auto merge of #15181 : steveklabnik/rust/hello_world, r=brson

This is built on top of https://github.com/rust-lang/rust/pull/15162 . cccae83d92 is the only new commit, you may want to look at that rather than the whole diff.

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.

src/doc/guide.md

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