Currently, the ipv6 reachability of websites operated by the rust team is pretty bad.
Sites with ipv6 support
Sites without IPv6 support
crates.io; probably the most important rust related site
These are just the sites that I can recall, but there are probably many more.
I’m not listing docs.rs here because its maintained by the community. But, that site does have ipv6.
I know that a large part if not all of those sites are hosted on amazon’s cloud. It seems they support ipv6 since August this year: https://aws.amazon.com/blogs/aws/now-available-ipv6-support-for-amazon-s3/