Send and Sync

Not everything obeys inherited mutability, though. Some types allow you to multiply alias 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 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:

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:

#![feature(optin_builtin_traits)]

// 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 possible 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.

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