Automate updates of subtree/library branch

There should not be a need for manual processes for this stage of the
subtree update. Merge conflicts (which might require a human to
intervene) will only happen once attempting to merge from
subtree/library back into main. Automation for that step will follow in
a separate PR.

.github/workflows/update-subtree.yml

@@ -0,0 +1,75 @@
+name: Kani Metrics Update
+
+on:
+  schedule:
+    - cron: '0 14 * * *'  # Run at 14:00 UTC every day
+  workflow_dispatch:
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  update-subtree-library:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout Repository
+      uses: actions/checkout@v4
+
+    - name: Fetch Kani toolchain version
+      run: |
+        mkdir kani-tmp
+        cd kani-tmp
+        git init
+        git remote add origin https://github.com/model-checking/kani
+        git fetch --depth 1 origin main
+        git checkout main
+        TOOLCHAIN_DATE=$(grep -oP 'channel = "nightly-\K\d{4}-\d{2}-\d{2}' rust-toolchain.toml)
+        COMMIT_HASH=$(curl https://static.rust-lang.org/dist/$TOOLCHAIN_DATE/channel-rust-nightly-git-commit-hash.txt)
+        if [ -z "$COMMIT_HASH" ]; then
+          echo "Could not find commit hash on static.rust-lang.org"
+          exit 1
+        fi
+        echo "Kani toolchain date: ${TOOLCHAIN_DATE}"
+        echo "TOOLCHAIN_DATE=${TOOLCHAIN_DATE}" >> $GITHUB_ENV
+        echo "Kani toolchain hash: ${COMMIT_HASH}"
+        echo "COMMIT_HASH=${COMMIT_HASH}" >> $GITHUB_ENV
+        cd ..
+        rm -r kani-tmp
+
+    - name: Update subtree/library locally
+      run: |
+        git remote add upstream https://github.com/rust-lang/rust
+        git fetch upstream
+
+        # Ensure "upstream/master" branch contains the target commit
+        if ! git show ${COMMIT_HASH} --oneline --no-patch; then
+          echo "Rust commit ${COMMIT_HASH} cannot be found."
+          exit 1
+        fi
+
+        # subtree/library has the subtree-split variant of a commit that previously
+        # went into upstream/master. Running `git subtree split` on top of the
+        # original commit is much faster (my experiment when working on 280k+
+        # commits: 113 seconds vs 613 seconds) than using the HEAD commit from
+        # subtree/library as base.
+        SUBTREE_HEAD=$(git log --format=%s -n 1 origin/subtree/library)
+        echo "SUBTREE_HEAD: ${SUBTREE_HEAD}"
+        UPSTREAM_ONTO=$(git log --grep=$SUBTREE_HEAD -n 1 --format=%H upstream/master)
+        echo "UPSTREAM_ONTO: ${UPSTREAM_ONTO}"
+
+        git checkout ${COMMIT_HASH}
+        git subtree split --prefix=library --onto $UPSTREAM_ONTO -b subtree/library
+
+    - name: Create Pull Request
+      uses: peter-evans/create-pull-request@v7
+      with:
+        commit-message: Update Kani metrics
+        title: 'Update subtree/library'
+        body: |
+          This is an automated PR to update the subtree/library branch to the changes
+          up to and including ${{ env.COMMIT_HASH }} of ${{ env.TOOLCHAIN_DATE }}.
+        branch: update-subtree/library
+        delete-branch: true
+        base: subtree/library