mdbook/book-example/src/for_developers/preprocessors.md

Preprocessors

A preprocessor is simply a bit of code which gets run immediately after the book is loaded and before it gets rendered, allowing you to update and mutate the book. Possible use cases are:

• Creating custom helpers like \{{#include /path/to/file.md}}
• Updating links so [some chapter](some_chapter.md) is automatically changed to [some chapter](some_chapter.html) for the HTML renderer
• Substituting in latex-style expressions ($$ \frac{1}{3} $$) with their mathjax equivalents

Implementing a Preprocessor

A preprocessor is represented by the Preprocessor trait.

pub trait Preprocessor {
    fn name(&self) -> &str;
    fn run(&self, ctx: &PreprocessorContext, book: &mut Book) -> Result<()>;
}

Where the PreprocessorContext is defined as

pub struct PreprocessorContext {
    pub root: PathBuf,
    pub config: Config,
}