docs(serde-spanned): display feature guarded items on docs.rs

crates/serde_spanned/Cargo.toml

@@ -34,5 +34,9 @@ pre-release-replacements = [
   {file="CHANGELOG.md", search="<!-- next-url -->", replace="<!-- next-url -->\n[Unreleased]: https://github.com/toml-rs/toml/compare/{{tag_name}}...HEAD", exactly=1},
 ]
 
+[package.metadata.docs.rs]
+rustdoc-args = ["--cfg", "docsrs"]
+all-features = true
+
 [dependencies]
 serde = { version = "1.0.145", optional = true }

crates/serde_spanned/src/lib.rs

@@ -12,6 +12,7 @@
 // and lets them ensure that there is indeed no unsafe code as opposed to
 // something they couldn't detect (e.g. unsafe added via macro expansion, etc).
 #![forbid(unsafe_code)]
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
 
 mod spanned;
 pub use crate::spanned::Spanned;