Fallow - Dead Code Analysis

Version updated for https://github.com/fallow-rs/fallow to version v1.3.0.

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Action Type

This is a Composite action.

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Action Summary

This GitHub Action, fallow, is a codebase analyzer for JavaScript and TypeScript, designed to detect and address unused code, circular dependencies, and code duplication with exceptional speed and efficiency. Built in Rust, it automates the identification of unused files, exports, dependencies, and types, as well as duplicate code blocks and import cycles, significantly improving code quality and maintainability. Its key capabilities include comprehensive dead code detection, duplicate code analysis (including semantic clones), and dependency auditing, without requiring a Node.js runtime.

Release notes

Performance

• Workspace discovery: 27% faster on large monorepos. The workspace glob expansion now checks for package.json existence before calling canonicalize() — reducing syscalls from 759 to ~20 on monorepos like vite that use deep playground/** workspace globs. Vite benchmark improved from 596ms → 435ms.

• Re-export chain propagation: zero-allocation string comparisons. Replaced ExportName::to_string() allocations in hot inner loops with a new matches_str() method that compares directly against the enum variant without heap allocation. Affects barrel-heavy monorepos with many re-export chains.

New detection: optionalDependencies

Fallow now parses optionalDependencies from package.json and detects unused optional dependencies separately from regular dependencies. Full pipeline support:

• New unused-optional-dependencies rule (defaults to error, suppressed in --production mode)
• All output formats: human, JSON, SARIF, compact, markdown
• Auto-fix via fallow fix (removes from optionalDependencies section)
• LSP diagnostics in VS Code
• Included in all_dependency_names() for unlisted dependency cross-referencing

This eliminates false “unlisted dependency” reports for packages like sharp that are listed in optionalDependencies.

TypeScript function overload deduplication

Function overload signatures are now deduplicated during extraction:

export function parse(): void;           // overload
export function parse(input: string): void;  // overload  
export function parse(input?: string): void {}  // implementation

Previously reported as 3 separate exports (and flagged as duplicate exports). Now correctly treated as a single export. On vite, this reduced unused export false positives from 99 → 84 and duplicate exports from 21 → 16.

Expanded benchmark suite

Added 5 large real-world monorepo projects to the comparative benchmark suite, bringing the total from 3 to 8 projects spanning 174 to 20,416 source files:

Knip errors out on the largest monorepos (next.js, vite) without producing results. Fallow completes on all projects.

Ecosystem test fix

Fixed the ecosystem CI workflow (tests/ecosystem/run.sh) which was failing on all 12 projects due to passing the project path as a positional argument instead of --root. Also fixed install commands for projects using yarn berry and pnpm version constraints.

Full Changelog: https://github.com/fallow-rs/fallow/compare/v1.2.0...v1.3.0

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