A default install of Neovim 0.6.1 includes the https://github.com/rust-lang/rust.vim plugin but it is a very old version. The problem that I noticed was that :RustFmt did not work. I got the following error:
Error detected while processing function rustfmt#Format..<SNR>40_RunRustfmt: line 48: E776: No location list
The fix was to install the plugin from https://github.com/rust-lang/rust.vim which overrides the built-in plugin. Although that was simple enough, it does make me question the logic of including the rust.vim plugin in the default Neovim build if it is not kept up-to-date. The user experience of broken functionality is worse than missing functionality. This issue is not unique to Neovim though, the latest release of Vim also has an old version of rust.vim.