Thank you for your interest in contributing to the Rust Reference!
There are a few ways of helping with the reference: critiquing the reference, editing the reference, fixing incorrect information, adding examples and glossary entries, and documenting new or otherwise undocumented features in Rust.
For a while, the Reference was basically ignored, and Rust continued gaining new features or changing old ones. It was also basically the introduction document before the first edition of the Rust book, and constantly in flux from the huge churn of the language design before v1.0.0. So there's a lot that's wrong, too teachy for people who should have basic understanding of Rust, or is too shallow for the Reference. As such, we have the warning saying there's work that needs to be done. Eventually, we plan to make sure everything is well documented enough that we can remove the warning.
It is encouraged for you to read the introduction to familiarize yourself with the kind of content the reference is expected to contain and the conventions it uses. Also, the style guide provides more detailed guidelines for formatting and content.
This is the easiest way to contribute. Basically, as you read the reference, if you find something confusing, incorrect, or missing, then you can file an issue against the reference explaining your concerns.
Typos and incorrect links get through from time to time. Should you find them, we welcome PRs to fix them. Additionally, larger editing jobs that help remove the number of parentheticals, remove comma splices, italicize term definitions and other similar tasks are helpful.
Examples are great. Many people will only read examples and ignore the prose. Ideally, every facet of every feature will have an example.
Likewise, the reference has a glossary. It doesn't need to explain every facet of every feature nor contain every definition, but it does need to be expanded upon. Ideally entries in the glossary link to the associated documentation.
There are a lot of features that are not documented at all or are documented poorly. This is the hardest, but definitely most valuable. Pick an unassigned issue from the issue tracker, and write about it.
While writing, you may find it handy to have a playpen open to test out what you are documenting.
Feel free to take information from the standard library and Rustonomicon as appropriate.
Note that we don't write documentation for purely library features such as
threads and IO and we don't write about Rust in the future. Documentation is
written as if the current stable release of Rust is the last release. The
master branch of the reference corresponds to what is stable on the
master branch ("nightly") of rust-lang/rust. If you want to write about
Rust in the future, you want the Unstable book.
When something that alters the language is stabilized, an issue should be opened on the reference issue tracker to track the documentation process. This should include links to any relevant information, such as the stabilization PR, the RFC, the tracking issue, and anything else that would be helpful for writing the documentation.