PhantomData to protect your pointers

https://rust-unofficial.github.io/too-many-lists/sixth-variance.html
아.................. 이건 또 뭔데, 빙한테 물어보자...
PhantomData is a Rust feature that is used when working with unsafe code. It is used when types or lifetimes are logically associated with a struct, but not actually part of a field. This most commonly occurs with lifetimes. For instance, the Iter for &'a [T] is (approximately) defined as follows:

struct Iter<'a, T: 'a> {
    ptr: *const T,
    end: *const T,
    _marker: PhantomData<&'a T>,
}

Here, _marker is a field of type PhantomData<&'a T>. The purpose of this field is to indicate that this struct has an associated lifetime 'a, even though it doesn’t actually contain any references with that lifetime.

Another important example is Vec, which is (approximately) defined as follows:

#![allow(unused)]
fn main() {
    struct Vec<T> {
        data: *const T,
        // *const for variance!
        len: usize,
        cap: usize,
        _marker: PhantomData<&'a [T]>,
    }
}

Here, _marker is a field of type PhantomData<&'a [T]>. The purpose of this field is to indicate that this struct has an associated lifetime 'a, even though it doesn’t actually contain any references with that lifetime.1 2

I hope this helps! Let me know if you have any more questions.