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A shell function: the project lives as long as the prompt

Level: 101 · for newcomers

One line: rs anyhow rand opens a nested shell inside a fresh Cargo project under $TMPDIR; exit brings you back to where you were and deletes it — no install, no config file, and the lifetime is the one thing every other route has to fake.

The function

Two copies, one per shell family. Fish is the login shell on this machine, so that is the one that autoloads:

  • rs.fish ↗ — copy to ~/.config/fish/functions/rs.fish; fish loads it on first use, no reload
  • rs.bash ↗ — source it from ~/.bashrc or ~/.zshrc; it is plain POSIX-ish shell and runs under either
function rs --description 'throwaway cargo project for this shell; deleted on exit'
    set -l dir (mktemp -d (string trim --right --chars=/ $TMPDIR)/rs.XXXXXX)
    cargo init --quiet --name scratch --vcs none --edition 2024 $dir
    or return
    printf '\n[lints.rust]\nunused_variables = "allow"\nunused_imports = "allow"\nunused_mut = "allow"\ndead_code = "allow"\n' >> $dir/Cargo.toml
    mkdir -p $dir/.cargo
    printf '[build]\nrustflags = ["--remap-path-prefix==%s/"]\n' $dir > $dir/.cargo/config.toml
    if test (count $argv) -gt 0
        cargo add --quiet --manifest-path $dir/Cargo.toml $argv
        or begin
            rm -rf $dir
            return 1
        end
    end
    echo "scratch: $dir  (exit deletes it)"
    fish --init-command "cd $dir; function fish_right_prompt; set_color yellow; echo scratch; set_color normal; end"
    rm -rf $dir
    echo "scratch: deleted"
end
Real output — rs, with pwd / ls -a / cargo run --quiet / exit typed at the nested prompt
scratch: /var/folders/bc/gzwjtz8d6mx3l2f9yyf_p8yw0000gn/T/rs.gC7I4m  (exit deletes it)
/var/folders/bc/gzwjtz8d6mx3l2f9yyf_p8yw0000gn/T/rs.gC7I4m
.
..
.cargo
Cargo.toml
src
Hello, world!
scratch: deleted

Two seconds from rs to a prompt inside a project with cargo run working, and nothing on disk afterwards.

What each line is for

Line Why it is there
mktemp -d …/rs.XXXXXX a unique folder under $TMPDIR; string trim strips the trailing / macOS puts on that variable, which would otherwise give T//rs.XXXXXX
cargo init … --vcs none a project with no .git — there is nothing to commit in a folder that deletes itself
[lints.rust] block the four scratch-noise lints off — unused_variables, unused_imports, unused_mut, dead_code — and not unused = "allow", which would also silence unused_must_use, the lint that catches a dropped Result
.cargo/config.toml remap --> /var/folders/…/rs.gC7I4m/src/main.rs:3:5 in every diagnostic instead of --> src/main.rs:3:5, so the path is clickable from any terminal when three scratch projects are open
cargo add … $argv the crates you named; an unknown crate name fails here, and the folder is removed rather than left half-made
fish --init-command "cd $dir; …" the nested shell, started inside the project, with a yellow scratch on the right of the prompt so you can tell which shell you are in
rm -rf $dir runs when the nested shell exits, however it exits

The trap: a shell inside a shell

You are one level deeper than you were. exit (or Ctrl-D) leaves the scratch shell and deletes the project; a second exit closes your real terminal. The right-prompt marker is there so the first exit is not a surprise, and $SHLVL counts the nesting if you want to check.

The second trap is an editor. open -a RustRover . from the nested shell works — the IDE opens the folder — but the folder is deleted when you exit, and RustRover will then be showing files that no longer exist. Close the project in the IDE first, then exit. If you want the deletion tied to the editor rather than the shell, that is what scratch.py --code does.

Versus the other routes

rs scratch.sh cargo temp
lifetime this shell session until reboot, or --prune the shell it opens, or the editor
setup one file in ~/.config/fish/functions/ one script on PATH cargo install
learner defaults yes yes no
crates rs anyhow rand --crates "anyhow rand", five by default cargo temp anyhow rand
opens the editor you do, from inside --rustrover / --code config file
leaves a project behind never by design, so you can come back only if you rm TO_DELETE

rs is the right one for "let me check what this method returns" — a question whose answer you will have in a minute and never need the file for. scratch.sh is for an afternoon: you will come back after lunch, and the project should still be there.

See also