rust持续学习 get_or_insert_with

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 logic
 let 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 T
    where
        F: ~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        // SAFETY: a `None` variant for `self` would have been replaced by a `Some`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() }
    }