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.

1 files changed, 0 insertions, 203 deletions

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