Fix ci

[kiendang]

Synopsis

Fix a couple of CI failures due to cargo/rustc updates

• replace doc_auto_cfg with doc_cfg Implement RFC 3631: add rustdoc doc_cfg features rust-lang/rust#138907
• update as_ref, as_mut compile error message

Solution

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)

[kiendang]

Hmm looks like the error message changes in nightly but not in stable yet hence updating it fails stable.

[kiendang]

Hmm I cannot reproduce locally. My stable-aarch64-apple-darwin toolchain, which is also rustc 1.90.0 (1159e78c4 2025-09-14) also uses the new error message.

[tyranron]

@kiendang thank you for taking care of it!

Hmm I cannot reproduce locally.

Don't bother. This flakiness is here for years now. Nothing we can do about it, though.