Fearless Concurrency

Ensuring memory safety isn't Rust's only goal: being a language that is better equipped to handle concurrent and parallel programming has always been another major goal of Rust. Concurrent programming, where different parts of a program execute independently, and parallel programming, where different parts of a program are executing at the same time, are becoming more important as more computers have multiple processors for our programs to take advantage of. Historically, programming in these contexts has been difficult and error prone: Rust hopes to change that.

Originally, we thought that memory safety and preventing concurrency problems were two separate challenges to be solved with different methods. However, over time, we discovered that ownership and the type system are a powerful set of tools that help in dealing with both memory safety and concurrency problems! By leveraging ownership and type checking, many concurrency errors are compile time errors in Rust, rather than runtime errors. We've nicknamed this aspect of Rust fearless concurrency. Fearless concurrency means Rust not only allows you to have confidence that your code is free of subtle bugs, but also lets you refactor this kind of code easily without worrying about introducing new bugs.

Note: given that Rust's slogan is fearless concurrency, we'll be referring to many of the problems here as concurrent rather than being more precise by saying concurrent and/or parallel, for simplicity's sake. If this were a book specifically about concurrency and/or parallelism, we'd be sure to be more specific. For this chapter, please mentally substitute concurrent and/or parallel whenever we say concurrent.

Many languages are strongly opinionated about the solutions they offer you to deal with concurrent problems. That's a very reasonable strategy, especially for higher-level languages, but lower-level languages don't have that luxury. Lower-level languages are expected to enable whichever solution would provide the best performance in a given situation, and they have fewer abstractions over the hardware. Rust, therefore, gives us a variety of tools for modeling our problems in whatever way is appropriate for our situation and requirements.

Here's what we'll cover in this chapter:

• How to create threads to run multiple pieces of code at the same time
• Message passing concurrency, where channels are used to send messages between threads.
• Shared state concurrency, where multiple threads have access to some piece of data.
• The Sync and Send traits, which allow Rust's concurrency guarantees to be extended to user-defined types as well as types provided by the standard library.