What's new in Infection 0.14.0

Sep 20, 2019

Release: https://github.com/infection/infection/releases/tag/0.14.0

Infection now requires PHP 7.2.9+. If you can’t for some reason upgrade, don’t worry, you can still use previous versions.

Most of the work in 0.14.0 was done to improve stability, performance and DX. However, there are a couple of new features and enhancements.

Performance and memory consumption

We made an update to dramatically reduce memory usage by using classes instead of object-like arrays. For example, running Infection for Psalm now takes 58% less memory (5G instead of 12G).

Read this blog post to understand why you should prefer classes.

Precision in metrics calculator

You can now use whatever precision you want in --min-msi and --covered-min-msi options without rounding.


The minimum required MSI percentage should be 71.43%, but actual is 71.428571428571%. Improve your tests!

New Mutators

Infection already mutates some of the new operators from PHP 7.4, and we added a couple of new mutators for this new syntax.

Spread Operator in Array Expression

RFC: https://wiki.php.net/rfc/spread_operator_for_array

- $array = [...$collection, 4];
+ $array = [[...$collection][0], 4];

Basically, it removes all elements from $collection except the first one to ensure your code uses everything from the $collection and spread operator is really needed here.

Leave one element in array

- return $collection;
+ return \count($collection) > 1 ? array_slice($collection, 0, 1, true) : $collection;

It is similar to the previous one, but works without spread operator. It leaves only one element in the returned array.

Unwrap mb_str_split

RFC: https://wiki.php.net/rfc/mb_str_split

In 0.13.0, we’ve added a new mutator that converts mb_* functions to usual ones. PHP 7.4 adds new function - mb_str_split which will be also mutated to str_split.

Removed functionality

Remove the self-update command

We have removed self-update command and recommend using PHIVE.

phive install infection

When you want to upgrade infection, run

phive update infection

Developer Experience (DX)

infection.json schema validation

Starting from 0.14.0, all mutators are validated by our JSON schema. No more incorrect configuration.


Our builds are faster now which is a good thing when providing PRs and developing Infection locally.

We recommend you read the mentioned blog post and apply the same approach for your project, or use pcov for collecting code coverage data.


Rework Infection command

We have reworked the main entry point of Infection application - InfectionCommand class.

This is how the mutation testing algorithm looks like:


And this is how InfectionCommand now looks like:

protected function execute(InputInterface $input, OutputInterface $output)
$adapter = $this->startUp();


if (!$this->checkMetrics()) {
return 1;

return 0;

This should help new contributors with understanding the processes inside Infection.

All other enhancements and bugfixes are listed in the changelog: https://github.com/infection/infection/blob/0.14/CHANGELOG.md

Next steps


We are working really hard to integrate Codeception into Infection. There were major blocker issues and we had to patch Codeception as well to fix them. We really hope to complete the work by 0.15.0 release in the next couple of months.

Split Infection to separate packages

Currently, Infection is a quite a big project with all the code placed inside one repository. We will definitely split it to several ones.

  1. First of all, we want to extract all Mutators to a separate package. Read the reasoning here.
  2. We need to extract Test Framework adapters to separate packages because now it’s not convenient to maintain the code, set package requirements for test framework adapters and update the code (e.g. we can’t say that Infection requires codeception/codeception: ^3.1.1 because not all the users will use Codeception, so it must be a separate package)

Have another great idea? Let us know!