Deny warnings in CI

The main crate already has `#![deny(warnings)]`. Set RUSTFLAGS in CI to
enforce this for other crates in the workspace.

.github/workflows/main.yml

@@ -1,6 +1,10 @@
 name: CI
 on: [push, pull_request]
 
+env:
+  RUSTDOCFLAGS: -Dwarnings
+  RUSTFLAGS: -Dwarnings
+
 jobs:
   docker:
     name: Docker

build.rs

@@ -2,10 +2,14 @@ use std::env;
 
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo::rustc-check-cfg=cfg(assert_no_panic)");
+    println!("cargo::rustc-check-cfg=cfg(feature, values(\"unstable\"))");
 
     #[cfg(feature = "musl-reference-tests")]
     musl_reference_tests::generate();
 
+    println!("cargo::rustc-check-cfg=cfg(feature, values(\"checked\"))");
+    #[allow(unexpected_cfgs)]
     if !cfg!(feature = "checked") {
         let lvl = env::var("OPT_LEVEL").unwrap();
         if lvl != "0" {

ci/run-docker.sh

@@ -16,6 +16,7 @@ run() {
     docker run \
            --rm \
            --user $(id -u):$(id -g) \
+           -e "RUSTFLAGS=${RUSTFLAGS:-}" \
            -e CARGO_HOME=/cargo \
            -e CARGO_TARGET_DIR=/target \
            -v "${HOME}/.cargo":/cargo \