Mewt

Mewt is a mutation testing tool. Mutation testing works by making small changes (mutations) to your source code — like replacing + with - or swapping true for false — and then running your test suite against each change. If your tests still pass after a mutation, that's a gap: the mutant "survived," meaning your tests didn't catch the change.

This tells you something code coverage alone can't: not just whether your tests execute a line, but whether they'd actually fail if that line were wrong.

Supported languages:

• C++
• DAML
• Go
• JavaScript/TypeScript
• Rust
• Solidity
• Move (dialects: sui, iota, aptos; use move, move/sui, move/iota, or move/aptos)

For details on how campaigns work under the hood, see How it works. For the language/dialect resolver contract, see Language resolution contract. To add support for a new language, see Adding a language. For mutation test suite structure and shared test helper conventions, see tests/README.md.

Installation

Prebuilt binaries (recommended)

curl --proto '=https' --tlsv1.2 -LsSf https://github.com/trailofbits/mewt/releases/latest/download/mewt-installer.sh | sh

Prebuilt installers are currently provided for macOS (aarch64) and Linux (x86_64). Windows is currently unsupported.

To build from source instead, see Building from source.

Quick start

• Mutate a single file (auto-detected language):

mewt run path/to/contract.rs

• Mutate all supported files in a directory (recursive):

mewt run path/to/project

• List available mutation slugs for a language:

mewt print mutations --language rust

• List Move mutation slugs for a specific dialect:

mewt print mutations --language move/iota

• Print all mutants for a target path:

mewt print mutants --target path/to/contract.rs

• Show mutation test results (optionally filtered by target):

mewt results --target path/to/contract.rs

• Test all mutants even if more severe ones were uncaught (disable skip optimization):

mewt run path/to/contract.rs --comprehensive

Configuration

Mewt reads configuration from the nearest mewt.toml found by walking up from the current working directory. CLI flags override config file values.

You can also point to an explicit config file with --config path/to/mewt.toml.

See Configuration for the full reference and src/example.toml for a commented example.

Choosing a Move dialect

For .move files, mewt resolves dialect in this order:

1. explicit language label, e.g. --language move/iota
2. per-target languages.move.dialect in mewt.toml
3. project-wide [languages.move].dialect in mewt.toml
4. default sui

Examples:

# Explicit dialect from CLI
mewt print mutations --language move/sui

Compatibility note:

• Use canonical Move names: move, move/sui, move/iota, and move/aptos.

Examples

This repo includes example files you can try:

• C++: tests/cpp/example.cpp
• DAML: tests/daml/example.daml
• Go: tests/go/example.go
• JavaScript/TypeScript/JSX/TSX: tests/javascript/example.js (plus example.ts, example.jsx, example.tsx)
• Rust: tests/rust/example.rs
• Solidity: tests/solidity/example.sol
• Move: tests/move/example.move

Notes

• Mixed-language projects are supported. When a directory is targeted, only files with supported extensions are considered.