Source

File: wp-includes/rest-api.php

function rest_ensure_request( $request ) {
	if ( $request instanceof WP_REST_Request ) {
		return $request;
	}

	if ( is_string( $request ) ) {
		return new WP_REST_Request( 'GET', $request );
	}

	return new WP_REST_Request( 'GET', '', $request );
}