Discussion about the compiler itself -- internal data structures and algorithms, optimizations, parsing, and so on. Home of the compiler subteam.
Discussion on designs for new language features and other user-facing language changes. Home of the language design team.
Discussion around libraries, either in the community or officially supported (hosted by <code>rust-lang</code>).
Discussion about the documentation, either the prose docs or the API docs.
Discussion about the Rust Guidelines. Available rendered <a href="http://aturon.github.io/style/whitespace.html">here</a> and <a href="https://github.com/rust-lang/rust-guidelines">as source</a>. Use the "style" subcategory as appropriate!
tools and infrastructure
Discussion about all "official" (rust-lang managed) non-rustc tools, and community infrastructure:
Discussion about this forum, its organization, how it works, and how we can improve it.
Low-traffic category for announcements about the decision making process, such as when RFCs are entering the final comment period, new meeting minutes are available, and so forth.
Community happenings, such as talks, meetups, group projects, etc.
Style and Formatting
A category for discussion about the behavior of rustfmt (with a focus on what the defaults ought to be). See <a href="https://github.com/rust-lang/rfcs/blob/master/text/1607-style-rfcs.md">RFC 1607</a> for more details.