src/doc/trpl/README.md

   1 % The Rust Programming Language
   2
   3 Welcome! This book will teach you about the [Rust Programming Language][rust].
   4 Rust is a systems programming language focused on three goals: safety, speed,
   5 and concurrency. It maintains these goals without having a garbage collector,
   6 making it a useful language for a number of use cases other languages aren’t
   7 good at: embedding in other languages, programs with specific space and time
   8 requirements, and writing low-level code, like device drivers and operating
   9 systems. It improves on current languages targeting this space by having a
  10 number of compile-time safety checks that produce no runtime overhead, while
  11 eliminating all data races. Rust also aims to achieve ‘zero-cost abstrations’
  12 even though some of these abstractions feel like those of a high-level
  13 language. Even then, Rust still allows precise control like a low-level
  14 language would.
  15
  16 [rust]: http://rust-lang.org
  17
  18 “The Rust Programming Language” is split into seven sections. This introduction
  19 is the first. After this:
  20
  21 * [Getting started][gs] - Set up your computer for Rust development.
  22 * [Learn Rust][lr] - Learn Rust programming through small projects.
  23 * [Effective Rust][er] - Higher-level concepts for writing excellent Rust code.
  24 * [Syntax and Semantics][ss] - Each bit of Rust, broken down into small chunks.
  25 * [Nightly Rust][nr] - Cutting-edge features that aren’t in stable builds yet.
  26 * [Glossary][gl] - A reference of terms used in the book.
  27
  28 [gs]: getting-started.html
  29 [lr]: learn-rust.html
  30 [er]: effective-rust.html
  31 [ss]: syntax-and-semantics.html
  32 [nr]: nightly-rust.html
  33 [gl]: glossary.html
  34
  35 After reading this introduction, you’ll want to dive into either ‘Learn Rust’
  36 or ‘Syntax and Semantics’, depending on your preference: ‘Learn Rust’ if you
  37 want to dive in with a project, or ‘Syntax and Semantics’ if you prefer to
  38 start small, and learn a single concept thoroughly before moving onto the next.
  39 Copious cross-linking connects these parts together.
  40
  41 ## A brief introduction to Rust
  42
  43 Is Rust a language you might be interested in? Let’s examine a few small code
  44 samples to show off a few of its strengths.
  45
  46 The main concept that makes Rust unique is called ‘ownership’. Consider this
  47 small example:
  48
  49 ```rust
  50 fn main() {
  51     let mut x = vec!["Hello", "world"];
  52 }
  53 ```
  54
  55 This program makes a [variable binding][var] named `x`. The value of this
  56 binding is a `Vec<T>`, a ‘vector’, that we create through a [macro][macro]
  57 defined in the standard library. This macro is called `vec`, and we invoke
  58 macros with a `!`. This follows a general principle of Rust: make things
  59 explicit. Macros can do significantly more complicated things than function
  60 calls, and so they’re visually distinct. The `!` also helps with parsing,
  61 making tooling easier to write, which is also important.
  62
  63 We used `mut` to make `x` mutable: bindings are immutable by default in Rust.
  64 We’ll be mutating this vector later in the example.
  65
  66 It’s also worth noting that we didn’t need a type annotation here: while Rust
  67 is statically typed, we didn’t need to explicitly annotate the type. Rust has
  68 type inference to balance out the power of static typing with the verbosity of
  69 annotating types.
  70
  71 Rust prefers stack allocation to heap allocation: `x` is placed directly on the
  72 stack. However, the `Vec<T>` type allocates space for the elements of the
  73 vector on the heap. If you’re not familiar with this distinction, you can
  74 ignore it for now, or check out [‘The Stack and the Heap’][heap]. As a systems
  75 programming language, Rust gives you the ability to control how your memory is
  76 allocated, but when we’re getting started, it’s less of a big deal.
