auto merge of #15181 : steveklabnik/rust/hello_world, r=brson
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Writing our first Rust program together. This is the most crucial step, so I go to a fairly deep level of detail. Future sections will move more quickly.
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src/doc/guide.md
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@ -106,4 +106,184 @@ call ourselves), and we can help you out. Other great resources include our
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## Hello, world!
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Now that you have Rust installed, let's write your first Rust program. It's
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traditional to make your first program in any new language one that prints the
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text "Hello, world!" to the screen. The nice thing about starting with such a
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simple program is that you can verify that your compiler isn't just installed,
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but also working properly. And printing information to the screen is a pretty
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common thing to do.
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The first thing that we need to do is make a file to put our code in. I like
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to make a projects directory in my home directory, and keep all my projects
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there. Rust does not care where your code lives.
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This actually leads to one other concern we should address: this tutorial will
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assume that you have basic familiarity with the command-line. Rust does not
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require that you know a whole ton about the command line, but until the
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language is in a more finished state, IDE support is spotty. Rust makes no
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specific demands on your editing tooling, or where your code lives.
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With that said, let's make a directory in our projects directory. Note that you
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don't need to type in the `$`s, they just indicate the start of each command:
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```{bash}
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$ mkdir ~/projects
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$ cd ~/projects
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$ mkdir hello_world
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$ cd hello_world
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```
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If you're on Windows and not using PowerShell, the `~` may not work. Consult
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the documentation for your shell for more details.
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Let's make a new source file next. I'm going to use the syntax `editor
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filename` to represent editing a file in these examples, but you should use
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whatever method you want. We'll call our file `hello_world.rs`:
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```{bash}
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$ editor hello_world.rs
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```
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Rust files always end in a `.rs` extension. If you're using more than one word
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in your file name, use an underscore. `hello_world.rs` versus `goodbye.rs`.
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Now that you've got your file open, type this in:
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```
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fn main() {
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println!("Hello, world");
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}
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```
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Save the file, and then type this into your terminal window:
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```{bash}
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$ rustc hello_world.rs
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$ ./hello_world # on Windows, this is ./hello_world.exe
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Hello, world
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```
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Success! Let's go over what just happened in detail.
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```
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fn main() {
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}
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```
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These two lines define a **function** in Rust. The `main` function is special:
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it's the beginning of every Rust program. The first line says "I'm declaring a
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function named `main`, which takes no arguments and returns nothing." If there
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were arguments, they would go inside the parentheses (`(` and `)`), and because
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we aren't returning anything from this function, we've dropped that notation
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entirely. We'll get to it later.
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You'll also note that the function is wrapped in curly braces (`{` and `}`).
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Rust requires these around all function bodies. It is also considered good
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style to put the curly brace on the same line as the function declaration, with
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one space in between.
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Next up is this line:
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```
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println!("Hello, world");
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```
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This line does all of the work in our little program. There are a number of
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details that are important here. The first is that it's indented with four
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spaces, not tabs. Please configure your editor of choice to insert four spaces
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with the tab key. We provide some sample configurations for various editors
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[here](https://github.com/rust-lang/rust/tree/master/src/etc).
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The second point is the `println!()` part. This is calling a Rust **macro**,
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which is how metaprogramming is done in Rust. If it were a function instead, it
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would look like this: `println()`. For our purposes, we don't need to worry
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about this difference. Just know that sometimes, you'll see a `!`, and that
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means that you're calling a macro instead of a normal function.
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Next, `"Hello, world"` is a **string**. Strings are a surprisingly
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complicated topic in a systems programming language, and this is a **staticly
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allocated** string. We will talk more about different kinds of allocation
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later. We pass this string as an argument to `println!`, which prints the
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string to the screen. Easy enough!
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Finally, the line ends with a semicolon (`;`). Rust is an **expression
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oriented** language, which means that most things are expressions. The `;` is
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used to indicate that this expression is over, and the next one is ready to
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begin. Most lines of Rust code end with a `;`. We will cover this in-depth
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later in the tutorial.
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Finally, actually **compiling** and **running** our program. We can compile
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with our compiler, `rustc`, by passing it the name of our source file:
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```{bash}
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$ rustc hello_world.rs
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```
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This is similar to `gcc` or `clang`, if you come from a C or C++ background. Rust
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will output a binary executable. You can see it with `ls`:
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```{bash}
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$ ls
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hello_world hello_world.rs
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```
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Or on Windows:
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```{bash}
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$ dir
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hello_world.exe hello_world.rs
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```
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There are now two files: our source code, with the `.rs`, and the executable.
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We ran the executable like this:
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```{bash}
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$ ./hello_world # or ./hello_world.exe on Windows
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```
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This prints out our `Hello, world!` text to our terminal.
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If you come from a dynamically typed language like Ruby, Python, or JavaScript,
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you may not be used to these two steps being separate. Rust is an
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**ahead-of-time compiled language**, which means that you can compile a
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program, give it to someone else, and they don't need to have Rust installed.
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If you give someone a `.rb` or `.py` or `.js` file, they need to have
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Ruby/Python/JavaScript installed, but you just need one command to both compile
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and run your program. Everything is a tradeoff in language design, and Rust has
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made its choice.
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Congratulations! You have officially written a Rust program. That makes you a
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Rust programmer! Welcome.
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Next, I'd like to introduce you to another tool, Cargo, which is used to write
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real-world Rust programs. Just using `rustc` is nice for simple things, but as
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your project grows, you'll want something to help you manage all of the options
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that it has, and to make it easy to share your code with other people and
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projects.
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## Hello, Cargo!
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