tests: add test for Rust formatting
We enforce formatting for Python and C. It makes sense to do it
for Rust as well.
Since our rustfmt.toml relies on unstable rustfmt features, we
need to use a Nightly rustfmt with --unstable-features in order
for it to work. This is a bit hacky and I would prefer we remove
this requirement. But for now, this commit assumes this is the
way things must be and we go out of our way to detect and use the
rustfmt from the "nightly" toolchain, as installed via rustup.
We had to add some environment variables to the tests to make
the Rust binaries happy. Otherwise when running rustfmt we get
an error about no default toolchain being installed.
Differential Revision: https://phab.mercurial-scm.org/D7579
This directory is meant to cache artifacts useful for tests (such as bundle).
Those artifacts need to be cached because they are slow to regenerate on each
test but too large to be tracked within the repository. They are not expected
to change between each run and can be cached.
The `./scripts/` contains code to generate the artifact while the `cache`
directory contains resulting artifact.