# rubyfast
[](https://github.com/7a6163/rubyfast/actions/workflows/ci.yml)
[](https://codecov.io/gh/7a6163/rubyfast)
[](https://crates.io/crates/rubyfast)
[](LICENSE)
A Ruby performance linter rewritten in Rust. Detects 19 common performance anti-patterns in Ruby code.
Rust rewrite of [fasterer](https://github.com/DamirSvrtan/fasterer) — same detection rules, but faster execution, parallel file scanning, and zero Ruby runtime dependency.
## Installation
```bash
cargo install --path .
```
## Usage
```bash
# Scan current directory
rubyfast
# Scan a specific file
rubyfast path/to/file.rb
# Scan a specific directory
rubyfast path/to/ruby/project
# Auto-fix safe offenses in-place
rubyfast --fix path/to/ruby/project
# Choose output format
rubyfast --format file path/to/project # group by file (default)
rubyfast --format rule path/to/project # group by rule
rubyfast --format plain path/to/project # one per line (for CI/grep/reviewdog)
```
Exit code `0` if no offenses, `1` if any offenses found.
## Output Formats
### `--format file` (default)
Groups offenses by file path — compact and easy to scan:
```
app/controllers/concerns/lottery_common.rb
L13 Hash#fetch with second argument is slower than Hash#fetch with block
L94 Hash#fetch with second argument is slower than Hash#fetch with block
app/controllers/api/v1/health_articles_controller.rb
L11 Hash#fetch with second argument is slower than Hash#fetch with block
```
Offenses that support `--fix` are tagged with `(fixable)`:
```
tests/fixtures/19_for_loop.rb
L1 For loop is slower than using each (fixable)
```
### `--format rule`
Groups offenses by rule — useful for understanding which patterns are most common:
```
Hash#fetch with second argument is slower than Hash#fetch with block. (3 offenses)
app/controllers/api/v1/health_articles_controller.rb:11
app/controllers/concerns/lottery_common.rb:13
app/controllers/concerns/lottery_common.rb:94
```
### `--format plain`
One offense per line — suitable for grep, reviewdog, and CI pipelines:
```
app/controllers/api/v1/health_articles_controller.rb:11 Hash#fetch with second argument is slower than Hash#fetch with block.
```
## Auto-fix
`rubyfast --fix` automatically corrects 8 safe offenses in-place:
| 1 | `.shuffle.first` | `.sample` |
| 2 | `.select{}.first` | `.detect{}` |
| 4 | `.reverse.each` | `.reverse_each` |
| 5 | `.keys.each` | `.each_key` |
| 6 | `.map{}.flatten(1)` | `.flat_map{}` |
| 7 | `.gsub("x","y")` | `.tr("x","y")` |
| 13 | `(1..10).include?` | `.cover?` |
| 19 | `for x in arr` | `arr.each do \|x\|` |
Fixes are applied in reverse byte order with syntax verification — if a fix would produce invalid Ruby, the file is left unchanged.
## Detected Offenses
| 1 | `.shuffle.first` | `.sample` | Yes |
| 2 | `.select{}.first` | `.detect{}` | Yes |
| 3 | `.select{}.last` | `.reverse.detect{}` | No |
| 4 | `.reverse.each` | `.reverse_each` | Yes |
| 5 | `.keys.each` | `.each_key` | Yes |
| 6 | `.map{}.flatten(1)` | `.flat_map{}` | Yes |
| 7 | `.gsub("x","y")` (single chars) | `.tr("x","y")` | Yes |
| 8 | `.sort { \|a,b\| ... }` | `.sort_by` | No |
| 9 | `.fetch(k, v)` | `.fetch(k) { v }` | No |
| 10 | `.merge!({k: v})` | `h[k] = v` | No |
| 11 | `.map { \|x\| x.foo }` | `.map(&:foo)` | No |
| 12 | `.each_with_index` | `while` loop | No |
| 13 | `(1..10).include?` | `.cover?` | Yes |
| 14 | `.module_eval("def ...")` | `define_method` | No |
| 15 | `rescue NoMethodError` | `respond_to?` | No |
| 16 | `def foo(&block); block.call; end` | `yield` | No |
| 17 | `def x; @x; end` | `attr_reader` | No |
| 18 | `def x=(v); @x=v; end` | `attr_writer` | No |
| 19 | `for x in arr` | `arr.each` | Yes |
## Inline Disable
Suppress specific offenses with inline comments (similar to RuboCop):
```ruby
# Disable on the same line
x = [].shuffle.first # rubyfast:disable shuffle_first_vs_sample
# Disable the next line
# rubyfast:disable-next-line shuffle_first_vs_sample
x = [].shuffle.first
# Block disable/enable
# rubyfast:disable for_loop_vs_each
for i in arr
puts i
end
# rubyfast:enable for_loop_vs_each
# Disable all rules
code # rubyfast:disable all
# Multiple rules
code # rubyfast:disable shuffle_first_vs_sample, for_loop_vs_each
# Backwards compatible with fasterer
code # fasterer:disable shuffle_first_vs_sample
```
## Configuration
Create a `.rubyfast.yml` (or `.fasterer.yml` for backwards compatibility) in your project root to disable specific checks or exclude files:
```yaml
speedups:
shuffle_first_vs_sample: true
for_loop_vs_each: false # disable this check
exclude_paths:
- vendor/**/*.rb
- tmp/**/*.rb
```
The config file is auto-discovered by walking up from the scan directory. `.rubyfast.yml` takes precedence if both exist.
## CI/CD Integration
### GitHub Action
```yaml
- uses: 7a6163/rubyfast-action@v1
```
With [reviewdog](https://github.com/reviewdog/reviewdog) inline PR comments (uses `--format plain` internally):
```yaml
rubyfast:
runs-on: ubuntu-latest
permissions:
contents: read
checks: write
pull-requests: write
steps:
- uses: actions/checkout@v4
- uses: 7a6163/rubyfast-action@v1
with:
reviewdog: "true"
github-token: ${{ secrets.GITHUB_TOKEN }}
reviewdog-reporter: github-pr-review
```
See [rubyfast-action](https://github.com/7a6163/rubyfast-action) for all options.
### GitLab CI
```yaml
rubyfast:
image:
name: ghcr.io/7a6163/rubyfast:latest
entrypoint: [""]
script:
- rubyfast .
```
### Docker
```bash
docker run --rm -v $(pwd):/workspace ghcr.io/7a6163/rubyfast:latest .
```
## Benchmark
Compared against the original [fasterer](https://github.com/DamirSvrtan/fasterer) Ruby gem (v0.11.0) using [hyperfine](https://github.com/sharkdp/hyperfine). Measured on Apple Silicon, macOS.
### Large project (17,091 Ruby files)
| `rubyfast` | 3,445 ms | 3,421 ms | 3,464 ms | **1.00** |
| `fasterer` | 150,122 ms | 148,309 ms | 151,609 ms | 43.57x slower |
### Small project: 22 test fixtures
| `rubyfast` | 4.6 ms | 3.3 ms | 15.1 ms | **1.00** |
| `fasterer` | 488.7 ms | 483.1 ms | 510.7 ms | 105.57x slower |
**~44–106x faster** on real-world codebases. The Rust implementation processes 17,000+ Ruby files in 3.4 seconds, while the Ruby gem takes over 2.5 minutes.
## Development
```bash
cargo build
cargo test
cargo clippy -- -D warnings
```
## License
MIT