Use rust-cache GitHub Actions For R Package

This is a quick note about how to use the rust-cache action on a repository of an R package.

Problem

rust-cache doesn’t work with the r-lib/actions/check-r-package action without a tweak. Why? This is because check-r-package runs the check on a new directory, "check" by default.

An naive idea is to set workspaces parameter of rust-cache to the check dir. Like this.

- uses: Swatinem/rust-cache@v2
  with:
    workspaces: check/src/rust

- uses: r-lib/actions/check-r-package@v2
  with:
    upload-results: 'never'
    error-on: '"error"'
    build_args: 'c("--no-manual","--compact-vignettes=gs+qpdf")'

But, this doesn’t work. check-r-package is done on the tarball of the package, so it’s not possible to inject the cache into it.

Idea

My workaround is to change the target dir of cargo build to an absolute path. On a GitHub Actions runner, there are many environmental variables are available. I chose GITHUB_WORKSPACE to detect if the check is executed on GitHub Actions and, at the same time, to know the location of the workspace.

You can check the actual settings on my repo, but here’s the simpler version:

configure:

# In order to use rust-cache action, target dir needs to be a fixed, existing path.
if [ -d "${GITHUB_WORKSPACE}" ]; then
  CARGO_TARGET_DIR="${GITHUB_WORKSPACE}/src/rust/target"
  echo "Detected GitHub Actions envvar. Using ${CARGO_TARGET_DIR} for the target dir of cargo"
else
  CARGO_TARGET_DIR='$(CURDIR)/rust/target'
fi

sed \
  -e "s|@CARGO_TARGET_DIR@|${CARGO_TARGET_DIR}|" \
  src/Makevars.in > src/Makevars

src/Makevars.in:

CARGO_BUILD_ARGS = ... --target-dir @CARGO_TARGET_DIR@
...
$(STATLIB):
    export PATH="$(PATH):$(HOME)/.cargo/bin" && \
      cargo build $(CARGO_BUILD_ARGS)

By doing above, cargo build invoked by r-lib/actions/check-r-package always refers to the existing directory, src/rust/target. So, we can restore the cache into it.

- uses: Swatinem/rust-cache@v2
  with:
    workspaces: src/rust

That’s all. This should work.

Caveats

While this works, I see some crates are always recompiled. It seems this happens on Windows more frequently. I tried to investigate, but couldn’t find the root cause… Probably some [rerun-if-changed] settings matters? If you know the answer, I’d really appreciate it.

Anyway, as this problem exists, please use this technique at your own risk. In some cases, using rust-cache might increase the CI time.

macOS:

Windows: