rust programming

Link: https://doc.rust-lang.org/beta/nomicon/phantom-data.html

struct Iter<'a, T: 'a> { // <-- Error, I have to associate the lifetime with something...
    ptr: *const T,
    end: *const T,
}

// iter is valid as long as the underlying elements it's iterating are valid and
// this relationship is not captured

// And that's why I'm using a dummy PhantomData type, so that I can use the
// lifetime

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

• They can enable the use of lifetimes, when you don't have fields to bind them to.
• They 're just special markers, they don't consume space but just ^simulate a field of the given type for the purpose of static analysis

The drop checker will generously determine that Vec<T> does not own any values of type T. This will in turn make it conclude that it doesn't need to worry about Vec dropping any T's in its destructor for determining drop check soundness. This will in turn allow people to create unsoundness using Vec's destructor.

In order to tell dropck that we do own values of type T, and therefore may drop some T's when we drop, we must add an extra PhantomData saying exactly that: