The main places to look are https://internals.rust-lang.org/, https://github.com/rust-lang/rfcs, https://github.com/rust-lang/rust (for actual implementation), and the Rust IRC channels such as #rust-internals on irc.mozilla.org.

IRC is used for quick casual conversation. The internals forum is used for discussion, "pre-RFC", and the like. The rfcs repo is use for actually discussing formal RFCs, as well as wishlist language and library bugs. The rust repo is for the actual implementation of rustc and the standard library itself.