Add rustfmt to CI workflow

[str4d]

#63 adds a workflow to check that the test vectors are up-to-date, and also hard-codes the test vectors. Once #65 is implemented, we'll want the hard-coded Rust test vectors to be nicely formatted, so we should add rustfmt to the CI workflow to ensure this.

[str4d]

Planned workflow change:

diff --git a/.github/workflows/test_vectors.yml b/.github/workflows/test_vectors.yml
index e2184b9..d53adf5 100644
--- a/.github/workflows/test_vectors.yml
+++ b/.github/workflows/test_vectors.yml
@@ -30,8 +30,20 @@ jobs:
       - name: Install dependencies
         run: poetry install --no-root
 
+      - uses: actions-rs/toolchain@v1
+        with:
+          toolchain: stable
+          components: rustfmt
+        if: matrix.kind == 'rust'
+
       - name: Regenerate test vectors
         run: ./regenerate.sh ${{ matrix.kind }} ${{ matrix.extension }}
 
+      - name: Check formatting
+        uses: actions-rs/cargo@v1
+        with:
+          command: fmt
+        if: matrix.kind == 'rust'
+
       - name: Verify there are no changes
         run: test -z "$(git status --porcelain)"

[daira]

Shouldn't regenerate.sh run rustfmt (when generating Rust)? Then the existing flow would just do the right thing.

[str4d]

That might end up being how we do this (though the above workflow change would still be necessary in order for CI to have rustfmt even be available). But that still won't work for the existing workflow, only after #65. The current Rust output is un-formattable, because it expects to be copy-pasted into a function (as I was doing with the first Rust test vectors we wrote for Sapling).