cargo-feat

Disclaimer: I am not a Rust professional and I am not claiming to be one - I built this simply because it was useful to me. The core logic was written by me; Some of the performance work and optimizations were done in collaboration with Claude.

A fast command-line tool for Rust developers to instantly look up the available features of any crate on crates.io - directly in your terminal, with no browser required.

Why

When adding a dependency, finding out what features it exposes usually means opening a browser, searching crates.io or docs.rs, and scrolling through documentation. cargo-feat removes that friction: one command gives you a color-coded list of every feature the crate exposes, which ones are enabled by default, and what each feature pulls in.

Demo

$ feat reqwest

— reqwest's features are in the following list —
    ★ default
         default-tls
         charset
         http2
         system-proxy

    — blocking
    — brotli
    — charset          (default)
    — cookies
    — default-tls      (default)
    — deflate
    — gzip
    — http2            (default)
    — ...

Installation

From crates.io

cargo install cargo-feat

From source

Requires a recent stable Rust toolchain.

git clone https://github.com/vunholy/cargo-feat.git
cd cargo-feat
cargo build --release

The compiled binary will be at target/release/feat (or feat.exe on Windows).

To make it available globally, copy it to a directory in your PATH:

# Linux / macOS
cp target/release/feat ~/.local/bin/feat

# Windows (PowerShell - adjust path as needed)
Copy-Item target\release\feat.exe "$env:USERPROFILE\.cargo\bin\feat.exe"

Usage

feat <crate-name> [version] [all|nd] [--deps] [--internals] [--include-internals|-ii] [--json]

All flags are order-independent and can be freely combined.

Examples

# List all features for the latest version of reqwest
feat reqwest

# List features without the default block
feat reqwest nd

# List features for a specific version
feat tokio 1.35.0

# Show what each feature pulls in
feat reqwest --deps

# Annotate features that pull in internal (__) deps
feat reqwest --internals

# Show internal (__) features as their own entries
feat reqwest -ii

# Combine: deps + internals annotation + internal entries
feat reqwest --deps --internals -ii

# Dump the features as JSON
feat serde --json

# Crate names with underscores work fine
feat proc_macro2

Output Format

— crate's features are in the following list —
    ★ default          <- the default feature set (lists which features it enables)
         feature-a
         feature-b

    — feature-a  (default)   <- enabled by default
    — feature-b  (default)
    — feature-c              <- opt-in feature
    — feature-d

With --deps:

    — blocking
         dep:futures-channel
         futures-channel?/sink
         tokio/sync
    — json
         dep:serde
         dep:serde_json

With --internals:

    — __tls                          <- internal feature, shown in grey
    — native-tls [[__native-tls]]    <- [[...]] lists the internal deps this feature pulls in
    — rustls     [[__rustls, __rustls-aws-lc-rs]]

• The ★ default block lists which features are enabled when you add the crate without specifying features.
• Features marked (default) are part of the default feature set.
• Using nd hides the ★ default block but keeps all features in the list, still marking the ones that belong to the default set.
• Internal features (names starting with __) are hidden by default. Use --internals to annotate which public features pull them in, or -ii / --include-internals to show them as their own entries.

Exit Codes

Performance

cargo-feat is built with performance as a first-class concern. Benchmarked against cargo info on a Ryzen 5 5500, Windows 11, warm local registry cache:

hyperfine --warmup 10 --runs 50 \
  -n 'cargo info (offline/warm)' 'cargo info reqwest --offline' \
  -n 'cargo info (network)'      'cargo info reqwest' \
  -n 'cargo feat'                'cargo feat reqwest'

	cargo feat                 49.2 ms ± 1.7 ms
	cargo info (offline/warm) 81.0 ms ± 4.0 ms    ~1.65x slower
	cargo info (network)      357.3 ms ± 11.4 ms  ~7.26x slower

The lookup time stays flat regardless of how many versions a crate has. serde (180+ versions in the index) benchmarks identically to reqwest:

  cargo feat serde   12.0 ms ±  1.6 ms
  cargo info serde   92.7 ms ±  7.7 ms    7.7x slower

The remaining ~12 ms is Windows process startup — unavoidable overhead from the OS loader. The actual feature lookup logic completes in ~2–3 ms.

How it achieves this:

• Sparse registry index — reads directly from index.crates.io (Cloudflare CDN), the same source Cargo uses, instead of the heavier crates.io REST API.
• Local cache first — on every lookup, cargo-feat checks your local Cargo registry cache (~/.cargo/registry/index/) before touching the network. If you've ever built a project that depends on the crate, the result is instant. After any network fetch, the result is written back to the cache, so the next run is always fast.
• MiMalloc — Microsoft's high-performance memory allocator replaces the system allocator globally.
• simd-json — SIMD-accelerated JSON parsing (AVX2/FMA when available).
• hashbrown + ahash — faster HashMap and HashSet than the standard library.
• Aggressive release profile — fat LTO, single codegen unit, opt-level = 3, stripped symbols.
• ureq with rustls (pure-Rust TLS) and gzip decompression.

Dependencies

Build Configuration

The .cargo/config.toml enables additional compiler flags for the Windows MSVC target:

• target-cpu=native — compile for the host CPU to unlock all supported ISA extensions.
• target-feature=+avx2,+fma — explicitly enable AVX2 and FMA for simd-json.
• rust-lld.exe — LLVM linker for faster link times.
• /OPT:REF, /OPT:ICF — dead-code elimination and identical-COMDAT-folding at link time.

If you are building for a different target or a CPU without AVX2, remove or adjust those flags in .cargo/config.toml.

License

Licensed under the Apache License, Version 2.0.