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+# Syncing changes between Clippy and [`rust-lang/rust`]
+
+Clippy currently gets built with a pinned nightly version.
+
+In the `rust-lang/rust` repository, where rustc resides, there's a copy of
+Clippy that compiler hackers modify from time to time to adapt to changes in the
+unstable API of the compiler.
+
+We need to sync these changes back to this repository periodically, and the
+changes made to this repository in the meantime also need to be synced to the
+`rust-lang/rust` repository.
+
+To avoid flooding the `rust-lang/rust` PR queue, this two-way sync process is
+done in a bi-weekly basis if there's no urgent changes. This is done starting on
+the day of the Rust stable release and then every other week. That way we
+guarantee that we keep this repo up to date with the latest compiler API, and
+every feature in Clippy is available for 2 weeks in nightly, before it can get
+to beta. For reference, the first sync following this cadence was performed the
+2020-08-27.
+
+This process is described in detail in the following sections. For general
+information about `subtree`s in the Rust repository see [Rust's
+`CONTRIBUTING.md`][subtree].
+
+## Patching git-subtree to work with big repos
+
+Currently, there's a bug in `git-subtree` that prevents it from working properly
+with the [`rust-lang/rust`] repo. There's an open PR to fix that, but it's
+stale. Before continuing with the following steps, we need to manually apply
+that fix to our local copy of `git-subtree`.
+
+You can get the patched version of `git-subtree` from [here][gitgitgadget-pr].
+Put this file under `/usr/lib/git-core` (making a backup of the previous file)
+and make sure it has the proper permissions:
+
+```bash
+sudo cp --backup /path/to/patched/git-subtree.sh /usr/lib/git-core/git-subtree
+sudo chmod --reference=/usr/lib/git-core/git-subtree~ /usr/lib/git-core/git-subtree
+sudo chown --reference=/usr/lib/git-core/git-subtree~ /usr/lib/git-core/git-subtree
+```
+
+> _Note:_ The first time running `git subtree push` a cache has to be built.
+> This involves going through the complete Clippy history once. For this you
+> have to increase the stack limit though, which you can do with `ulimit -s
+> 60000`. Make sure to run the `ulimit` command from the same session you call
+> git subtree.
+
+> _Note:_ If you are a Debian user, `dash` is the shell used by default for
+> scripts instead of `sh`. This shell has a hardcoded recursion limit set to
+> 1000. In order to make this process work, you need to force the script to run
+> `bash` instead. You can do this by editing the first line of the `git-subtree`
+> script and changing `sh` to `bash`.
+
+## Defining remotes
+
+You may want to define remotes, so you don't have to type out the remote
+addresses on every sync. You can do this with the following commands (these
+commands still have to be run inside the `rust` directory):
+
+```bash
+# Set clippy-upstream remote for pulls
+$ git remote add clippy-upstream https://github.com/rust-lang/rust-clippy
+# Make sure to not push to the upstream repo
+$ git remote set-url --push clippy-upstream DISABLED
+# Set a local remote
+$ git remote add clippy-local /path/to/rust-clippy
+```
+
+> Note: The following sections assume that you have set those remotes with the
+> above remote names.
+
+## Performing the sync from [`rust-lang/rust`] to Clippy
+
+Here is a TL;DR version of the sync process (all of the following commands have
+to be run inside the `rust` directory):
+
+1. Clone the [`rust-lang/rust`] repository or make sure it is up to date.
+2. Checkout the commit from the latest available nightly. You can get it using
+   `rustup check`.
+3. Sync the changes to the rust-copy of Clippy to your Clippy fork:
+    ```bash
+    # Make sure to change `your-github-name` to your github name in the following command. Also be
+    # sure to either use a net-new branch, e.g. `sync-from-rust`, or delete the branch beforehand
+    # because changes cannot be fast forwarded and you have to run this command again.
+    git subtree push -P src/tools/clippy clippy-local sync-from-rust
+    ```
+
+    > _Note:_ Most of the time you have to create a merge commit in the
+    > `rust-clippy` repo (this has to be done in the Clippy repo, not in the
+    > rust-copy of Clippy):
+    ```bash
+    git fetch upstream  # assuming upstream is the rust-lang/rust remote
+    git checkout sync-from-rust
+    git merge upstream/master --no-ff
+    ```
+    > Note: This is one of the few instances where a merge commit is allowed in
+    > a PR.
+4. Bump the nightly version in the Clippy repository by changing the date in the
+   rust-toolchain file to the current date and committing it with the message:
+   ```bash
+   git commit -m "Bump nightly version -> YYYY-MM-DD"
+   ```
+5. Open a PR to `rust-lang/rust-clippy` and wait for it to get merged (to
+   accelerate the process ping the `@rust-lang/clippy` team in your PR and/or
+   ask them in the [Zulip] stream.)
+
+[Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/clippy
+
+## Performing the sync from Clippy to [`rust-lang/rust`]
+
+All of the following commands have to be run inside the `rust` directory.
+
+1. Make sure you have checked out the latest `master` of `rust-lang/rust`.
+2. Sync the `rust-lang/rust-clippy` master to the rust-copy of Clippy:
+    ```bash
+    git checkout -b sync-from-clippy
+    git subtree pull -P src/tools/clippy clippy-upstream master
+    ```
+3. Open a PR to [`rust-lang/rust`]
+
+[gitgitgadget-pr]: https://github.com/gitgitgadget/git/pull/493
+[subtree]: https://rustc-dev-guide.rust-lang.org/contributing.html#external-dependencies-subtree
+[`rust-lang/rust`]: https://github.com/rust-lang/rust