Speeding up Rust CI Builds by 4x using Smart Caching

Rust compiles are notoriously slow. In developer environments, cargo's incremental compiler hides this, but in fresh CI environments, compiles can easily drag on for 10+ minutes. Here is how we reduced our build pipeline from 8 minutes to under 2 minutes.

The Problem: Incremental Builds in CI

GitHub Actions spins up a fresh virtual machine on every run. By default, Cargo compiles all dependency crates from scratch.

The Solution: Smart Caching

We can leverage swatinem/rust-cache or GitHub's default actions/cache. The trick is to cache both the ~/.cargo/registry and the target/ directory.

Here is a snippet of a highly optimized workflow file:

name: CI
on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - name: Install Rust
        uses: dtolnay/rust-toolchain@stable

      - name: Rust Cache
        uses: swatinem/rust-cache@v2
        with:
          # Shared key across runs
          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}

      - name: Run Tests
        run: cargo test --workspace --all-features

Using rust-cache ensures your builds only compile crates that have changed, saving massive developer time and CI runner minutes.