How it works...

The instructions for this recipe are a bit more complex than the others, as we need to manage two separate crates. If your code doesn't compile, compare your version with the one used in the book at https://github.com/SirRade/rust-standard-library-cookbook/tree/master/chapter_five. We need to separate the code into two crates because providing a custom derive requires creating a procedural macro, as indicated by all of the instances of proc_macro in the code. A procedural macro is Rust code that runs alongside the compiler and interacts directly with it. Because of the special nature and unique restrictions of such code, it needs to be in a separate crate that is annotated with the following:

[lib]proc-macro = true

This crate ...

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