Deprecate passing a string as third argument of `add_index`

This was there due historical reasons since
7dc45818dc to give the user the
possibility to create unique indexes passing "UNIQUE" as the third
argument

activerecord/CHANGELOG.md

@@ -1,5 +1,12 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Deprecate the possibility to pass a string as third argument of `add_index`.
+    Pass `unique: true` instead.
+
+        add_index(:users, :organization_id, unique: true)
+
+    *Rafael Mendonça França*
+
 *   Raise an `ArgumentError` when passing an invalid option to `add_index`.
 
     *Rafael Mendonça França*

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -629,6 +629,12 @@ module ActiveRecord
               index_options = options[:where] ? " WHERE #{options[:where]}" : ""
             end
           else
+            message = "Passing a string as third argument of `add_index` is deprecated and will" +
+              " be removed in Rails 4.1." +
+              " Use add_index(#{table_name.inspect}, #{column_name.inspect}, unique: true) instead"
+
+            ActiveSupport::Deprecation.warn message
+
             index_type = options
           end
 

activerecord/test/cases/migration/index_test.rb

@@ -97,6 +97,16 @@ module ActiveRecord
         end
       end
 
+      def test_deprecated_type_argument
+        message = "Passing a string as third argument of `add_index` is deprecated and will" +
+          " be removed in Rails 4.1." +
+          " Use add_index(:testings, [:foo, :bar], unique: true) instead"
+
+        assert_deprecated message do
+          connection.add_index :testings, [:foo, :bar], "UNIQUE"
+        end
+      end
+
       def test_unique_index_exists
         connection.add_index :testings, :foo, :unique => true