A project I started while working on Leaf, to check for broken links in your documentation.
We took a lot of care to have good documentation of the codebase, which for us also meant often linking from e.g. one method to another it likely will used together with in the rustdoc. However with a lot of refactoring going on too, this could often lead to broken links because the item you were linking to wasn't in the same module anymore. To ease that pain I created cargo-deadlinks, so you could automatically check for those broken links in CI/CD, and ensure high-quality documentation.
Since it's inception, it has gotten a few more features, notably checking HTTP links (which can be useful if you want to link to external reference docs) and also better support of checking non-rustdoc files.
In late 2020 I transfered maintainership to Joshua Nelson of the Rustdoc team, and around that time intra-doc links landed in Rustdoc stable as feature that fills a similar niche.