Disclaimer: If you didn't see my profile description. I am a biologist. I have some computer science background, but not coding. I am publishing some cargo crates and other little repos to (hopefully) meaningfully contribute to open-source projects (tactfully, I hope) and rust in general with any extra claude credits I have available. I am trying to ensure that any contributions I make are actually helpful so any criticism or feedback of my approach would be greatly appreciated.

cargo-why-slow

A cargo subcommand that answers "why is my build slow?" by running cargo build with timing data and presenting results in plain English with actionable recommendations.

Features

• Runs cargo build --message-format=json to collect per-crate build durations
• Ranks dependencies by compile time and shows the top N slowest
• Identifies proc-macro crates (which can't be parallelized well)
• Analyzes your Cargo.toml feature flags for known heavy crates
• Generates plain-English recommendations to speed up your build
• Color-coded terminal output for quick visual scanning

Installation

cargo install --path .

Usage

# In your project directory:
cargo why-slow

# Show top 20 slowest crates:
cargo why-slow -n 20

# Analyze a release build:
cargo why-slow --release

# Analyze a project in a different directory:
cargo why-slow --path /path/to/project

How It Works

1. Runs cargo build --message-format=json to get structured build output
2. Parses compiler-artifact JSON messages to extract crate names and compile durations
3. Identifies proc-macro crates by naming convention (_derive, _macros suffixes)
4. Reads your Cargo.toml to check for known heavy feature combinations (e.g., tokio with full, syn with full)
5. Generates a sorted report with color-coded durations and actionable recommendations

Tips

• Run cargo clean before cargo why-slow to get accurate timings (cached builds show 0s)
• Proc-macro crates compile serially and often dominate build times
• Consider replacing heavy dependencies with lighter alternatives
• Use only the features you need (e.g., tokio = { features = ["rt", "net"] } instead of features = ["full"])

License

Licensed under either of:

• Apache License, Version 2.0 (LICENSE-APACHE)
• MIT License (LICENSE-MIT)

at your option.