How do I tell Cargo to build files other than main.rs?

Here is my directory structure:

lowks@lowkster ~/src/rustlang/gettingrusty $ tree .
.
├── Cargo.lock
├── Cargo.toml
├── foo.txt
├── src
│   ├── boolean_example.rs
│   ├── function_goodbye_world.rs
│   ├── listdir.rs
│   ├── looping.rs
│   ├── main.rs
│   ├── pattern_match.rs
│   └── write_to_file.rs
└── target
    ├── build
    ├── deps
    ├── examples
    ├── gettingrusty
    └── native

6 directories, 11 files

When I run 'cargo build', it seems to only build main.rs. How should I change Cargo.toml to build the rest of the files too?

The Rust compiler compiles all the files at the same time to build a crate, which is either an executable or a library. To add files to your crate, add mod items to your crate root (here, main.rs) or to other modules:

mod boolean_example;
mod function_goodbye_world;
mod listdir;
mod looping;
mod pattern_match;
mod write_to_file;

To access items defined in another module from your crate root, you must qualify that item with the module name. For example, if you have a function named foo in module looping, you must refer to it as looping::foo.

You can also add use statements to import names in the module's scope. For example, if you add use looping::foo;, then you can just use foo to refer to looping::foo.

For more information, see Separating Modules into Different Files in The Rust Programming Language.