SlackBuildsOrg/development/rustup

rustup - The Rust toolchain installer

If you want to use the versions of rust installed through rustup by
default for everything, you'll need to add links to the rustup binary
on your path before the system rust.

You are free to choose where, but here is an example for a single user:

mkdir -p $HOME/.rustup/shims
for lnk in cargo cargo-fmt rls rustc rustdoc rustfmt rust-gdb rust-lldb
do
  (
    cd $HOME/.rustup/shims
    ln -s /usr/bin/rustup $lnk
  )
done

Then add the directory to your path (.bashrc for example):
export PATH="$HOME/.rustup/shims:$PATH"