Better CI advice

book/src/ci.md

@@ -1,11 +1,25 @@
 # Continuous integration
 
+You might want to use cargo-mutants in your continuous integration (CI) system, to ensure that no uncaught mutants are merged into your codebase.
+
+There are at least two complementary ways to use cargo-mutants in CI:
+
+1. [Check for mutants produced in the code changed in a pull request](pr-diff.md). This is typically _much_ faster than testing all mutants, and is a good way to ensure  that newly merged code is well tested, and to facilitate conversations about how to test the PR.
+
+2. Checking that all mutants are caught, on PRs or on the development branch.
+
 Here is an example of a GitHub Actions workflow that runs mutation tests and uploads the results as an artifact. This will fail if it finds any uncaught mutants.
 
+The recommended way to install cargo-mutants is using [install-action](https://github.com/taiki-e/install-action), which will fetch a binary from cargo-mutants most recent GitHub release, which is faster than building from source. You could alternatively use [baptiste0928/cargo-install](https://github.com/baptiste0928/cargo-install) which will build it from source in your worker and cache the result.
+
 ```yml
 name: cargo-mutants
 
-on: [pull_request, push]
+on:
+  push:
+    branches:
+      - main
+  pull_request:
 
 jobs:
   cargo-mutants:
@@ -15,17 +29,19 @@ jobs:
       - uses: actions-rs/toolchain@v1
         with:
           toolchain: stable
-      - name: Install cargo-mutants
-        run: cargo install --locked cargo-mutants
+      - uses: taiki-e/install-action@v2
+        name: Install cargo-mutants using install-action
+        with:
+          tool: cargo-mutants
       - name: Run mutant tests
-        run: cargo mutants -- --all-features
+        run: cargo mutants -vV
       - name: Archive results
         uses: actions/upload-artifact@v3
-        if: failure()
+        if: always()
         with:
-          name: mutation-report
+          name: mutants-out
           path: mutants.out
 ```
 
 The workflow used by cargo-mutants on itself can be seen at
-<https://github.com/sourcefrog/cargo-mutants/blob/main/.github/workflows/mutate-self.yaml>.
+<https://github.com/sourcefrog/cargo-mutants/blob/main/.github/workflows/mutate-self.yaml>, but this is different from what you will typically want to use, because it runs cargo-mutants from HEAD.