roave/infection-static-analysis-plugin

Static analysis on top of mutation testing - prevents escaped mutants from being invalid according to static analysis

1.7.1 2021-03-04 16:33 UTC

README

This plugin is designed to run static analysis on top of infection/infection test runs in order to discover if escaped mutants are valid mutations, or if they do not respect the type signature of your program. If the mutation would result in a type error, it is "killed".

TL;DR:

  • This will improve your mutation score, since mutations which result in type errors become killed.
  • This is very hacky, and replaces vendor/bin/infection essentially. Please read the Stability section below first for details.
  • This is currently much slower than running infection by itself. There are ideas/suggestions to improve this in the future.

Usage

The current design of this tool requires you to run vendor/bin/roave-infection-static-analysis-plugin instead of running vendor/bin/infection:

composer require --dev roave/infection-static-analysis-plugin

vendor/bin/roave-infection-static-analysis-plugin

Background

If you come from a statically typed language with AoT compilers, you may be confused about the scope of this project, but in the PHP ecosystem, producing runnable code that does not respect the type system is very easy, and mutation testing tools do this all the time.

Take for example following snippet:

/**
 * @template T
 * @param array<T> $values
 * @return list<T>
 */
function makeAList(array $values): array
{
    return array_values($values);
}

Given a valid test as follows:

function test_makes_a_list(): void
{
    $list = makeAList(['a' => 'b', 'c' => 'd']);
 
    assert(count($list) === 2);
    assert(in_array('b', $list, true));
    assert(in_array('d', $list, true));
}

The mutation testing framework will produce following mutation, since we failed to verify the output in a more precise way:

/**
 * @template T
 * @param array<T> $values
 * @return list<T>
 */
function makeAList(array $values): array
{
-    return array_values($values);
+    return $values;
}

The code above is valid PHP, but not valid according to our type declarations. While we can indeed write a test for this, such test would probably be unnecessary, as existing type checkers can detect that our actual return value is no longer a list<T>, but a map of array<int|string, T>, which is in conflict with what we declared.

This plugin detects such mutations, and prevents them from making you write unnecessary tests, leveraging the full power of existing PHP type checkers such as phpstan and psalm.

Stability

Since infection/infection is not yet designed to support plugins, this tool uses a very aggressive approach to bootstrap itself, and relies on internal details of the underlying runner.

To prevent compatibility issues, it therefore always pins to a very specific version of infection/infection, so please be patient when you wish to use the latest and greatest version of infection/infection, as we may still be catching up to it.

Eventually, we will contribute patches to infection/infection so that there is a proper way to design and use plugins, without the need for dirty hacks.

PHPStan? Psalm? Where's my favourite static analysis tool?

Our initial scope of work for 1.0.x is to provide vimeo/psalm support as a start, while other static analysers will be included at a later point in time.