I recently installed Rust on my Gentoo machine and was interested in creating two separate installations: one for the system to build Rust-based packages like Firefox, and one for local development. Separating the installations in this way makes a lot of sense to me, since this is what Gentoo forces you to do with Python installations.

The recommended tool to manage rust installations is rustup. If there is an existing installation, (e.g. from portage) rustup will complain and refuse to install anything without an override. rustup recommends to "link" your system installation so rustup can learn about it. If you do this, you can instruct rustup to build sources with either your system or local toolchains. I wasn't interested in doing this, since I wasn't sure if rustup would attempt to update things behind the package manager's back.

rustup installs all binaries in $HOME/.cargo/bin by default. So in order to have isolation we just have to play with the paths. Gentoo's documentation shows you how to do this with bash, but I'm using fish so the solution was marginally different.

I added the following function to ~/.config/fish/functions:

function localrust --description 'Use a local installation of rust for this command'
    set -lp PATH $HOME/.cargo/bin
    $argv
end
        

It prepends $HOME/.cargo/bin to the path before executing any command passed to the function. The important part here is the -l flag to make the change local to the function's scope. This way, the only way to access local binaries (including rustup itself) is with localrust <command>.