  77
  78 [var]: variable-bindings.html
  79 [macro]: macros.html
  80 [heap]: the-stack-and-the-heap.html
  81
  82 Earlier, we mentioned that ‘ownership’ is the key new concept in Rust. In Rust
  83 parlance, `x` is said to ‘own’ the vector. This means that when `x` goes out of
  84 scope, the vector’s memory will be de-allocated. This is done deterministically
  85 by the Rust compiler, rather than through a mechanism such as a garbage
  86 collector. In other words, in Rust, you don’t call functions like `malloc` and
  87 `free` yourself: the compiler statically determines when you need to allocate
  88 or deallocate memory, and inserts those calls itself. To err is to be human,
  89 but compilers never forget.
  90
  91 Let’s add another line to our example:
  92
  93 ```rust
  94 fn main() {
  95     let mut x = vec!["Hello", "world"];
  96
  97     let y = &x[0];
  98 }
  99 ```
 100
 101 We’ve introduced another binding, `y`. In this case, `y` is a ‘reference’ to
 102 the first element of the vector. Rust’s references are similar to pointers in
 103 other languages, but with additional compile-time safety checks. References
 104 interact with the ownership system by [‘borrowing’][borrowing] what they point
 105 to, rather than owning it. The difference is, when the reference goes out of
 106 scope, it will not deallocate the underlying memory. If it did, we’d
 107 de-allocate twice, which is bad!
 108
 109 [borrowing]: references-and-borrowing.html
 110
 111 Let’s add a third line. It looks innocent enough, but causes a compiler error:
 112
 113 ```rust,ignore
 114 fn main() {
 115     let mut x = vec!["Hello", "world"];
 116
 117     let y = &x[0];
 118
 119     x.push("foo");
 120 }
 121 ```
 122
 123 `push` is a method on vectors that appends another element to the end of the
 124 vector. When we try to compile this program, we get an error:
 125
 126 ```text
 127 error: cannot borrow `x` as mutable because it is also borrowed as immutable
 128     x.push(4);
 129     ^
 130 note: previous borrow of `x` occurs here; the immutable borrow prevents
 131 subsequent moves or mutable borrows of `x` until the borrow ends
 132     let y = &x[0];
 133              ^
 134 note: previous borrow ends here
 135 fn main() {
 136
 137 }
 138 ^
 139 ```
 140
 141 Whew! The Rust compiler gives quite detailed errors at times, and this is one
 142 of those times. As the error explains, while we made our binding mutable, we
 143 still cannot call `push`. This is because we already have a reference to an
 144 element of the vector, `y`. Mutating something while another reference exists
 145 is dangerous, because we may invalidate the reference. In this specific case,
 146 when we create the vector, we may have only allocated space for three elements.
 147 Adding a fourth would mean allocating a new chunk of memory for all those elements,
 148 copying the old values over, and updating the internal pointer to that memory.
 149 That all works just fine. The problem is that `y` wouldn’t get updated, and so
 150 we’d have a ‘dangling pointer’. That’s bad. Any use of `y` would be an error in
 151 this case, and so the compiler has caught this for us.
 152
 153 So how do we solve this problem? There are two approaches we can take. The first
 154 is making a copy rather than using a reference:
 155
 156 ```rust
 157 fn main() {
 158     let mut x = vec!["Hello", "world"];
 159
 160     let y = x[0].clone();
 161
 162     x.push("foo");
 163 }
 164 ```
 165
 166 Rust has [move semantics][move] by default, so if we want to make a copy of some
 167 data, we call the `clone()` method. In this example, `y` is no longer a reference
 168 to the vector stored in `x`, but a copy of its first element, `"hello"`. Now
 169 that we don’t have a reference, our `push()` works just fine.
 170
 171 [move]: move-semantics.html
 172
 173 If we truly want a reference, we need the other option: ensure that our reference
 174 goes out of scope before we try to do the mutation. That looks like this:
 175
 176 ```rust
 177 fn main() {
 178     let mut x = vec!["Hello", "world"];
 179
 180     {
 181         let y = &x[0];
 182     }
 183
 184     x.push("foo");
 185 }
 186 ```
 187
 188 We created an inner scope with an additional set of curly braces. `y` will go out of
 189 scope before we call `push()`, and so we’re all good.
 190
 191 This concept of ownership isn’t just good for preventing danging pointers, but an
 192 entire set of related problems, like iterator invalidation, concurrency, and more.