One usage of PhantomData in Rust.

phantom_data.md

According to the doc of PhantomData, one usafe of PhantomData is for unused lifetime parameters like:

struct Slice<'a, T: 'a> {
    start: *mut T,
    end: *mut T,
    _invariant: PhantomData<&'a T>,
}

fn foo<T>(v: &mut Vec<T>) -> Slice<'_, T> {
    let ptr = v.as_mut_ptr();
    unsafe { Slice { start: ptr, end: ptr.add(v.len()), _invariant: PhantomData } }
}

pub fn main() {
    let mut v = vec!(1, 2, 3);
    let slice = foo(&mut v);

    drop(v);

    let s1 = unsafe { &mut *slice.start };
    let s2 = slice.start;

    *s1 += 1;
    println!("{}", s1);

    unsafe { *s2 += 1 };
    println!("{}", unsafe { *s2 });
}

It causes an compile error:

error[E0505]: cannot move out of `v` because it is borrowed
  --> phantom.rs:35:10
   |
33 |     let slice = foo(&mut v);
   |                     ------ borrow of `v` occurs here
34 | 
35 |     drop(v);
   |          ^ move out of `v` occurs here
...
38 |     let s1 = unsafe { &mut *slice.start };
   |                       ----------------- borrow later used here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0505`.

If we don't use PhantomData, it generates no compile error.

struct Slice<T> {
    start: *mut T,
    end: *mut T,
}

fn foo<T>(v: &mut Vec<T>) -> Slice<T> {
    let ptr = v.as_mut_ptr();
    unsafe { Slice { start: ptr, end: ptr.add(v.len()) } }
}

pub fn main() {
    let mut v = vec!(1, 2, 3);
    let slice = foo(&mut v);

    drop(v);
    // drop(slice);

    let s1 = unsafe { &mut *slice.start };
    let s2 = slice.start;

    *s1 += 1;
    println!("{}", s1);

    unsafe { *s2 += 1 };
    println!("{}", unsafe { *s2 });
}

Borrow in foo is important here. Without it, no compile error occurs.

use std::marker::PhantomData;

struct Slice<'a, T: 'a> {
    start: *mut T,
    end: *mut T,
    _invariant: PhantomData<&'a T>,
}

pub fn main() {
    let mut v = vec!(1, 2, 3);
    let ptr = v.as_mut_ptr(); // pointer can be copied, so borrow doesn't occur.
    let slice = unsafe { Slice { start: ptr, end: ptr.add(v.len()), _invariant: PhantomData } };

    drop(v);
    // drop(slice);

    let s1 = unsafe { &mut *slice.start };
    let s2 = slice.start;

    *s1 += 1;
    println!("{}", s1);

    unsafe { *s2 += 1 };
    println!("{}", unsafe { *s2 });
}