New cargo subcommand: sync-readme

cargo, subcommand, readme
Mon Feb 25 18:50:00 2019 UTC, by Dimitri Sabadie — feed

Have you ever struggled with welcoming people on your project pages, be them on GitHub or One major problem we’re facing when developping crates is that we need to maintain several places where documentation is important:

  • The / files, that get rendered as front page on This is important because we want people to have nice onboardings when hitting the official documentations of our crates.
  • The README file — often, that gets automatically rendered as HTML when you end up on the project’s GitHub page. This page should provide as many as possible hints and information about what the project is and how to use it.

However, sometimes, I just don’t want to bother writing twice the same thing and honestly, I don’t really know whether the README file is a good place to write an onboarding section, since that should also be included in your Rust documentation on

So… I’ve been thinking of a way to fix that, without really investing too much time in it. But lately, I came to the realization that I often have this pattern:

  1. Write the onboarding documentation in my or For instance, I’m pretty proud of the onboarding documentation of warmy. It’s exhaustive, easy for newcomers, it has plenty of links and explanations and it renders pretty nice on
  2. Write a header in the README file with badges etc. and then copy the Rust onboarding documentation in the readme.

Here, (2.) is a manual and annoying task: I open my editor, make a block selection of the documentation, store it in a buffer, apply a smart macro on it to remove the Rust annotation and then paste the result in the README after having purged it from the previous documentation… That’s tedious and not very interesting.

cargo sync-readme to the rescue!

So, yesterday, I decided to start working on a small tool that would automate all this for me. The idea is the following:

  • cargo sync-readme synchronizes your README (the file specified by the readme key in your Cargo.toml, or just by default) with the entrypoint of your library or binary crate (by default, or, or what is defined at the path key in your manifest).
  • In order to perform the synchronization, you need to have put a special marker so that the command is instructed where to put the synchronized documentation.
    • The <!-- cargo-sync-readme --> marker must lie on a single line where you want the documentation to be inserted in your README.
    • Post generation, this marker will disappear and will get replaced by the synchronized documentation, surrounded by two markers: <!-- cargo-sync-readme start> and <!-- cargo-sync-readme end -->.

This is really all you have to do. cargo sync-readme will take care of the rest for you. There’re two hypothesis that the command requires to be true, though:

  • Your README file should be a Markdown-formatted file. It doesn’t have to be, but since Rust’s documentation is written in Markdown, it should be.
  • You use the //! annotation to write your documentation. This is currently how cargo sync-readme works. It doesn’t have to be solely using this annotation on longer term, but currently, this is the only annotation supported. More can be added if needed.

Basically, insert the marker once, and run cargo sync-readme. Every time you change your Rust documentation, just call cargo sync-readme once again to synchronize the documentation in your README file.

On workspace crates

Currently, cargo sync-readme doesn’t work with workspace crates. You will have to go into each of the workspace’s members to run cargo sync-readme if you want to synchronize their respective READMEs.


cargo sync-readme is already available on for you to use. You can install it as a development tool with the following command:

cargo install cargo-sync-readme

Disclaimer: after having published cargo-sync-readme, I was told that there is already another cargo plugin, cargo-readme, that already does that. Indeed, that crate does more or less the same job. However, the way it does it is very different. First, it uses a template file while cargo-sync-readme lets you use your README file without having to depend on a template. Also, cargo-readme has special semantics in its template (like {{crate_name}}, etc.) while cargo-sync-readme is simpler and just requires a single marker. To sum up: cargo-readme is heavier and is likely to require you to break your file into two separate files but contains more customization options while cargo-sync-readme only requires a single line in your README and will synchronize from within that same file.

Feel free to comment, open issues, drink beers, share that awesome bear driving a sausage podracer and most of all… keep the vibes!