# rust-build-times

Guides coding assistants through profiling and optimizing Rust build times. Teaches the agent to configure caching, faster linkers, and compiler backends.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 468 installs · ★ 267
- Author: mohitmishra786
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/rust/rust-build-times/SKILL.md
- Updated: 2026-06-27
- HTML: https://affpapa.org/en/nevibecode/skills/rust-build-times

## What it does

This skill guides agents through locating compile bottlenecks and applying proven fixes. It instructs the assistant to inspect build profiles via `cargo build --timings` and inspect monomorphization bloat with `cargo-llvm-lines`. Heavy proc-macros become visible immediately.

The instructions cover practical configurations for development and release profiles. The model learns how to enable `sccache` with local or S3 storage, switch to fast linkers like `mold` or `lld`, and reorganize single-crate projects into workspaces to unlock parallel compilation.

Because the Cranelift codegen backend requires a Rust `nightly` toolchain, it will not run on standard stable compiler releases. The guide also details LTO trade-offs so release builds do not spend unnecessary minutes in link phases.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill rust-build-times -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/rust-build-times/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/rust-build-times/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill rust-build-times -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/rust-build-times/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill rust-build-times -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/rust-build-times/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install mohitmishra786/low-level-dev-skills rust-build-times
```

2. **Manually: put the files in place**
- File: `.github/skills/rust-build-times/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill rust-build-times -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/mohitmishra786/low-level-dev-skills --path skills/rust/rust-build-times
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/rust-build-times/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add mohitmishra786/low-level-dev-skills --skill rust-build-times -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/rust/rust-build-times/SKILL.md

## When to use it
- identify slow compilation bottlenecks using `cargo build --timings`
- configure local and cloud compiler caching with `sccache`
- accelerate dev iteration cycles with the Cranelift codegen backend
- split monolithic crates into workspace members for parallel builds
- configure `mold` or `lld` linkers to cut linking overhead

## Example prompts
- Profile our project with cargo-timings and point out the slowest crates
- Configure sccache and mold in .cargo/config.toml to speed up our dev builds
- Help me set up thin LTO and split debuginfo to reduce release link times

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 2490d52343abac66.

BEGIN THIRD-PARTY FILE 2490d52343abac66
````text
---
name: rust-build-times
description: Rust build time optimization skill for reducing slow compilation. Use when using cargo-timings to profile builds, configuring sccache for Rust, using the Cranelift backend, splitting workspaces for parallelism, choosing between thin LTO and fat LTO, or using the mold linker with Rust. Activates on queries about slow Rust compilation, cargo-timings, sccache Rust, cranelift backend, Rust workspace splitting, LTO tradeoffs, or mold linker with Rust.
---

# 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

```bash
# 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
```

```bash
# 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

```bash
# 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
```

### 3. Cranelift codegen backend

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

```bash
# Install nightly (Cranelift requires nightly for now)
rustup toolchain install nightly
````
END THIRD-PARTY FILE 2490d52343abac66

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First 2638 of 6841 bytes. © mohitmishra786 · MIT · https://github.com/mohitmishra786/low-level-dev-skills/tree/HEAD/skills/rust/rust-build-times/SKILL.md

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