Detect warnings in CI

Author: dtolnay

.github/workflows/ci.yml

@@ -5,6 +5,9 @@ on:
   pull_request:
   schedule: [cron: "40 1 * * *"]
 
+env:
+  RUSTFLAGS: '-Dwarnings'
+
 jobs:
   test:
     name: Rust ${{matrix.rust}}