## Extensible Concurrency with the `Sync` and `Send` Traits

One interesting aspect of Rust's concurrency model is that the language knows very little about concurrency. Almost everything we've been talking about so far has been part of the standard library, not the language itself. Because we don't need the language to provide everything we need to program in a concurrent context, we're not limited to the concurrency options that the standard library or language provide: we can write our own or use ones others have written.

We said almost everything wasn't in the language, so what is? There are two traits, both in `std::marker`: `Sync` and `Send`.

### `Send` for Indicating Ownership May Be Transferred to Another Thread

The `Send` marker trait indicates that ownership of that type may be transferred between threads. Almost every Rust type is `Send`, but there are some exceptions. One type provided by the standard library that is not `Send` is `Rc<T>`: if we clone an `Rc<T>` value and try to transfer ownership of the clone to another thread, both threads might update the reference count at the same time. As we mentioned in the previous section, `Rc<T>` is implemented for use in single-threaded situations where you don't want to pay the performance penalty of having a threadsafe reference count.

Because `Rc<T>` is not marked `Send`, Rust's type system and trait bounds ensure that we can never forget and accidentally send an `Rc<T>` value across threads unsafely. We tried to do this in Listing 16-14, and we got an error that said `the trait Send is not implemented for Rc<Mutex<i32>>`. When we switched to `Arc<T>`, which is `Send`, the code compiled.

Any type that is composed entirely of `Send` types is automatically marked as `Send` as well. Almost all primitive types are `Send`, aside from raw pointers, which we'll discuss in Chapter 19. Most standard library types are `Send`, aside from `Rc<T>`.

### `Sync` for Indicating Access from Multiple Threads is Safe

The `Sync` marker trait indicates that a type is safe to have references to a value from multiple threads. Another way to say this is for any type `T`, `T` is `Sync` if `&T` (a reference to `T`) is `Send` so that the reference can be sent safely to another thread. In a similar manner as `Send`, primitive types are `Sync` and types composed entirely of types that are `Sync` are also `Sync`.

`Rc<T>` is also not `Sync`, for the same reasons that it's not `Send`. `RefCell<T>` (which we talked about in Chapter 15) and the family of related `Cell<T>` types are not `Sync`. The implementation of the borrow checking at runtime that `RefCell<T>` does is not threadsafe. `Mutex<T>` is `Sync`, and can be used to share access with multiple threads as we saw in the previous section.

### Implementing `Send` and `Sync` Manually is Unsafe

Usually, we don't need to implement the `Send` and `Sync` traits, since types that are made up of `Send` and `Sync` traits are automatically also `Send` and `Sync`. Because they're marker traits, they don't even have any methods to implement. They're just useful for enforcing concurrency-related invariants.

Implementing the guarantees that these traits are markers for involves implementing unsafe Rust code. We're going to be talking about using unsafe Rust code in Chapter 19; for now, the important information is that building new concurrent types that aren't made up of `Send` and `Sync` parts requires careful thought to make sure the safety guarantees are upheld. The Nomicon has more information about these guarantees and how to uphold them.

## Summary

This isn't the last we'll see of concurrency in this book; the project in Chapter 20 will use these concepts in a more realistic situation than the smaller examples we discussed in this chapter.

As we mentioned, since very little of how Rust deals with concurrency has to be part of the language, there are many concurrency solutions implemented as crates. These evolve more quickly than the standard library; search online for the current state-of-the-art crates for use in multithreaded situations.

Rust provides channels for message passing and smart pointer types like `Mutex<T>` and `Arc<T>` that are safe to use in concurrent contexts. The type system and the borrow checker will make sure the code we write using these solutions won't have data races or invalid references. Once we get our code compiling, we can rest assured that our code will happily run on multiple threads without the kinds of hard-to-track-down bugs common in other programming languages. Concurrent programming is no longer something to be afraid of: go forth and make your programs concurrent, fearlessly!

Next, let's talk about idiomatic ways to model problems and structure solutions as your Rust programs get bigger, and how Rust's idioms relate to those you might be familiar with from Object Oriented Programming.