Automate cargo-generate branch updates

.github/workflows/cargo-generate.yaml

@@ -0,0 +1,110 @@
+# This workflow is used to automatically update the `cargo-generate` branch. If you copied the
+# template, you can safely delete this. :)
+#
+# BE VERY CAREFUL about what this workflow runs, since IT HAS THE ABILITY TO DELETE THE ENTIRE
+# REPOSITORY'S CONTENTS. For reason, stick to pre-installed tools and trusted actions (such as
+# those hosted from the `actions` organization.)
+
+name: Update `cargo-generate` branch
+
+on:
+  # Update `cargo-generate` from `main` every time `main` is updated.
+  push:
+    branches: main
+  # Allow this workflow to be manually triggered.
+  workflow_dispatch:
+
+# This workflows also has write-access for content and workflow, but it is through
+# CARGO_GENERATE_PAT.
+permissions:
+  # We need write permissions to create new issues.
+  issues: write
+
+jobs:
+  update:
+    runs-on: ubuntu-latest
+    # Only run this job on the source repository, skipping forks.
+    if: ${{ github.repository == 'TheBevyFlock/bevy_quickstart' }}
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          # Fetch entire history, since we need it all to successfully merge.
+          fetch-depth: 0
+          # Specify a personal access token, since the default one does not let workflows modify
+          # the `.github/workflows` folder. This PAT needs write permissions for Contents and
+          # Workflows. I recommend creating a scoped token that only has access to this repository.
+          token: ${{ secrets.CARGO_GENERATE_PAT }}
+
+      - name: Configure Git user
+        env:
+          # From https://github.com/actions/checkout/pull/1707.
+          NAME: github-actions[bot]
+          EMAIL: 41898282+github-actions[bot]@users.noreply.github.com
+        run: |
+          git config user.name $NAME
+          git config user.email $EMAIL
+
+      - name: Switch to `cargo-generate` branch
+        run: git checkout cargo-generate
+
+      - name: Merge `main` branch into `cargo-generate`
+        id: merge
+        # We use `--no-edit` to prevent any editor from being opened, since we can't close it.
+        run: git merge main --no-edit --verbose
+
+      - name: Handle merge conflict
+        # Only run this if merging failed.
+        if: ${{ failure() && steps.merge.outcome == 'failure' }}
+        env:
+          TITLE: Failed to auto-update `cargo-generate`
+          BODY: |
+            There were merge conflicts while trying to update `cargo-generate` from `main`. You can
+            do it manually by running:
+
+            ```bash
+            # Update your local copy of the `main` branch.
+            git switch main
+            git pull
+            # Checkout the `cargo-generate` branch.
+            git checkout cargo-generate
+            # Merge changes from `main` to `cargo-generate`. You will have to fix merge conflicts.
+            git merge main
+            ```
+
+            <details>
+              <summary><code>git status</code></summary>
+
+              ```
+              GIT_STATUS_IDENTIFIER
+              ```
+            </details>
+
+            <details>
+              <summary><code>git diff --diff-filter=U</code></summary>
+
+              ```diff
+              GIT_DIFF_IDENTIFIER
+              ```
+            </details>
+
+            _This is an automated message created using `cargo-generate.yaml`._
+          # This is required to use the `gh` CLI.
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          GIT_STATUS=$(git status)
+
+          # Filter the diff to only include files with merge conflicts.
+          GIT_DIFF=$(git diff --diff-filter=U)
+
+          # Replace `GIT_STATUS_IDENTIFIER` and `GIT_DIFF_IDENTIFIER` with their outputs.
+          BODY=${BODY/GIT_STATUS_IDENTIFIER/"$GIT_STATUS"}
+          BODY=${BODY/GIT_DIFF_IDENTIFIER/"$GIT_DIFF"}
+
+          # Create the issue. The text must be in quotes, since it contains spaces.
+          gh issue create --title "$TITLE" --body "$BODY"
+
+      - name: Push changes
+        # Only run if merging succeeded.
+        if: ${{ success() && steps.merge.outcome == 'success' }}
+        run: git push