Handling concurrent programming safely and efficiently is another of Rust’s major goals. 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 increasingly important as more computers have multiple processors to take advantage of. Historically, programming in these contexts has been difficult and error prone: Rust hopes to change that.

Initially, the Rust team thought that ensuring memory safety and preventing concurrency problems were two separate challenges to be solved with different methods. Over time, they discovered that the ownership and type systems are a powerful set of tools to 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. Rather than spending lots of time trying to reproduce the exact circumstances under which a runtime concurrency bug occurs, incorrect code will refuse to compile with an error explaining the problem. This lets you fix your code while you’re working on it, rather than potentially after it’s been shipped to production. We’ve nicknamed this aspect of Rust fearless concurrency. Fearless concurrency allows you to write code that’s free of subtle bugs and is easy to refactor without introducing new bugs.

Note: 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 for dealing with concurrent problems. For example, Erlang has elegant functionality for message passing concurrency, but only obscure ways to share state between threads. Only supporting a subset of possible solutions is a reasonable strategy for higher-level languages to take, because a higher-level language promises benefits from giving up some control in order to gain abstractions. However, lower-level languages are expected to provide the solution with the best performance in any given situation, and have fewer abstractions over the hardware. Rust, therefore, gives us a variety of tools for modeling your problems in whatever way is appropriate for your 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 extend Rust’s concurrency guarantees to user-defined types as well as types provided by the standard library.