Add Rust checks to Makefile and CircleCI

These lint and test solely the Rust code (no Python). The following
tools are enabled:

* rustfmt: code formatting (like black)
* clippy: linting (like flake8+pylint+mypy)
* cargo test: Rust test suite

Running maturin to verify the Python integration will happen in the next
commit, when it's hooked into the development environment.

Refs #6814.

.circleci/config.yml

@@ -94,6 +94,24 @@ jobs:
             DOCKER_BUILD_ARGUMENTS="--cache-from securedrop-test-focal-py3:${fromtag:-latest}" securedrop/bin/dev-shell \
                   bash -c "/opt/venvs/securedrop-app-code/bin/pip3 install --require-hashes -r requirements/python3/develop-requirements.txt && make -C .. lint"
 
+  rust:
+    # Keep version in sync with rust-toolchain.toml
+    docker:
+      - image: rust:1.69.0
+    steps:
+      - checkout
+      - run:
+          name: Install dependencies
+          command: |
+            apt-get update && apt-get install make --yes
+            rustup component add rustfmt
+            rustup component add clippy
+      - run:
+          name: Lint and test Rust code
+          command: |
+            make rust-lint
+            make rust-test
+
   app-tests:
     machine:
       image: ubuntu-2004:202010-01
@@ -357,6 +375,10 @@ workflows:
           context:
             - circleci-slack
           <<: *slack-fail-post-step
+      - rust:
+          context:
+            - circleci-slack
+          <<: *slack-fail-post-step
       - app-tests:
           filters:
             branches:

Makefile

@@ -119,6 +119,12 @@ html-lint:  ## Validate HTML in web application template files.
 		securedrop/source_templates/*.html securedrop/journalist_templates/*.html
 	@echo
 
+.PHONY: rust-lint
+rust-lint: ## Lint Rust code
+	@echo "███ Linting Rust code..."
+	cargo fmt --check
+	cargo clippy
+
 .PHONY: shellcheck
 shellcheck:  ## Lint shell scripts.
 	@echo "███ Linting shell scripts..."
@@ -282,6 +288,11 @@ test:  ## Run the test suite in a Docker container.
 .PHONY: test-focal
 test-focal:  test
 
+.PHONY: rust-test
+rust-test:
+	@echo "███ Running Rust tests..."
+	cargo test
+
 .PHONY: validate-test-html
 validate-test-html:
 	@echo "███ Validating HTML source from $(shell find securedrop/tests/functional/pageslayout/html -name "*.html" | wc -l | xargs echo -n) page-layout test(s)"