.,...It works?

build-support/bin/rust/bootstrap_code.sh

now looks like:

function _run_bootstrapped_pants_command() {
  PANTS_VERSION=2.18.0rc0 pants \
    --no-pantsd \
    --no-verify-config \
    --backend-packages='["pants.backend.shell"]' \
    --pants-ignore='[ \
          "/BUILD", \
          "/3rdparty/jvm", \
          "/3rdparty/python", \
          "/testprojects", \
          "/build-support", \
          "/tests", \
          "/pants-plugins", \
          "/pants-plugins", \
          "/.github", \
      ]' \
    --pythonpath="[]" \
    --environments-preview-names="{}" \
    "$@"
}

function _calculate_current_hash() {
  _run_bootstrapped_pants_command peek src/rust/engine:engine-and-client | grep -o '"sources_fingerprint": "[^"]*' | cut -d'"' -f4
}

function _build_native_code() {
  banner "Building native code..."
  _run_bootstrapped_pants_command export-codegen src/rust/engine:engine-and-client
}

... and it works. and changing branches with Rust changes is instant on the Nth run