Send and Sync

Not everything obeys inherited mutability, though. Some types allow you to have multiple aliases of a location in memory while mutating it. Unless these types use synchronization to manage this access, they are absolutely not thread-safe. Rust captures this through the Send and Sync traits.

A type is Send if it is safe to send it to another thread.
A type is Sync if it is safe to share between threads (T is Sync if and only if &T is Send).

Send and Sync are fundamental to Rust's concurrency story. As such, a substantial amount of special tooling exists to make them work right. First and foremost, they're unsafe traits. This means that they are unsafe to implement, and other unsafe code can assume that they are correctly implemented. Since they're marker traits (they have no associated items like methods), correctly implemented simply means that they have the intrinsic properties an implementor should have. Incorrectly implementing Send or Sync can cause Undefined Behavior.

Send and Sync are also automatically derived traits. This means that, unlike every other trait, if a type is composed entirely of Send or Sync types, then it is Send or Sync. Almost all primitives are Send and Sync, and as a consequence pretty much all types you'll ever interact with are Send and Sync.

Major exceptions include:

raw pointers are neither Send nor Sync (because they have no safety guards).
UnsafeCell isn't Sync (and therefore Cell and RefCell aren't).
Rc isn't Send or Sync (because the refcount is shared and unsynchronized).

Rc and UnsafeCell are very fundamentally not thread-safe: they enable unsynchronized shared mutable state. However raw pointers are, strictly speaking, marked as thread-unsafe as more of a lint. Doing anything useful with a raw pointer requires dereferencing it, which is already unsafe. In that sense, one could argue that it would be "fine" for them to be marked as thread safe.

However it's important that they aren't thread-safe to prevent types that contain them from being automatically marked as thread-safe. These types have non-trivial untracked ownership, and it's unlikely that their author was necessarily thinking hard about thread safety. In the case of Rc, we have a nice example of a type that contains a *mut that is definitely not thread-safe.

Types that aren't automatically derived can simply implement them if desired:

#![allow(unused)]
fn main() {
struct MyBox(*mut u8);

unsafe impl Send for MyBox {}
unsafe impl Sync for MyBox {}
}

In the incredibly rare case that a type is inappropriately automatically derived to be Send or Sync, then one can also unimplement Send and Sync:

#![allow(unused)]
#![feature(negative_impls)]

fn main() {
// I have some magic semantics for some synchronization primitive!
struct SpecialThreadToken(u8);

impl !Send for SpecialThreadToken {}
impl !Sync for SpecialThreadToken {}
}

Note that in and of itself it is impossible to incorrectly derive Send and Sync. Only types that are ascribed special meaning by other unsafe code can possibly cause trouble by being incorrectly Send or Sync.

Most uses of raw pointers should be encapsulated behind a sufficient abstraction that Send and Sync can be derived. For instance all of Rust's standard collections are Send and Sync (when they contain Send and Sync types) in spite of their pervasive use of raw pointers to manage allocations and complex ownership. Similarly, most iterators into these collections are Send and Sync because they largely behave like an & or &mut into the collection.

Example

Box is implemented as its own special intrinsic type by the compiler for various reasons, but we can implement something with similar-ish behavior ourselves to see an example of when it is sound to implement Send and Sync. Let's call it a Carton.

We start by writing code to take a value allocated on the stack and transfer it to the heap.

#![allow(unused)]
fn main() {
pub mod libc {
   pub use ::std::os::raw::{c_int, c_void};
   #[allow(non_camel_case_types)]
   pub type size_t = usize;
   extern "C" { pub fn posix_memalign(memptr: *mut *mut c_void, align: size_t, size: size_t) -> c_int; }
}
use std::{
    mem::{align_of, size_of},
    ptr,
    cmp::max,
};

struct Carton<T>(ptr::NonNull<T>);

impl<T> Carton<T> {
    pub fn new(value: T) -> Self {
        // Allocate enough memory on the heap to store one T.
        assert_ne!(size_of::<T>(), 0, "Zero-sized types are out of the scope of this example");
        let mut memptr: *mut T = ptr::null_mut();
        unsafe {
            let ret = libc::posix_memalign(
                (&mut memptr as *mut *mut T).cast(),
                max(align_of::<T>(), size_of::<usize>()),
                size_of::<T>()
            );
            assert_eq!(ret, 0, "Failed to allocate or invalid alignment");
        };

        // NonNull is just a wrapper that enforces that the pointer isn't null.
        let ptr = {
            // Safety: memptr is dereferenceable because we created it from a
            // reference and have exclusive access.
            ptr::NonNull::new(memptr)
                .expect("Guaranteed non-null if posix_memalign returns 0")
        };

        // Move value from the stack to the location we allocated on the heap.
        unsafe {
            // Safety: If non-null, posix_memalign gives us a ptr that is valid
            // for writes and properly aligned.
            ptr.as_ptr().write(value);
        }

        Self(ptr)
    }
}
}

This isn't very useful, because once our users give us a value they have no way to access it. Box implements Deref and DerefMut so that you can access the inner value. Let's do that.

