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Faster Builds with cargo-slicer

cargo-slicer is a RUSTC_WRAPPER that stubs unreachable library functions at the MIR level, skipping LLVM codegen for code the final binary never calls.

Benchmark Results

Environment Mode Baseline With cargo-slicer Wall-time savings
48-core server syn pre-analysis 3m 52s 3m 31s -9.1%
48-core server MIR-precise 3m 52s 2m 49s -27.2%
Raspberry Pi 4 syn pre-analysis 25m 03s 17m 54s -28.6%

All measurements are clean cargo +nightly build --release. MIR-precise mode reads actual compiler MIR to build a more accurate call graph, stubbing 1,060 mono items vs 799 with syn-based analysis.

CI Integration

The workflow .github/workflows/ci-build-fast.yml runs an accelerated release build alongside the standard one. It triggers on Rust-code changes and workflow changes, does not gate merges, and runs in parallel as a non-blocking check.

CI uses a resilient two-path strategy:

  • Fast path: install cargo-slicer plus the rustc-driver binaries and run the MIR-precise sliced build.
  • Fallback path: if rustc-driver install fails (for example due to nightly rustc API drift), run a plain cargo +nightly build --release instead of failing the check.

This keeps the check useful and green while preserving acceleration whenever the toolchain is compatible.

Local Usage

# One-time install
cargo install cargo-slicer
rustup component add rust-src rustc-dev llvm-tools-preview --toolchain nightly
cargo +nightly install cargo-slicer --profile release-rustc \
  --bin cargo-slicer-rustc --bin cargo_slicer_dispatch \
  --features rustc-driver

# Build with syn pre-analysis (from zeroclaw root)
cargo-slicer pre-analyze
CARGO_SLICER_VIRTUAL=1 CARGO_SLICER_CODEGEN_FILTER=1 \
  RUSTC_WRAPPER=$(which cargo_slicer_dispatch) \
  cargo +nightly build --release

# Build with MIR-precise analysis (more stubs, bigger savings)
# Step 1: generate .mir-cache (first build with MIR_PRECISE)
CARGO_SLICER_MIR_PRECISE=1 CARGO_SLICER_WORKSPACE_CRATES=zeroclaw,zeroclaw_robot_kit \
  CARGO_SLICER_VIRTUAL=1 CARGO_SLICER_CODEGEN_FILTER=1 \
  RUSTC_WRAPPER=$(which cargo_slicer_dispatch) \
  cargo +nightly build --release
# Step 2: subsequent builds automatically use .mir-cache

How It Works

  1. Pre-analysis scans workspace sources via syn to build a cross-crate call graph (~2 s).
  2. Cross-crate BFS from main() identifies which public library functions are actually reachable.
  3. MIR stubbing replaces unreachable bodies with Unreachable terminators — the mono collector finds no callees and prunes entire codegen subtrees.
  4. MIR-precise mode (optional) reads actual compiler MIR from the binary crate's perspective, building a ground-truth call graph that identifies even more unreachable functions.

No source files are modified. The output binary is functionally identical.