[ci] Add fast benchmark dry run

Author: patrick-ogrady

.github/workflows/benchmark.yml

@@ -3,6 +3,11 @@ name: Benchmark
 on:
   workflow_dispatch:
     inputs:
+      dry_run:
+        description: "Generate synthetic benchmark output instead of running cargo bench"
+        required: false
+        default: true
+        type: boolean
       publish:
         description: "Publish results to the benchmarks repository"
         required: false
@@ -68,17 +73,24 @@ jobs:
       - name: Run setup
         uses: ./.github/actions/setup
       - name: Compile benchmarks
+        if: github.event_name == 'schedule' || !inputs.dry_run || inputs.publish
         env:
           RUSTFLAGS: "--cfg full_bench"
         run: cargo bench ${{ matrix.cargo_flags }} --benches -p ${{ matrix.package }} --no-run
       - name: Run benchmarks
+        if: github.event_name == 'schedule' || !inputs.dry_run || inputs.publish
         env:
           RUSTFLAGS: "--cfg full_bench"
         run: |
           cargo bench ${{ matrix.cargo_flags }} \
             --benches -p ${{ matrix.package }} \
             -- --output-format bencher \
             | tee "${{ matrix.benchmark_name }}.txt"
+      - name: Generate dry-run benchmark result
+        if: github.event_name == 'workflow_dispatch' && inputs.dry_run && !inputs.publish
+        run: |
+          output="${{ matrix.benchmark_name }}.txt"
+          printf 'test dry_run::mechanics ... bench:           1 ns/iter (+/- 0)\n' | tee "$output"
       - name: Upload benchmark result
         uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
         with: