[ci] Roll pinned nightly toolchain automatically

Closes #348

.github/workflows/roll-pinned-toolchain-versions.yml

@@ -0,0 +1,54 @@
+name: Roll pinned toolchain versions
+on:
+  schedule:
+    - cron: '29 15 * * *'
+  # TODO: Remove this before merging!!!
+  pull_request:
+
+# TODO: Once we confirm this works, try changing this to `read-all` to see if
+# the job still works.
+permissions: write-all
+
+jobs:
+  roll:
+    name: Roll pinned toolchain versions
+    runs-on: ubuntu-latest
+    steps:
+      - name: "Checkout code"
+        uses: actions/checkout@3df4ab11eba7bda6032a0b82a6bb43b11571feac # v4.0.0
+        # TODO: Once we confirm this works, try uncommenting this to see if the
+        # job still works.
+        # with:
+        #   persist-credentials: false
+      - name: "Calculate target version"
+        # Use yesterday's date (`-d '-1 day'`) so we're sure the nightly for that
+        # date has actually been published yet. This allows us to not worry
+        # about what time of day this job runs.
+        run: echo "ZC_TARGET_NIGHTLY=nightly-$(date -d '-1 day' +%Y-%m-%d)" >> $GITHUB_ENV
+      - name: Install Rust with ${{ env.ZC_TARGET_NIGHTLY }} toolchain
+        uses: dtolnay/rust-toolchain@00b49be78f40fba4e87296b2ead62868750bdd83 # stable
+        with:
+            toolchain: ${{ env.ZC_TARGET_NIGHTLY }}
+      - name: "Update files"
+        run: |
+          set -eox pipefail
+
+          # Confirm that `Cargo.toml` lists the pinned nightly version in the
+          # expected format. This is a prerequisite for the subsequent `sed`
+          # command. If this fails, the preceding `set -eo pipefail` will cause
+          # the script to exit with an error.
+          REGEX='^pinned-nightly = "[a-z0-9-]*"$'
+          grep "$REGEX" Cargo.toml >/dev/null
+          sed -i -e "s/$REGEX/pinned-nightly = \"$ZC_TARGET_NIGHTLY\"/" Cargo.toml
+
+          # Confirm that the update didn't bork `Cargo.toml`.
+          grep "$REGEX" Cargo.toml >/dev/null
+
+          # Update `.stderr` files as needed for the new nightly version.
+          TRYBUILD=overwrite ./cargo.sh +nightly test --all-features --package zerocopy
+          TRYBUILD=overwrite ./cargo.sh +nightly test --all-features --package zerocopy-derive
+      
+      - name: "Submit PR"
+        uses: peter-evans/create-pull-request@153407881ec5c347639a548ade7d8ad1d6740e38 # v5.0.2
+        with:
+          commit-message: "[ci] Roll pinned nightly toolchain"