Cargo Book

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A draft RFC for adding a book subcommand to Rust’s Cargo tool.


  • Feature Name: cargo-book
  • Start Date: 2018-03-12
  • RFC PR: (leave this empty)
  • Rust Issue: (leave this empty)


The Rust distribution has an official book generator, mdBook, in the nursery. This generator should be able to be used in Cargo projects to build the external documentation into a book independent of the artifacts produced by rustdoc.


External documentation is very good, and standalone Markdown files are a much nicer editing environment than are comments in Rust source files. Furthermore, the external documentation set for a project can include a more comprehensive suite of project documentation, in contrast to the rustdoc-generated docs which are primarily technical.

As rustdoc supports an external documentation file now via doc(include = "path"), documentation files can even be reused!

Guide-level explanation

Rust projects often follow certain conventions about where items are located. For example, the source code for a project lives under src/1, integration tests in tests/, and example uses in examples/. Documentation for a project can be written directly in source code, as documentation comments (///, //!, /** **/, or /*! !*/2), or can be written in Markdown files that live under doc/ and are referenced by the Rust code (see RFC #1990 and issue #44732).

The cargo doc subcommand builds documentation from your source code, and the cargo book subcommand builds a manual from the contents of your doc/ folder with the mdBook tool.

This manual is built from the doc/ root file by default. A custom root file, or multiple books, can be set in your Cargo.toml file, like this:

# Title of the book
name = "User’s Guide"
# Path to the book
# `/` is implicitly appended
path = "doc/user"

The contents of a book directory are laid out in the mdBook documentation. The output of cargo book can be found in target/book.

Book vs Docs

While Rust docs are superheroes, this isn’t a Hulk-vs-Thor gladiator scene. Books and docs are teammates.

The book is meant to be a complement to, not a replacement of, the source code documentation that we already have. Source code documentation provides an excellent set of technical information for the APIs and behavior found in a project, but is less well-suited for non-technical documents such as a user’s manual, or a comprehensive guide, or an explanation of the theory behind a library.

Every project you’ve seen that has a guide distinct from the API docs — Rocket, Diesel, Turtle, and more — can benefit from having these manuals directly in the project repository.


The cargo test command knows how to ask cargo book for all the book files in the project, and pull out any pieces of Rust code inside them. All the Rust code snippets in your book is part of your test suite, just like the snippets in your documentation comments!

Reference-level explanation

Since mdBook is in the Rust Nursery and considered an official project, and used to render everything in The Rust Bookshelf, it is a good candidate for inclusion into the Cargo workflow and the Rust distribution.

Cargo will add the creation of a doc/ directory to its cargo new command, and add a cargo book command that uses the mdbook library to drive the book generation. The default book configuration for cargo book differs from the defaults that mdBook expects; currently, the cargo book configuration is equivalent to the following book.toml in the root of a Cargo project:

title = "$crate_name"
src = "doc"
description = "$crate_description"

build-dir = "target/book/$crate_name"

Cargo will add a [[book]] table to its Cargo.toml specification that takes the same keys as the [book] table currently in mdBook’s book.toml. The only difference is that [book] src will be changed to [[book]] path in order to match existing conventions in Cargo.toml.

When executing cargo book, Cargo will map each [[book]] table into an MDBook struct and build the book into target/book. If more than one [[book]] table exists in Cargo.toml, then each book will receive its own subdirectory named by its title and hash.

The mdBook project does not need to be altered from its present state for this RFC to land. The book.toml settings above are sufficient for mdbook to correctly operate in the root of a Cargo project directory that has one book in doc/.

Cargo will need to be changed in four areas:

  • the Cargo.toml specification, to include [[book]]
  • the addition of a cargo book subcommand
  • the expansion of cargo new to create doc/{SUMMARY,README}.md (perhaps optional, such as behind a --with-book flag)
  • the expansion of cargo test to extract all Rust code from doc/**/*.md

The Cargo documentation will also need to be expanded with a chapter on using cargo book.

This RFC should require comparatively little technical work, as the components required already exist and require “only” integration, rather than full construction.


This expands the scope of Cargo’s work in a project directory, and may result in more complexity in both the codebase and the user interface. Furthermore, it may not be desirable to have Cargo accumulate more and more features that can be managed by distinct programs.

The author’s counterargument to the above paragraph is that Cargo ate Rustdoc and everyone is fine with that. Add a cargo mdbook parallel to cargo rustdoc to control the tool directly with Cargo support for paths if desired, but Cargo is very clearly a project manager, and Rust’s culture very clearly holds that documentation is a first-class citizen of a project that is an equal peer of source code.

Rationale and alternatives

  • Why is this design the best in the space of possible designs?

    It integrates with existing workflows for Cargo, uses existing unofficial conventions, and existing tools.

  • What other designs have been considered and what is the rationale for not choosing them?

    None to my knowledge

  • What is the impact of not doing this?

    Projects have to write their user manuals separately from their API docs and don’t have native testing support for any code in them, aka, the status quo.

Prior art

I don’t know off the top of my head of any language toolchains that have a book builder in their tool set, be it in the official project manager or not.

Unresolved questions

  • What parts of the design do you expect to resolve through the RFC process before this gets merged?

    Human interface details like “should we call it book/ instead of doc/ and use doc/ for API documentation files” or “what should the Cargo interface be”

  • What parts of the design do you expect to resolve through the implementation of this feature before stabilization?

    None come to mind.

  • What related issues do you consider out of scope for this RFC that could be addressed in the future independently of the solution that comes out of this RFC?

    Everything related to how mdBook actually functions is out of scope of this RFC. If mdBook grows new features because it is integrated into Cargo by this RFC, those new features are still mdBook’s responsibility.

  1. I firmly believe src/bin/ should have been app/ instead.

  2. Doc comments are block comments, fight me.