ci: add 'cargo hack check' step

Ensure that our stated Rust version compatibility stays correct over
time.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>

Author: tycho

.github/workflows/ci.yml

@@ -23,6 +23,8 @@ jobs:
 
       - uses: Swatinem/rust-cache@v2
 
+      - uses: taiki-e/install-action@cargo-hack
+
       # Build first — catches template include_str! failures and compile errors
       # before running the slower test step.
       - name: Build
@@ -31,15 +33,18 @@ jobs:
       - name: Check formatting
         run: cargo fmt --check
 
-      - name: Clippy
+      - name: Run clippy
         run: cargo clippy --all-targets -- -D warnings
 
-      - name: Audit
+      - name: Cargo audit
         uses: rustsec/audit-check@858dc40f52ca2b8570b7a997c1c4e35c6fc9a432
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
 
       # Integration tests invoke the binary and compile the generated C.
       # gcc is pre-installed on ubuntu-latest.
-      - name: Test
+      - name: Run test suite
         run: cargo test --release
+
+      - name: Check Rust version compatibility
+        run: cargo hack check --rust-version --all-targets