Libraries and binaries

It took a bit of code to implement the Error trait for TicketNewError, didn’t it?
A manual Display implementation, plus an Error impl block.

We can remove some of the boilerplate by using thiserror, a Rust crate that provides a procedural macro to simplify the creation of custom error types.
But we’re getting ahead of ourselves: thiserror is a third-party crate, it’d be our first dependency!

Let’s take a step back to talk about Rust’s packaging system before we dive into dependencies.

What is a package?

A Rust package is defined by the [package] section in a Cargo.toml file, also known as its manifest. Within [package] you can set the package’s metadata, such as its name and version.

Go check the Cargo.toml file in the directory of this section’s exercise!

What is a crate?

Inside a package, you can have one or more crates, also known as targets.
The two most common crate types are binary crates and library crates.

Binaries

A binary is a program that can be compiled to an executable file.
It must include a function named main—the program’s entry point. main is invoked when the program is executed.

Libraries

Libraries, on the other hand, are not executable on their own. You can’t run a library, but you can import its code from another package that depends on it.
A library groups together code (i.e. functions, types, etc.) that can be leveraged by other packages as a dependency.

All the exercises you’ve solved so far have been structured as libraries, with a test suite attached to them.

Conventions

There are some conventions around Rust packages that you need to keep in mind:

The package’s source code is usually located in the src directory.
If there’s a src/lib.rs file, cargo will infer that the package contains a library crate.
If there’s a src/main.rs file, cargo will infer that the package contains a binary crate.

You can override these defaults by explicitly declaring your targets in the Cargo.toml file—see cargo’s documentation for more details.

Keep in mind that while a package can contain multiple crates, it can only contain one library crate.

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