通常使用一个值
if(x===null)x=some_valid_value
忽然今天看见一段代码

pub fn get_id() -> u64 
{
let mut res = struct.data.borrow_mut();
*res.get_or_insert_with(||{let mut xx = ...... some logiclet id = xx.id; id});
}

感觉这个名字蛮奇怪的 insert
然后翻了一下代码，是来自option.rs，原来就是空则赋值，这个写法对我来说蛮新奇的

    #[inline]#[stable(feature = "option_entry", since = "1.20.0")]#[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]pub const fn get_or_insert_with<F>(&mut self, f: F) -> &mut TwhereF: ~const FnOnce() -> T,F: ~const Destruct,{if let None = *self {// the compiler isn't smart enough to know that we are not dropping a `T`// here and wants us to ensure `T` can be dropped at compile time.mem::forget(mem::replace(self, Some(f())))}// SAFETY: a `None` variant for `self` would have been replaced by a `Some`// variant in the code above.unsafe { self.as_mut().unwrap_unchecked() }}