rust-build-times

Rust Build Times

Purpose

Guide agents through diagnosing and improving Rust compilation speed: cargo-timings for build profiling, sccache for caching, the Cranelift codegen backend for faster dev builds, workspace crate splitting, LTO configuration trade-offs, and fast linkers (mold/lld).

Triggers

• "My Rust project takes too long to compile"
• "How do I profile which crates are slow to build?"
• "How do I set up sccache for Rust?"
• "What is the Cranelift backend and how does it help?"
• "Should I use thin LTO or fat LTO?"
• "How do I use the mold linker with Rust?"

Workflow

1. Diagnose with cargo-timings

# Build with timing report
cargo build --timings

# Opens build/cargo-timings/cargo-timing.html
# Shows: crate compilation timeline, parallelism, bottlenecks

# For release builds
cargo build --release --timings

# Key things to look for in the timing report:
# - Long sequential chains (no parallelism)
# - Individual crates taking > 10s (candidates for optimization)
# - Proc-macro crates blocking everything downstream

# cargo-llvm-lines — count LLVM IR lines per function (monomorphization)
cargo install cargo-llvm-lines
cargo llvm-lines --release | head -20
# Shows functions generating the most LLVM IR (template explosion)

2. sccache — compilation caching for Rust

# Install
cargo install sccache
# or: brew install sccache

# Configure for Rust builds
export RUSTC_WRAPPER=sccache

# Add to .cargo/config.toml (project or global)
# ~/.cargo/config.toml
[build]
rustc-wrapper = "sccache"

# Check cache stats
sccache --show-stats

# S3 backend for CI teams
export SCCACHE_BUCKET=my-rust-cache
export SCCACHE_REGION=us-east-1
export AWS_ACCESS_KEY_ID=xxx
export AWS_SECRET_ACCESS_KEY=yyy
sccache --start-server

# GitHub Actions with sccache
# - uses: mozilla-actions/[email protected]

3. Cranelift codegen backend

Cranelift is a fast codegen backend (vs LLVM) — produces slower code but compiles much faster. Ideal for development builds: