minor: use new rustdoc:: warn prefix (#457)

Author: abr-egn

.evergreen/check-rustdoc.sh

@@ -3,6 +3,6 @@
 set -o errexit
 
 . ~/.cargo/env
-cargo rustdoc -- -D warnings
-cargo rustdoc --no-default-features --features async-std-runtime -- -D warnings
-cargo rustdoc --no-default-features --features sync -- -D warnings
+cargo +nightly rustdoc -- -D warnings --cfg docsrs
+cargo +nightly rustdoc --no-default-features --features async-std-runtime -- -D warnings --cfg docsrs
+cargo +nightly rustdoc --no-default-features --features sync -- -D warnings --cfg docsrs

src/lib.rs

@@ -282,7 +282,10 @@
 //! it will only happen in a minor or major version release.
 
 #![warn(missing_docs)]
-#![warn(missing_crate_level_docs)]
+// `missing_crate_level_docs` was renamed with a `rustdoc::` prefix in rustc 1.55, but isn't
+// supported in the MSRV.
+// TODO: remove the wrapping cfg_attr if/when the MSRV is 1.55+.
+#![cfg_attr(docsrs, warn(rustdoc::missing_crate_level_docs))]
 #![cfg_attr(
     feature = "cargo-clippy",
     allow(