#![allow(unused)]
fn main() {
use std::ops::{Deref, DerefMut};

struct Carton<T>(std::ptr::NonNull<T>);

impl<T> Deref for Carton<T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        unsafe {
            // Safety: The pointer is aligned, initialized, and dereferenceable
            //   by the logic in [`Self::new`]. We require readers to borrow the
            //   Carton, and the lifetime of the return value is elided to the
            //   lifetime of the input. This means the borrow checker will
            //   enforce that no one can mutate the contents of the Carton until
            //   the reference returned is dropped.
            self.0.as_ref()
        }
    }
}

impl<T> DerefMut for Carton<T> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        unsafe {
            // Safety: The pointer is aligned, initialized, and dereferenceable
            //   by the logic in [`Self::new`]. We require writers to mutably
            //   borrow the Carton, and the lifetime of the return value is
            //   elided to the lifetime of the input. This means the borrow
            //   checker will enforce that no one else can access the contents
            //   of the Carton until the mutable reference returned is dropped.
            self.0.as_mut()
        }
    }
}
}

Finally, let's think about whether our Carton is Send and Sync. Something can safely be Send unless it shares mutable state with something else without enforcing exclusive access to it. Each Carton has a unique pointer, so we're good.

#![allow(unused)]
fn main() {
struct Carton<T>(std::ptr::NonNull<T>);
// Safety: No one besides us has the raw pointer, so we can safely transfer the
// Carton to another thread if T can be safely transferred.
unsafe impl<T> Send for Carton<T> where T: Send {}
}

What about Sync? For Carton to be Sync we have to enforce that you can't write to something stored in a &Carton while that same something could be read or written to from another &Carton. Since you need an &mut Carton to write to the pointer, and the borrow checker enforces that mutable references must be exclusive, there are no soundness issues making Carton sync either.

#![allow(unused)]
fn main() {
struct Carton<T>(std::ptr::NonNull<T>);
// Safety: Since there exists a public way to go from a `&Carton<T>` to a `&T`
// in an unsynchronized fashion (such as `Deref`), then `Carton<T>` can't be
// `Sync` if `T` isn't.
// Conversely, `Carton` itself does not use any interior mutability whatsoever:
// all the mutations are performed through an exclusive reference (`&mut`). This
// means it suffices that `T` be `Sync` for `Carton<T>` to be `Sync`:
unsafe impl<T> Sync for Carton<T> where T: Sync  {}
}

When we assert our type is Send and Sync we usually need to enforce that every contained type is Send and Sync. When writing custom types that behave like standard library types we can assert that we have the same requirements. For example, the following code asserts that a Carton is Send if the same sort of Box would be Send, which in this case is the same as saying T is Send.

#![allow(unused)]
fn main() {
struct Carton<T>(std::ptr::NonNull<T>);
unsafe impl<T> Send for Carton<T> where Box<T>: Send {}
}

Right now Carton<T> has a memory leak, as it never frees the memory it allocates. Once we fix that we have a new requirement we have to ensure we meet to be Send: we need to know free can be called on a pointer that was yielded by an allocation done on another thread. We can check this is true in the docs for libc::free.

#![allow(unused)]
fn main() {
struct Carton<T>(std::ptr::NonNull<T>);
mod libc {
    pub use ::std::os::raw::c_void;
    extern "C" { pub fn free(p: *mut c_void); }
}
impl<T> Drop for Carton<T> {
    fn drop(&mut self) {
        unsafe {
            libc::free(self.0.as_ptr().cast());
        }
    }
}
}

A nice example where this does not happen is with a MutexGuard: notice how it is not Send. The implementation of MutexGuard uses libraries that require you to ensure you don't try to free a lock that you acquired in a different thread. If you were able to Send a MutexGuard to another thread the destructor would run in the thread you sent it to, violating the requirement. MutexGuard can still be Sync because all you can send to another thread is an &MutexGuard and dropping a reference does nothing.

TODO: better explain what can or can't be Send or Sync. Sufficient to appeal only to data races?

The Rustonomicon

Send and Sync

Example