ci: seed shared Rust caches from main

.github/workflows/branch-checks.yml

@@ -109,8 +109,6 @@ jobs:
           # Keep branch-check caches partitioned by runner architecture; lint
           # and test intentionally share the same job-local target directory.
           shared-key: rust-checks-${{ matrix.runner }}
-          # Cache the sccache directory too
-          cache-directories: .cache/sccache
           # Preserve compiled artifacts from failed lint/test runs so the next
           # push to the same PR branch does not start from a cold cache.
           cache-on-failure: "true"

.github/workflows/rust-cache-seed.yml

@@ -0,0 +1,72 @@
+name: Rust Cache Seed
+
+on:
+  push:
+    branches:
+      - main
+  workflow_dispatch:
+
+env:
+  CARGO_TERM_COLOR: always
+  CARGO_INCREMENTAL: "0"
+  MISE_GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+  SCCACHE_GHA_ENABLED: "true"
+
+permissions:
+  contents: read
+  packages: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  rust:
+    name: Rust (${{ matrix.runner }})
+    strategy:
+      fail-fast: false
+      matrix:
+        runner: [linux-amd64-cpu8, linux-arm64-cpu8]
+    runs-on: ${{ matrix.runner }}
+    env:
+      SCCACHE_GHA_VERSION: branch-checks-rust-${{ matrix.runner }}
+    container:
+      image: ghcr.io/nvidia/openshell/ci:latest
+      credentials:
+        username: ${{ github.actor }}
+        password: ${{ secrets.GITHUB_TOKEN }}
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
+
+      - name: Install tools
+        run: mise install --locked
+
+      - name: Configure GHA sccache backend
+        uses: mozilla-actions/sccache-action@9e7fa8a12102821edf02ca5dbea1acd0f89a2696 # v0.0.10
+
+      - name: Cache Rust target and registry
+        uses: Swatinem/rust-cache@c19371144df3bb44fab255c43d04cbc2ab54d1c4 # v2
+        with:
+          shared-key: rust-checks-${{ matrix.runner }}
+          cache-on-failure: true
+
+      - name: Format
+        run: mise run rust:format:check
+
+      - name: Lint
+        run: mise run rust:lint
+
+      - name: Test
+        run: mise run test:rust
+
+      - name: sccache stats
+        if: always()
+        run: |
+          set +e
+          stats_bin="${SCCACHE_PATH:-sccache}"
+          "$stats_bin" --show-stats
+          status=$?
+          if [ "$status" -ne 0 ]; then
+            echo "::warning::sccache stats unavailable (exit $status)"
+          fi
+          exit 0