use_nix_fast.sh

# Usage: use nix_shell
#
# Works like use_nix, except that it's only rebuilt if the shell.nix or
# default.nix file changes.  This avoids scenarios where the nix-channel is
# being updated and all the projects now need to be re-built.
#
# To force the reload the derivation, run `touch shell.nix`
use_nix() {
  local shellfile=shell.nix
  local wd=$PWD/.direnv/nix
  local drvfile=$wd/shell.drv

  # same heuristic as nix-shell
  if [[ ! -f $shellfile ]]; then
    shellfile=default.nix
  fi

  if [[ ! -f $shellfile ]]; then
    fail "use nix_shell: shell.nix or default.nix not found in the folder"
  fi

  if [[ -f $drvfile && $(stat -c %Y "$shellfile") -gt $(stat -c %Y "$drvfile") ]]; then
    log_status "use nix_shell: removing stale drv"
    rm "$drvfile"
  fi

  if [[ ! -f $drvfile ]]; then
    mkdir -p "$wd"
    # instanciate the drv like it was in a nix-shell
    IN_NIX_SHELL=1 nix-instantiate \
      --show-trace \
      --add-root "$drvfile" --indirect \
      "$shellfile" >/dev/null
  fi

  direnv_load nix-shell "$drvfile" --run "$(join_args "$direnv" dump)"
  watch_file "$shellfile"
}

A version that works on MacOS: zupo/dotfiles@8129398#diff-87eeee0cef907adfcb47fef055323a46

I now use https://github.com/nix-community/nix-direnv/, that I installed on my MacOS like so: https://github.com/nix-community/nix-direnv/