Decimal :visit_Arel_Attribute
-
-        def visit_Arel_Table(o)
-          visit o.name
-        end
-
-        def terminal(o)
-        end
-        alias :visit_ActiveSupport_Multibyte_Chars :terminal
-        alias :visit_ActiveSupport_StringInquirer  :terminal
-        alias :visit_Arel_Nodes_Lock               :terminal
-        alias :visit_Arel_Nodes_Node               :terminal
-        alias :visit_Arel_Nodes_SqlLiteral         :terminal
-        alias :visit_Arel_Nodes_BindParam          :terminal
-        alias :visit_Arel_Nodes_Window             :terminal
-        alias :visit_Arel_Nodes_True               :terminal
-        alias :visit_Arel_Nodes_False              :terminal
-        alias :visit_BigDecimal                    :terminal
-        alias :visit_Class                         :terminal
-        alias :visit_Date                          :terminal
-        alias :visit_DateTime                      :terminal
-        alias :visit_FalseClass                    :terminal
-        alias :visit_Float                         :terminal
-        alias :visit_Integer                       :terminal
-        alias :visit_NilClass                      :terminal
-        alias :visit_String                        :terminal
-        alias :visit_Symbol                        :terminal
-        alias :visit_Time                          :terminal
-        alias :visit_TrueClass                     :terminal
-
-        def visit_Arel_Nodes_InsertStatement(o)
-          visit o.relation
-          visit o.columns
-          visit o.values
-        end
-
-        def visit_Arel_Nodes_SelectCore(o)
-          visit o.projections
-          visit o.source
-          visit o.wheres
-          visit o.groups
-          visit o.windows
-          visit o.havings
-        end
-
-        def visit_Arel_Nodes_SelectStatement(o)
-          visit o.cores
-          visit o.orders
-          visit o.limit
-          visit o.lock
-          visit o.offset
-        end
-
-        def visit_Arel_Nodes_UpdateStatement(o)
-          visit o.relation
-          visit o.values
-          visit o.wheres
-          visit o.orders
-          visit o.limit
-        end
-
-        def visit_Arel_Nodes_Comment(o)
-          visit o.values
-        end
-
-        def visit_Array(o)
-          o.each { |i| visit i }
-        end
-        alias :visit_Set :visit_Array
-
-        def visit_Hash(o)
-          o.each { |k, v| visit(k); visit(v) }
-        end
-
-        DISPATCH = dispatch_cache
-
-        def get_dispatch_cache
-          DISPATCH
-        end
-    end
-  end
-end