write 'what is rustc'

src/doc/rustc/src/what-is-rustc.md

@@ -1 +1,68 @@
 # What is rustc?
+
+Welcome to "The rustc book"! `rustc` is the compiler for the Rust programming
+language, provided by the project itself. Compilers take your source code and
+produce binary code, either as a library or executable.
+
+Most Rust programmers don't invoke `rustc` directly, but instead do it through
+[Cargo](../cargo/index.html). It's all in service of `rustc` though! If you
+want to see how Cargo calls `rustc`, you can
+
+```bash
+$ cargo build --verbose
+```
+
+And it will print out each `rustc` invocation. This book can help you
+understand what each of these options does. Additionally, while most
+Rustaceans use Cargo, not all do: sometimes they integrate `rustc` into other
+build systems. This book should provide a guide to all of the options you'd
+need to do so.
+
+## Basic usage
+
+Let's say you've got a little hello world program in a file `hello.rs`:
+
+```rust
+fn main() {
+    println!("Hello, world!");
+}
+```
+
+To turn this source code into an executable, you can use `rustc`:
+
+```bash
+$ rustc hello.rs
+$ ./hello # on a *NIX
+$ .\hello.exe # on Windows
+```
+
+Note that we only ever pass `rustc` the *crate root*, not every file we wish
+to compile. For example, if we had a `main.rs` that looked like this:
+
+```rust,ignore
+mod foo;
+
+fn main() {
+    foo::hello();
+}
+```
+
+And a `foo.rs` that had this:
+
+```rust,ignore
+fn hello() {
+    println!("Hello, world!");
+}
+```
+
+To compile this, we'd run this command:
+
+```bash
+$ rustc main.rs
+```
+
+No need to tell `rustc` about `foo.rs`; the `mod` statements give it
+everything that it needs. This is different than how you would use a C
+compiler, where you invoke the compiler on each file, and then link
+everything together. In other words, the *crate* is a translation unit, not a
+particular module.