Function warn_once

packages/svelte/src/compiler/validate-options.js:199–204

@param {(node: null) => void} fn

(fn)

/** @param {(node: null) => void} fn */
function warn_once(fn) {
	if (!warned.has(fn)) {
		warned.add(fn);
		fn(null);
	}
}

/**
 * @param {(node: null) => void} fn