rust/src/doc/trpl

% The Rust Programming Language

Welcome! This book will teach you about the Rust Programming Language. Rust is a systems programming language focused on three goals: safety, speed, and concurrency. It maintains these goals without having a garbage collector, making it a useful language for a number of use cases other languages aren’t good at: embedding in other languages, programs with specific space and time requirements, and writing low-level code, like device drivers and operating systems. It improves on current languages targeting this space by having a number of compile-time safety checks that produce no runtime overhead, while eliminating all data races. Rust also aims to achieve ‘zero-cost abstractions’ even though some of these abstractions feel like those of a high-level language. Even then, Rust still allows precise control like a low-level language would.

“The Rust Programming Language” is split into eight sections. This introduction is the first. After this:

• Getting started - Set up your computer for Rust development.
• Learn Rust - Learn Rust programming through small projects.
• Effective Rust - Higher-level concepts for writing excellent Rust code.
• Syntax and Semantics - Each bit of Rust, broken down into small chunks.
• Nightly Rust - Cutting-edge features that aren’t in stable builds yet.
• Glossary - A reference of terms used in the book.
• Bibliography - Background on Rust's influences, papers about Rust.

After reading this introduction, you’ll want to dive into either ‘Learn Rust’ or ‘Syntax and Semantics’, depending on your preference: ‘Learn Rust’ if you want to dive in with a project, or ‘Syntax and Semantics’ if you prefer to start small, and learn a single concept thoroughly before moving onto the next. Copious cross-linking connects these parts together.

Contributing

The source files from which this book is generated can be found on Github: github.com/rust-lang/rust/tree/master/src/doc/trpl

A brief introduction to Rust

Is Rust a language you might be interested in? Let’s examine a few small code samples to show off a few of its strengths.

The main concept that makes Rust unique is called ‘ownership’. Consider this small example:

fn main() {
    let mut x = vec!["Hello", "world"];
}

This program makes a variable binding named x. The value of this binding is a Vec<T>, a ‘vector’, that we create through a macro defined in the standard library. This macro is called vec, and we invoke macros with a !. This follows a general principle of Rust: make things explicit. Macros can do significantly more complicated things than function calls, and so they’re visually distinct. The ! also helps with parsing, making tooling easier to write, which is also important.

We used mut to make x mutable: bindings are immutable by default in Rust. We’ll be mutating this vector later in the example.

It’s also worth noting that we didn’t need a type annotation here: while Rust is statically typed, we didn’t need to explicitly annotate the type. Rust has type inference to balance out the power of static typing with the verbosity of annotating types.

Rust prefers stack allocation to heap allocation: x is placed directly on the stack. However, the Vec<T> type allocates space for the elements of the vector on the heap. If you’re not familiar with this distinction, you can ignore it for now, or check out ‘The Stack and the Heap’. As a systems programming language, Rust gives you the ability to control how your memory is allocated, but when we’re getting started, it’s less of a big deal.

Earlier, we mentioned that ‘ownership’ is the key new concept in Rust. In Rust parlance, x is said to ‘own’ the vector. This means that when x goes out of scope, the vector’s memory will be de-allocated. This is done deterministically by the Rust compiler, rather than through a mechanism such as a garbage collector. In other words, in Rust, you don’t call functions like malloc and free yourself: the compiler statically determines when you need to allocate or deallocate memory, and inserts those calls itself. To err is to be human, but compilers never forget.

Let’s add another line to our example:

fn main() {
    let mut x = vec!["Hello", "world"];

    let y = &x[0];
}

We’ve introduced another binding, y. In this case, y is a ‘reference’ to the first element of the vector. Rust’s references are similar to pointers in other languages, but with additional compile-time safety checks. References interact with the ownership system by ‘borrowing’ what they point to, rather than owning it. The difference is, when the reference goes out of scope, it will not deallocate the underlying memory. If it did, we’d de-allocate twice, which is bad!

Let’s add a third line. It looks innocent enough, but causes a compiler error:

fn main() {
    let mut x = vec!["Hello", "world"];

    let y = &x[0];

    x.push("foo");
}

push is a method on vectors that appends another element to the end of the vector. When we try to compile this program, we get an error:

error: cannot borrow `x` as mutable because it is also borrowed as immutable
    x.push("foo");
    ^
note: previous borrow of `x` occurs here; the immutable borrow prevents
subsequent moves or mutable borrows of `x` until the borrow ends
    let y = &x[0];
             ^
note: previous borrow ends here
fn main() {

}
^

Whew! The Rust compiler gives quite detailed errors at times, and this is one of those times. As the error explains, while we made our binding mutable, we still cannot call push. This is because we already have a reference to an element of the vector, y. Mutating something while another reference exists is dangerous, because we may invalidate the reference. In this specific case, when we create the vector, we may have only allocated space for two elements. Adding a third would mean allocating a new chunk of memory for all those elements, copying the old values over, and updating the internal pointer to that memory. That all works just fine. The problem is that y wouldn’t get updated, and so we’d have a ‘dangling pointer’. That’s bad. Any use of y would be an error in this case, and so the compiler has caught this for us.

So how do we solve this problem? There are two approaches we can take. The first is making a copy rather than using a reference:

fn main() {
    let mut x = vec!["Hello", "world"];

    let y = x[0].clone();

    x.push("foo");
}

Rust has move semantics by default, so if we want to make a copy of some data, we call the clone() method. In this example, y is no longer a reference to the vector stored in x, but a copy of its first element, "Hello". Now that we don’t have a reference, our push() works just fine.

If we truly want a reference, we need the other option: ensure that our reference goes out of scope before we try to do the mutation. That looks like this:

fn main() {
    let mut x = vec!["Hello", "world"];

    {
        let y = &x[0];
    }

    x.push("foo");
}

We created an inner scope with an additional set of curly braces. y will go out of scope before we call push(), and so we’re all good.

This concept of ownership isn’t just good for preventing dangling pointers, but an entire set of related problems, like iterator invalidation, concurrency, and more.