falcon

falcon is a fast, standalone linter for Dart and Flutter, written in Rust. It parses Dart itself — no Dart SDK, no analyzer package, no analysis server — so a whole project lints in a single pass with no warm-up.

falcon ships 79 lint rules: 76 across five groups (complexity, correctness, performance, style, suspicious) plus 3 cross-file project rules (unused-files, unused-code, unnecessary-nullable) that reason about the whole module graph rather than one file at a time.

Configuration is a single biome-shaped falcon.json: grouped rules, per-rule severities and options, per-path overrides, a flutter domain, and a separate project section for cross-file analysis. Diagnostics are suppressed inline with // falcon-ignore lint/<group>/<rule>: <reason> — the reason is mandatory.

Status: pre-1.0. falcon is usable today but the rule set, rule ids, and config surface are still moving. Expect breaking changes before 1.0 (see the roadmap). Pin a revision if you depend on stable behaviour.

Installation

falcon is distributed as a Nix flake whose default package is a prebuilt static binary fetched from GitHub Releases — zero compilation, no Rust toolchain.

Add it as a flake input:

{
  inputs.falcon.url = "github:JacobDevelops/dart_falcon";
}

Then reference falcon.packages.${system}.default in a devShell or package list. Or run it directly without installing anything:

nix run github:JacobDevelops/dart_falcon -- check .

See docs/installation.md for supported systems, the build-from-source package, and how releases are cut.

Build from source

With a stable Rust toolchain and Cargo:

cargo build --release
./target/release/falcon check .

Usage

falcon check .            # lint the current directory
falcon check lib/ test/   # lint specific paths
falcon check . --format json
falcon lsp                # start the language server (JSON-RPC over stdin)
falcon version            # print version information

falcon check discovers falcon.json automatically: first the current directory, then the enclosing git root, then ~/.falcon.json. With no config found, every rule runs at its default severity. Pass --config <path> to point at a specific file.

Configuration

falcon.json is biome 2.x-shaped — rules are grouped, and each entry is either a severity string or { "level": ..., "options": { ... } }:

{
  "linter": {
    "rules": {
      "recommended": true,
      "complexity": { "max_lines_for_file": "off" },
      "style": { "prefer-trailing-comma": { "level": "error", "options": {} } }
    },
    "domains": { "flutter": "recommended" }
  },
  "project": {
    "enabled": true,
    "rules": {
      "correctness": { "unused-files": "warn", "unused-code": "warn" }
    }
  }
}

Full reference — every rule, option, per-path overrides, and the flutter domain — is in docs/configuration.md.

Suppressions

Suppress a diagnostic inline with a reason (modelled on Biome's biome-ignore):

dynamic payload = decode(bytes); // falcon-ignore lint/suspicious/avoid-dynamic: interop boundary

The reason after the colon is required; a malformed or reasonless directive is itself reported. Use // falcon-ignore-all lint/<group>/<rule>: <reason> to suppress a rule across a whole file.

Editor support

falcon speaks LSP (falcon lsp). First-party extensions live in extensions/:

• VS Code — extensions/falcon-vscode
• Zed — extensions/falcon-zed

Roadmap

Near-term plans and the road to 1.0 are tracked in ROADMAP.md.

Contributing

Contributions are welcome — see CONTRIBUTING.md for the dev environment, the CI gates, and how to add a rule end-to-end.

Credits

falcon's configuration design is inspired by Biome. Many rules are ports of dart_code_linter and pyramid_lint; provenance is tracked per-rule in the rule metadata (RuleSource).

License

MIT © 2026-present Jacob Sanderson and falcon contributors.