[WIP RFC] Enforce rustfmt on automation.

etc/ci/buildbot_steps.yml

@@ -103,6 +103,7 @@ linux-dev:
     CCACHE: sccache
     RUSTC_WRAPPER: sccache
   commands:
+    - ./etc/ci/check_rustfmt.sh
     - ./mach clean-nightlies --keep 3 --force
     - ./mach clean-cargo-cache --keep 3 --force
     - ./etc/ci/clean_build_artifacts.sh

etc/ci/check_rustfmt.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+cd "$(git rev-parse --show-toplevel)"
+
+# NOTE(emilio): This assumes that HEAD is a merge commit, which is true for
+# builbot stuff. Locally you probably want to use `-1`.
+LAST_MERGE="$(git log --merges --format=%h -2 | tail -1)"
+
+cargo install --force rustfmt-nightly
+git diff ${LAST_MERGE} | rustfmt-format-diff -p 1
+
+git add -u
+git diff @
+git diff-index --quiet HEAD