Book: Improve chapter on CI

Recommend the -Dwarnings and --all-targets/--all-features more strongly.

Author: flip1995

book/src/continuous_integration/README.md

@@ -1,7 +1,8 @@
 # Continuous Integration
 
-It is recommended to run Clippy on CI, preferably with `-Dwarnings`, so that
-Clippy lints prevent CI from passing.
+It is recommended to run Clippy on CI with `-Dwarnings`, so that Clippy lints
+prevent CI from passing. To enforce errors on warnings on all `cargo` commands
+not just `cargo clippy`, you can set the env var `RUSTFLAGS="-Dwarnings"`.
 
 We recommend to use Clippy from the same toolchain, that you use for compiling
 your crate for maximum compatibility. E.g. if your crate is compiled with the

book/src/continuous_integration/github_actions.md

@@ -6,11 +6,16 @@ pre-installed. So all you have to do is to run `cargo clippy`.
 ```yml
 on: push
 name: Clippy check
+
+# Make sure CI fails on all warnings, including Clippy lints
+env:
+  RUSTFLAGS: "-Dwarnings"
+
 jobs:
   clippy_check:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v1
       - name: Run Clippy
-        run: cargo clippy
+        run: cargo clippy --all-targets --all-features
 ```