FWIW, I just needed to install the default nvim-lspconfig plugin (it's provided by the Neovim maintainers, and it doesn't do anything besides provide default LSP server configs for a lot of languages to the builtin Neovim LSP client) and call lspconfig.rust_analyzer.setup in init.lua. Configures itself to start whenever rust_analyzer is in PATH. Seems to work well enough! I don't think it's missing any functionality.