# cargo-feat
[](https://crates.io/crates/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](https://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
```sh
cargo install cargo-feat
```
### From source
Requires a recent stable [Rust toolchain](https://rustup.rs/).
```sh
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`:
```sh
# 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]
```
| `<crate-name>` | Yes | Name of the crate to look up. Underscores are automatically normalized to hyphens. |
| `[all\|nd]` | No | Feature filter. `all` (default) shows every feature. `nd` hides the default feature block but still lists all features and marks the ones that are part of the default set. |
| `[version]` | No | Specific crate version to query. Defaults to the latest stable release. |
| `--deps` | No | Show the full dependency list for each feature, printed below it. |
| `--internals` | No | Annotate features that pull in internal (`__`-prefixed) deps with `[[...]]` next to their name. |
| `--include-internals`, `-ii` | No | Show internal (`__`-prefixed) features as their own entries in the list (greyed out). |
| `--json` | No | Print the raw features map as JSON and exit. |
All flags are order-independent and can be freely combined.
### Examples
```sh
# 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
| `0` | Success |
| `103` | Crate not found or bad response |
| `104` | The specified version was not found for the crate |
| `105` | No stable version found for the crate |
---
## 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](https://github.com/microsoft/mimalloc)** — Microsoft's high-performance memory allocator replaces the system allocator globally.
- **[simd-json](https://github.com/simd-lite/simd-json)** — SIMD-accelerated JSON parsing (AVX2/FMA when available).
- **[hashbrown](https://github.com/rust-lang/hashbrown)** + **[ahash](https://github.com/tkaitchuck/ahash)** — faster `HashMap` and `HashSet` than the standard library.
- **Aggressive release profile** — fat LTO, single codegen unit, `opt-level = 3`, stripped symbols.
- **[ureq](https://github.com/algesten/ureq)** with rustls (pure-Rust TLS) and gzip decompression.
---
## Dependencies
| `ureq` | Lightweight HTTP client with rustls TLS and gzip support |
| `simd-json` | SIMD-accelerated JSON deserialization |
| `serde` | Derive macros for JSON deserialization |
| `mimalloc` | Global high-performance memory allocator |
---
## 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](LICENSE-APACHE).