Don't consider unique index as complex

When detecting "complex" indexes to require an explicit index name,
don't raise a rubocop offense for indexes with a unique option as the
only option.

db/migrate/20200813143356_remove_old_external_diff_migration_index.rb

@@ -15,12 +15,11 @@ class RemoveOldExternalDiffMigrationIndex < ActiveRecord::Migration[6.0]
   end
 
   def down
-    # rubocop:disable Migration/ComplexIndexesRequireName
     add_concurrent_index(
       :merge_request_diffs,
       [:merge_request_id, :id],
-      where: { stored_externally: [nil, false] }
+      where: 'NOT stored_externally OR stored_externally IS NULL',
+      name: 'index_merge_request_diffs_on_merge_request_id_and_id_partial'
     )
-    # rubocop:enable Migration/ComplexIndexesRequireName
   end
 end

ee/db/geo/migrate/20170906182752_add_unique_index_to_project_id_on_project_registry.rb

@@ -7,9 +7,7 @@ class AddUniqueIndexToProjectIdOnProjectRegistry < ActiveRecord::Migration[4.2]
 
   def up
     remove_concurrent_index :project_registry, :project_id if index_exists? :project_registry, :project_id
-    # rubocop:disable Migration/ComplexIndexesRequireName
     add_concurrent_index :project_registry, :project_id, unique: true
-    # rubocop:enable Migration/ComplexIndexesRequireName
   end
 
   def down

rubocop/cop/migration/complex_indexes_require_name.rb

@@ -18,6 +18,10 @@ module RuboCop
           (pair {(sym :name) (str "name")} _)
         PATTERN
 
+        def_node_matcher :unique_option?, <<~PATTERN
+          (pair {(:sym :unique) (str "unique")} _)
+        PATTERN
+
         def on_def(node)
           return unless in_migration?(node)
 
@@ -35,8 +39,14 @@ module RuboCop
         end
 
         def needs_name_option?(option_nodes)
+          return false if only_unique_option?(option_nodes)
+
           option_nodes.none? { |node| name_option?(node) }
         end
+
+        def only_unique_option?(option_nodes)
+          option_nodes.size == 1 && unique_option?(option_nodes.first)
+        end
       end
     end
   end

spec/rubocop/cop/migration/complex_indexes_require_name_spec.rb

@@ -35,6 +35,8 @@ RSpec.describe RuboCop::Cop::Migration::ComplexIndexesRequireName, type: :ruboco
 
             def down
               add_concurrent_index :test_indexes, :column4, 'unique' => true
+
+              add_concurrent_index :test_indexes, :column4, 'unique' => true, where: 'column4 IS NOT NULL'
               ^^^^^^^^^^^^^^^^^^^^ #{described_class::MSG}
 
               add_concurrent_index :test_indexes, :column5, using: :gin, name: INDEX_NAME