The idea here is if you don't want rust in /usr/local
you can put something like this is your .profile:
```
export RUSTUP_PREFIX=$HOME/.local/rust
export PATH=$PATH:${RUSTUP_PREFIX}/bin
export DYLD_LIBRARY_PATH=$DYLD_LIBRARY_PATH:${RUSTUP_PREFIX}/lib
```
Then when you run rustup, it will update the install
in ${RUSTUP_PREFIX} without having to remember to pass
an explicit --prefix argument every time.
VAL_OPTIONS=""
flag uninstall "only uninstall from the installation prefix"
-valopt prefix "" "set installation prefix"
+valopt prefix "${RUSTUP_PREFIX}" "set installation prefix"
valopt date "" "use the YYYY-MM-DD nightly instead of the current nightly"
valopt channel "beta" "use the selected release channel [beta]"
flag save "save the downloaded nightlies to ~/.rustup"