thiserror

That was a bit of detour, wasn't it? But a necessary one!
Let's get back on track now: custom error types and thiserror.

Custom error types

We've seen how to implement the Error trait "manually" for a custom error type.
Imagine that you have to do this for most error types in your codebase. That's a lot of boilerplate, isn't it?

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.

#[derive(thiserror::Error, Debug)]
enum TicketNewError {
    #[error("{0}")]
    TitleError(String),
    #[error("{0}")]
    DescriptionError(String),
}

You can write your own macros

All the derive macros we've seen so far were provided by the Rust standard library.
thiserror::Error is the first example of a third-party derive macro.

derive macros are a subset of procedural macros, a way to generate Rust code at compile time. We won't get into the details of how to write a procedural macro in this course, but it's important to know that you can write your own!
A topic to approach in a more advanced Rust course.

Custom syntax

Each procedural macro can define its own syntax, which is usually explained in the crate's documentation. In the case of thiserror, we have:

• #[derive(thiserror::Error)]: this is the syntax to derive the Error trait for a custom error type, helped by thiserror.
• #[error("{0}")]: this is the syntax to define a Display implementation for each variant of the custom error type. {0} is replaced by the zero-th field of the variant (String, in this case) when the error is displayed.

Exercise

The exercise for this section is located in 05_ticket_v2/12_thiserror