CI: test docs build with --cfg docsrs (#2191)

Ensures our docs will actually build on docs.rs.

After publishing `kdf` I noticed since it's an ancient PR it still had
`doc_auto_cfg`. This should reproduce the failure.

Author: tarcieri

.github/workflows/workspace.yml

@@ -39,9 +39,11 @@ jobs:
       - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: stable
+          toolchain: nightly
       # TODO(tarcieri): remove `--exclude crypto` after new release series
-      - run: cargo doc --workspace --all-features --no-deps --exclude crypto
+      - env:
+          RUSTDOCFLAGS: "--cfg docsrs"
+        run: cargo doc --workspace --all-features --no-deps --exclude crypto
 
   rustfmt:
     runs-on: ubuntu-latest

kdf/src/lib.rs

@@ -1,5 +1,5 @@
 #![no_std]
-#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(docsrs, feature(doc_cfg))]
 #![doc = include_str!("../README.md")]
 #![doc(
     html_logo_url = "https://raw.githubusercontent.com/RustCrypto/media/8f1a9894/logo.svg",