Add formatting check to CI

Add code to the CI script, guarded on env var `DO_FMT` to run the
formatter.

Add a formatting job to the nightly CI job as a separate step, in a
similar fashion to how the other nightly steps are done.

.github/workflows/rust.yml

@@ -18,6 +18,10 @@ jobs:
           toolchain: nightly
           override: true
           components: rust-src
+      - name: Check formatting
+        env:
+          DO_FMT: true
+        run: ./contrib/test.sh
       - name: Running address sanitizer
         env:
           DO_ASAN: true

contrib/test.sh

@@ -88,6 +88,15 @@ if [ "$DO_ASAN" = true ]; then
     cargo run --release --features=alloc --manifest-path=./no_std_test/Cargo.toml | grep -q "Verified alloc Successfully"
 fi
 
+# Run formatter if told to.
+if [ "$DO_FMT" = true ]; then
+    if [ "$NIGHTLY" = false ]; then
+        echo "DO_FMT requires a nightly toolchain (consider using RUSTUP_TOOLCHAIN)"
+        exit 1
+    fi
+    rustup component add rustfmt
+    cargo fmt --check || exit 1
+fi
 
 # Bench if told to, only works with non-stable toolchain (nightly, beta).
 if [ "$DO_BENCH" = true ]