Layout

First off, we need to come up with the struct layout. A Vec has three parts: a pointer to the allocation, the size of the allocation, and the number of elements that have been initialized.

Naively, this means we just want this design:

pub struct Vec<T> { ptr: *mut T, cap: usize, len: usize, } fn main() {}
pub struct Vec<T> {
    ptr: *mut T,
    cap: usize,
    len: usize,
}

And indeed this would compile. Unfortunately, it would be incorrect. First, the compiler will give us too strict variance. So a &Vec<&'static str> couldn't be used where an &Vec<&'a str> was expected. More importantly, it will give incorrect ownership information to the drop checker, as it will conservatively assume we don't own any values of type T`T`. See the chapter on ownership and lifetimes for all the details on variance and drop check.

As we saw in the ownership chapter, we should use Unique<T> in place of *mut T`*mut T` when we have a raw pointer to an allocation we own. Unique is unstable, so we'd like to not use it if possible, though.

As a recap, Unique is a wrapper around a raw pointer that declares that:

We can implement all of the above requirements except for the last one in stable Rust:

use std::marker::PhantomData; use std::ops::Deref; use std::mem; struct Unique<T> { ptr: *const T, // *const for variance _marker: PhantomData<T>, // For the drop checker } // Deriving Send and Sync is safe because we are the Unique owners // of this data. It's like Unique<T> is "just" T. unsafe impl<T: Send> Send for Unique<T> {} unsafe impl<T: Sync> Sync for Unique<T> {} impl<T> Unique<T> { pub fn new(ptr: *mut T) -> Self { Unique { ptr: ptr, _marker: PhantomData } } } impl<T> Deref for Unique<T> { type Target = *mut T; fn deref(&self) -> &*mut T { // There's no way to cast the *const to a *mut // while also taking a reference. So we just // transmute it since it's all "just pointers". unsafe { mem::transmute(&self.ptr) } } } fn main() {}
use std::marker::PhantomData;
use std::ops::Deref;
use std::mem;

struct Unique<T> {
    ptr: *const T,              // *const for variance
    _marker: PhantomData<T>,    // For the drop checker
}

// Deriving Send and Sync is safe because we are the Unique owners
// of this data. It's like Unique<T> is "just" T.
unsafe impl<T: Send> Send for Unique<T> {}
unsafe impl<T: Sync> Sync for Unique<T> {}

impl<T> Unique<T> {
    pub fn new(ptr: *mut T) -> Self {
        Unique { ptr: ptr, _marker: PhantomData }
    }
}

impl<T> Deref for Unique<T> {
    type Target = *mut T;
    fn deref(&self) -> &*mut T {
        // There's no way to cast the *const to a *mut
        // while also taking a reference. So we just
        // transmute it since it's all "just pointers".
        unsafe { mem::transmute(&self.ptr) }
    }
}

Unfortunately the mechanism for stating that your value is non-zero is unstable and unlikely to be stabilized soon. As such we're just going to take the hit and use std's Unique:

#![feature(unique)] use std::ptr::{Unique, self}; pub struct Vec<T> { ptr: Unique<T>, cap: usize, len: usize, } fn main() {}
#![feature(unique)]

use std::ptr::{Unique, self};

pub struct Vec<T> {
    ptr: Unique<T>,
    cap: usize,
    len: usize,
}

If you don't care about the null-pointer optimization, then you can use the stable code. However we will be designing the rest of the code around enabling the optimization. In particular, Unique::new is unsafe to call, because putting null`nullinside of it is Undefined Behaviour. Our stable Unique doesn't neednew` to be unsafe because it doesn't make any interesting guarantees about its contents.