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>).
tools and infrastructure
Discussion about all "official" (rust-lang managed) non-rustc tools, and community infrastructure:
Discussion about the documentation, either the prose docs or the API docs.
Community happenings, such as talks, meetups, group projects, etc.
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.
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.
Discussion about this forum, its organization, how it works, and how we can improve it.
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!