ci: Run cargo mutants on changed code

See https://mutants.rs/, esp. https://mutants.rs/pr-diff.html

.github/actions/rust/action.yml

@@ -30,7 +30,7 @@ runs:
 
     - name: Install Rust tools
       shell: bash
-      run: cargo +${{ inputs.version }} binstall --no-confirm cargo-llvm-cov cargo-nextest flamegraph cargo-hack
+      run: cargo +${{ inputs.version }} binstall --no-confirm cargo-llvm-cov cargo-nextest flamegraph cargo-hack cargo-mutants
 
     # sccache slows CI down, so we leave it disabled.
     # Leaving the steps below commented out, so we can re-evaluate enabling it later.

.github/workflows/mutants.yml

@@ -0,0 +1,36 @@
+name: Find mutants
+on:
+  pull_request:
+    branches: ["main"]
+    paths: [".github/**"]
+  merge_group:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref_name }}
+  cancel-in-progress: true
+
+jobs:
+  incremental-mutants:
+    runs-on: ubuntu-latest
+    if: github.event_name == 'pull_request'
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Relative diff
+        run: git diff origin/${{ github.base_ref }}.. | tee git.diff
+
+      - name: Install Rust
+        uses: ./.github/actions/rust
+        with:
+          version: stable
+          components: rustfmt, clippy, llvm-tools-preview
+
+      - name: Mutants
+        run: cargo mutants --no-shuffle -j 2 -vV --in-diff git.diff
+
+      - name: Archive mutants.out
+        uses: actions/upload-artifact@v3
+        if: always()
+        with:
+          name: mutants-incremental.out
+          path: mutants.out