Prevent dirty Cargo.lock changes from install (#18693)

Spawning from https://github.com/element-hq/synapse/pull/18689

Example CI failure that will stop people from leaving stray `Cargo.lock`
changes behind,

```
Error: Cargo.lock has uncommitted changes after install. Please run 'poetry install --extras all' and commit the Cargo.lock changes.
```

.github/workflows/tests.yml

@@ -263,6 +263,43 @@ jobs:
 
       - run: cargo clippy --all-features -- -D warnings
 
+  lint-rust:
+    runs-on: ubuntu-latest
+    needs: changes
+    if: ${{ needs.changes.outputs.rust == 'true' }}
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+
+      - name: Install Rust
+        uses: dtolnay/rust-toolchain@b3b07ba8b418998c39fb20f53e8b695cdcc8de1b # master
+        with:
+          toolchain: ${{ env.RUST_VERSION }}
+      - uses: Swatinem/rust-cache@98c8021b550208e191a6a3145459bfc9fb29c4c0 # v2.8.0
+
+      - name: Setup Poetry
+        uses: matrix-org/setup-python-poetry@5bbf6603c5c930615ec8a29f1b5d7d258d905aa4 # v2.0.0
+        with:
+          # Install like a normal project from source with all optional dependencies
+          extras: all
+          install-project: "true"
+          poetry-version: "2.1.1"
+
+      - name: Ensure `Cargo.lock` is up to date (no stray changes after install)
+        # The `::error::` syntax is using GitHub Actions' error annotations, see
+        # https://docs.github.com/en/actions/reference/workflow-commands-for-github-actions
+        run: |
+          if git diff --quiet Cargo.lock; then
+            echo "Cargo.lock is up to date"
+          else
+            echo "::error::Cargo.lock has uncommitted changes after install. Please run 'poetry install --extras all' and commit the Cargo.lock changes."
+            git diff --exit-code Cargo.lock
+            exit 1
+          fi
+
+  # This job is split from `lint-rust` because it requires a nightly Rust toolchain
+  # for some of the unstable options we use in `.rustfmt.toml`.
   lint-rustfmt:
     runs-on: ubuntu-latest
     needs: changes
@@ -274,7 +311,8 @@ jobs:
       - name: Install Rust
         uses: dtolnay/rust-toolchain@b3b07ba8b418998c39fb20f53e8b695cdcc8de1b # master
         with:
-          # We use nightly so that it correctly groups together imports
+          # We use nightly so that we can use some unstable options that we use in
+          # `.rustfmt.toml`.
           toolchain: nightly-2025-04-23
           components: rustfmt
       - uses: Swatinem/rust-cache@98c8021b550208e191a6a3145459bfc9fb29c4c0 # v2.8.0
@@ -309,6 +347,7 @@ jobs:
       - check-lockfile
       - lint-clippy
       - lint-clippy-nightly
+      - lint-rust
       - lint-rustfmt
       - lint-readme
     runs-on: ubuntu-latest
@@ -327,6 +366,7 @@ jobs:
             lint-pydantic
             lint-clippy
             lint-clippy-nightly
+            lint-rust
             lint-rustfmt
             lint-readme
 

.rustfmt.toml

@@ -1 +1,6 @@
+# Unstable options are only available on a nightly toolchain and must be opted into
+unstable_features = true
+
+# `group_imports` is an unstable option that requires nightly Rust toolchain. Tracked by
+# https://github.com/rust-lang/rustfmt/issues/5083
 group_imports = "StdExternalCrate"

changelog.d/18693.misc

@@ -0,0 +1 @@
+Prevent dirty `Cargo.lock` changes from install.