ci: Add rustfmt warning

We are all human and forget to run formatters and checks. Let the CI
help us to recognize and remedy our human failings!

Signed-off-by: Peter Neuroth <[email protected]>

.github/workflows/check-formatting.yml

@@ -0,0 +1,26 @@
+name: Check Formatting
+
+on:
+  pull_request:
+    types:
+      - synchronize
+      - opened
+  workflow_dispatch:
+
+jobs:
+  check-formatting:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+    steps:
+    - uses: actions/checkout@v4
+    - name: Install Rust Toolchain
+      uses: actions-rs/toolchain@v1
+      with:
+        toolchain: 1.73
+        components: rustfmt
+
+    - name: Check Rust Formatting
+      continue-on-error: true
+      run: |
+        make check-rustfmt

Makefile

@@ -1,3 +1,5 @@
+.PHONY: check-rustfm
+
 ifdef GL_DOCKER
 REPO_ROOT=/repo
 else
@@ -23,6 +25,8 @@ endif
 ARTIFACTS = \
 	.coverage
 
+CHANGED_RUST_SOURCES=$(shell git diff --name-only origin/main | grep '\.rs')
+
 # Variable to collect all generated files into, so we can clean and
 # rebuild them easily.
 GENALL =
@@ -66,6 +70,12 @@ check-self: ensure-docker
 	  /repo/libs/gl-testing \
 	  ${PYTEST_OPTS}
 
+check-rustfmt:
+	@if [ -n "${CHANGED_RUST_SOURCES}" ]; then \
+		rustfmt --edition 2021 --check ${CHANGED_RUST_SOURCES}; else \
+		echo "skip rustfmt check no changes detected ${CHANGED_RUST_SOURCES}"; \
+	fi
+
 ensure-docker:
 	@if [ "x${GL_DOCKER}" != "x1" ]; then \
 		echo "We are not running in the gl-testing docker container, refusing to run"; \