New Packages

This chapter sketches out how to add a new package to the cargo workspace.


Choose the relevant parent directory

  • credential/ for credential-process related packages
  • benches/ for benchmarking of cargo itself
  • crates/ for everything else

Run cargo new <name>

  • <name>:
    • We tend to use - over _
    • For internal APIs, to avoid collisions with third-party subcommands, we can use the cargo-util- prefix
    • For xtasks, we use the xtask- prefix
  • package.rust-version
    • Internal packages tend to have a policy of “latest” with a # MSRV:1 comment
    • Ecosystem packages tend to have a policy of “N-2” with a # MSRV:3 comment
    • If the right choice is inherited from the workspace, feel free to keep it that way
  • If running without cargo new automatically adding to workspace, add it as a workspace member if not already captured by a glob

If its an xtask,

  • Add it to .cargo/config.tomls [alias] table
  • Mark package.publish = false

If needed to be published with cargo, add the package to in the repo root, in dependency order.

Note: by adding the package to the workspace, you automatically get

  • CI running cargo test
  • CI verifying MSRV
  • CI checking for cargo doc warnings

MSRV Policy

Our MSRV policies are

  • Internal packages: support latest version
  • Ecosystem packages: support latest 3 versions

We proactively update the MSRV

  • So contributors don’t shy away from using newer features, either assuming they can’t ask or feeling like they have to have a justification when asking
  • To avoid a de facto MSRV developing from staying on a version for a long period of time, leaving users unhappy when their expectations aren’t met

To proactively update the MSRV, we use RenovateBot with the configuration file in .github/renovatebot.json5. To know what MSRV policy to use, it looks for comments of the form # MSRV:N, where N is the number of supported rust versions.