In the rust-toolchain file, how do I set up a default Rust version and also specify a custom nightly version with a specific date? In my configuration, I use unstable features for formatting, but I want to use stable for building.
There is a better way to configure it outside of the rust-toolchain
It seems that it is not easy to do at present, but you can design two toml files at the same time, one is the default and the other is nightly, and use a command wrapper to switch which toml file to use before cargo is executed.
I think that is your best option at the moment, as there is no way to configure the toolchain for specific Cargo commands only, other than the toolchain override +<toolchain> as the first argument of the command (which is actually handled by rustup and not Cargo).