I'm using Fedora 35, and no matter what location I've tried for the Rust Toolchain in IntelliJ IDEA I get "Invalid toolchain location: can't find Cargo in..." I've tried /bin and many, many other locations. I prefer not to use rustup and would like to do this with the standard Fedora Rust installation.
It seems like it has to do with the sandbox effect of Flatpaks. I reinstalled IDEA from Fedora's Copr instead of the Flatpak and everything was read automatically.