Using External Repositories

The rust-lang/rust git repository depends on several other repos in the rust-lang organization. There are three main ways we use dependencies:

  1. As a Cargo dependency through (e.g. rustc-rayon)
  2. As a git subtree (e.g. clippy)
  3. As a git submodule (e.g. cargo)

As a general rule, use for libraries that could be useful for others in the ecosystem; use subtrees for tools that depend on compiler internals and need to be updated if there are breaking changes; and use submodules for tools that are independent of the compiler.

External Dependencies (subtree)

As a developer to this repository, you don't have to treat the following external projects differently from other crates that are directly in this repo:

In contrast to submodule dependencies (see below for those), the subtree dependencies are just regular files and directories which can be updated in tree. However, if possible, enhancements, bug fixes, etc. specific to these tools should be filed against the tools directly in their respective upstream repositories. The exception is that when rustc changes are required to implement a new tool feature or test, that should happen in one collective rustc PR.

Synchronizing a subtree

Periodically the changes made to subtree based dependencies need to be synchronized between this repository and the upstream tool repositories.

Subtree synchronizations are typically handled by the respective tool maintainers. Other users are welcome to submit synchronization PRs, however, in order to do so you will need to modify your local git installation and follow a very precise set of instructions. These instructions are documented, along with several useful tips and tricks, in the syncing subtree changes section in Clippy's Contributing guide. The instructions are applicable for use with any subtree based tool, just be sure to use the correct corresponding subtree directory and remote repository.

The synchronization process goes in two directions: subtree push and subtree pull.

A subtree push takes all the changes that happened to the copy in this repo and creates commits on the remote repo that match the local changes. Every local commit that touched the subtree causes a commit on the remote repo, but is modified to move the files from the specified directory to the tool repo root.

A subtree pull takes all changes since the last subtree pull from the tool repo and adds these commits to the rustc repo along with a merge commit that moves the tool changes into the specified directory in the Rust repository.

It is recommended that you always do a push first and get that merged to the tool master branch. Then, when you do a pull, the merge works without conflicts. While it's definitely possible to resolve conflicts during a pull, you may have to redo the conflict resolution if your PR doesn't get merged fast enough and there are new conflicts. Do not try to rebase the result of a git subtree pull, rebasing merge commits is a bad idea in general.

You always need to specify the -P prefix to the subtree directory and the corresponding remote repository. If you specify the wrong directory or repository you'll get very fun merges that try to push the wrong directory to the wrong remote repository. Luckily you can just abort this without any consequences by throwing away either the pulled commits in rustc or the pushed branch on the remote and try again. It is usually fairly obvious that this is happening because you suddenly get thousands of commits that want to be synchronized.

Creating a new subtree dependency

If you want to create a new subtree dependency from an existing repository, call (from this repository's root directory!)

git subtree add -P src/tools/clippy master

This will create a new commit, which you may not rebase under any circumstances! Delete the commit and redo the operation if you need to rebase.

Now you're done, the src/tools/clippy directory behaves as if Clippy were part of the rustc monorepo, so no one but you (or others that synchronize subtrees) actually needs to use git subtree.

External Dependencies (submodules)

Building Rust will also use external git repositories tracked using git submodules. The complete list may be found in the .gitmodules file. Some of these projects are required (like stdarch for the standard library) and some of them are optional (like src/doc/book).

Usage of submodules is discussed more in the Using Git chapter.

Some of the submodules are allowed to be in a "broken" state where they either don't build or their tests don't pass, e.g. the documentation books like The Rust Reference. Maintainers of these projects will be notified when the project is in a broken state, and they should fix them as soon as possible. The current status is tracked on the toolstate website. More information may be found on the Forge Toolstate chapter. In practice, it is very rare for documentation to have broken toolstate.

Breakage is not allowed in the beta and stable channels, and must be addressed before the PR is merged. They are also not allowed to be broken on master in the week leading up to the beta cut.