Add a CI workflow

.github/workflows/ci.yml

@@ -0,0 +1,48 @@
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+  merge_group:
+
+name: CI
+
+env:
+  CARGO_TERM_COLOR: always
+  CARGO_INCREMENTAL: 0
+
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        include:
+          - rust: 1.6.0 # MSRV
+          - rust: stable
+          - rust: beta
+          - rust: nightly
+
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@master
+        with:
+          toolchain: ${{ matrix.rust }}
+      - name: Tests
+        run: |
+          cargo build --verbose
+          cargo test --verbose
+
+  # One job that "summarizes" the success state of this pipeline. This can then be added to branch
+  # protection, rather than having to add each job separately.
+  success:
+    name: Success
+    runs-on: ubuntu-latest
+    needs: [tests]
+    # Github branch protection is exceedingly silly and treats "jobs skipped because a dependency
+    # failed" as success. So we have to do some contortions to ensure the job fails if any of its
+    # dependencies fails.
+    if: always() # make sure this is never "skipped"
+    steps:
+      # Manually check the status of all dependencies. `if: failure()` does not work.
+      - name: check if any dependency failed
+        run: jq --exit-status 'all(.result == "success")' <<< '${{ toJson(needs) }}'