Move broken-intra-doc-link lint config to command line

The docs lint `broken-intra-doc-links` has been changed but the new name
is not available in our MSRV, this means we get a build warning. We only
build docs with the nightly toolchain so we can move this lint control
to the docs build command in `test.sh` instead of doing it crate wide.

With this patch applied devs risk not noticing docs link issues until
they hit them on CI _if_ they do not build with the test script or
explicitly pass in `-- -D rustdoc::broken-intra-doc-links`, which no one
is going to do. Hence we add a line to the readme with a shell alias
that can be used to check docs, taken directly from `test.sh`.

Author: tcharding

README.md

@@ -101,6 +101,13 @@ cargo test
 
 Please refer to the [`cargo` documentation](https://doc.rust-lang.org/stable/cargo/) for more detailed instructions.
 
+### Building the docs
+
+We build docs with the nightly toolchain, you may wish to use the following
+shell alias to check your documentation changes build correctly.
+
+```alias build-docs='RUSTDOCFLAGS="--cfg docsrs" cargo +nightly rustdoc --features="$FEATURES" -- -D rustdoc::broken-intra-doc-links'```
+
 ## Pull Requests
 
 Every PR needs at least two reviews to get merged. During the review phase

contrib/test.sh

@@ -56,7 +56,7 @@ done
 
 # Build the docs if told to (this only works with the nightly toolchain)
 if [ "$DO_DOCS" = true ]; then
-    RUSTDOCFLAGS="--cfg docsrs" cargo doc --all --features="$FEATURES"
+    RUSTDOCFLAGS="--cfg docsrs" cargo +nightly rustdoc --features="$FEATURES" -- -D rustdoc::broken-intra-doc-links
 fi
 
 # Fuzz if told to

src/lib.rs

@@ -57,7 +57,6 @@
 #![deny(unused_imports)]
 #![deny(missing_docs)]
 #![deny(unused_must_use)]
-#![deny(broken_intra_doc_links)]
 
 #[cfg(not(any(feature = "std", feature = "no-std")))]
 compile_error!("at least one of the `std` or `no-std` features must be enabled");