syntax: enable 'doc_auto_cfg'

I wish this feature were stable and enabled by default. I suspect that
it maybe doesn't work correctly 100% of the time, but it's super useful.
And manually annotating APIs is a huge pain, so it's worth at least
attempting.

Author: BurntSushi

regex-syntax/Cargo.toml

@@ -32,3 +32,16 @@ unicode-gencat = []
 unicode-perl = []
 unicode-script = []
 unicode-segment = []
+
+[package.metadata.docs.rs]
+# We want to document all features.
+all-features = true
+# Since this crate's feature setup is pretty complicated, it is worth opting
+# into a nightly unstable option to show the features that need to be enabled
+# for public API items. To do that, we set 'docsrs', and when that's enabled,
+# we enable the 'doc_auto_cfg' feature.
+#
+# To test this locally, run:
+#
+#     RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --all-features
+rustdoc-args = ["--cfg", "docsrs"]

regex-syntax/src/lib.rs

@@ -162,6 +162,7 @@ The following features are available:
 #![forbid(unsafe_code)]
 #![deny(missing_docs)]
 #![warn(missing_debug_implementations)]
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
 #![no_std]
 
 #[cfg(any(test, feature = "std"))]