Rust Build Times

# 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)

# 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/sccache-action@v0.0.4

# Install nightly (Cranelift requires nightly for now)
rustup toolchain install nightly
rustup component add rustc-codegen-cranelift-preview --toolchain nightly

# Use Cranelift for dev builds only
# .cargo/config.toml
[unstable]
codegen-backend = true

[profile.dev]
codegen-backend = "cranelift"

# Use per-build
CARGO_PROFILE_DEV_CODEGEN_BACKEND=cranelift \
RUSTFLAGS="-Zunstable-options" \
cargo +nightly build

# Before: one giant crate
[package]
name = "monolith"    # everything in one crate = sequential compile

# After: workspace with parallel crates
[workspace]
members = [
    "core",          # compiled in parallel
    "networking",    # no deps on ui → parallel with ui
    "ui",            # no deps on networking → parallel
    "server",        # depends on core + networking
    "cli",           # depends on core + ui
]

# Visualize dependency graph
cargo tree | head -30
cargo tree --graph | dot -Tsvg > deps.svg   # visual graph

# Check how many crates compile in parallel
cargo build -j$(nproc) --timings    # maximize parallelism

# Cargo.toml profile configuration
[profile.release]
lto = "thin"         # thin LTO: good performance, much faster than "fat"
codegen-units = 1    # needed for best optimization (but disables parallelism)

[profile.release-fast]
inherits = "release"
lto = "fat"          # full LTO: maximum performance, very slow link

[profile.dev]
lto = "off"          # never use LTO in dev (compilation speed)
codegen-units = 16   # maximize parallel codegen in dev

# mold — fastest general-purpose linker (Linux)
sudo apt-get install mold

# .cargo/config.toml
[target.x86_64-unknown-linux-gnu]
linker = "clang"
rustflags = ["-C", "link-arg=-fuse-ld=mold"]

# Or use cargo-zigbuild (uses zig cc as linker)
cargo install cargo-zigbuild
cargo zigbuild --release

# lld — LLVM's linker (faster than GNU ld, available everywhere)
# .cargo/config.toml
[target.x86_64-unknown-linux-gnu]
rustflags = ["-C", "link-arg=-fuse-ld=lld"]

# On macOS: zld or the default lld
[target.x86_64-apple-darwin]
rustflags = ["-C", "link-arg=-fuse-ld=/usr/local/bin/zld"]

# Reduce debug info level (faster but less debuggable)
# Cargo.toml
[profile.dev]
debug = 1           # 0=off, 1=line tables, 2=full (default)
# debug=1 saves 20-40% on debug build time

# Split debug info (reduces linker input)
[profile.dev]
split-debuginfo = "unpacked"   # macOS: equivalent of gsplit-dwarf

# Disable incremental compilation (sometimes faster for full rebuilds)
CARGO_INCREMENTAL=0 cargo build

# Reduce proc-macro compile time (pin heavy proc-macro deps)
# Heavy proc-macros: serde, tokio, axum — keep versions stable