Use doc_cfg to mark Error::new

Cargo.toml

@@ -25,3 +25,4 @@ trybuild = { version = "1.0.19", features = ["diff"] }
 
 [package.metadata.docs.rs]
 targets = ["x86_64-unknown-linux-gnu"]
+rustdoc-args = ["--cfg", "doc_cfg"]

src/error.rs

@@ -19,6 +19,7 @@ impl Error {
     /// If the error type does not provide a backtrace, a backtrace will be
     /// created here to ensure that a backtrace exists.
     #[cfg(feature = "std")]
+    #[cfg_attr(doc_cfg, doc(cfg(feature = "std")))]
     pub fn new<E>(error: E) -> Self
     where
         E: StdError + Send + Sync + 'static,

src/lib.rs

@@ -191,6 +191,7 @@
 
 #![doc(html_root_url = "https://docs.rs/anyhow/1.0.27")]
 #![cfg_attr(backtrace, feature(backtrace))]
+#![cfg_attr(doc_cfg, feature(doc_cfg))]
 #![cfg_attr(not(feature = "std"), no_std)]
 #![allow(
     clippy::needless_doctest_main,