Inline tests in Testo (#[TestInline])

#[TestInline] lets you attach example inputs and an expected return value directly to a method in production code. The Testo runner discovers and executes them like normal tests, but they live next to the implementation — handy for pure functions and library primitives.

This requires the InlineTestPlugin to be enabled for the suite that scans the relevant directory. Fetch https://php-testo.github.io/llms.txt for the attribute signature and llms-full.txt for the plugin wiring details — verify them before generating code, the API surface is small but specific.

Canonical shape

<?php
declare(strict_types=1);

namespace App\Math;

use Testo\Inline\TestInline;

final class Vector
{
    #[TestInline([1, 2],  3)]
    #[TestInline([5, 10], 15)]
    #[TestInline([-1, 1], 0)]
    public static function sum(int $a, int $b): int
    {
        return $a + $b;
    }
}

• Each #[TestInline] attribute = one case.
• First argument: array of method arguments (positional).
• Second argument: expected return value (compared with strict equality).
• The decorated method must be deterministic and side-effect-free — inline tests aren't for I/O or stateful code.

When inline tests are the right choice

Use #[TestInline] when all of the following hold:

• The method is pure (same inputs → same output, no side effects).
• The examples document behaviour that readers of the source will benefit from seeing.
• Each example fits comfortably on one line.

Otherwise prefer a regular #[Test] class — testo-write-tests skill.

When inline tests are wrong

• The method has dependencies that need wiring (use DI + a normal test).
• The expected value isn't a simple literal (no fluent chain, no objects with identity).
• The test needs setup/teardown — #[TestInline] cannot use #[BeforeTest] hooks.
• You want to assert exceptions — use Expect::exception(...) inside a regular test.

Configuration check

For inline tests to run, the suite scanning the production directory must include InlineTestPlugin. Excerpt from testo.php:

use Testo\Application\Config\Plugin\SuitePlugins;
use Testo\Inline\InlineTestPlugin;

new SuiteConfig(
    name: 'Sources',
    location: ['src'],
    plugins: SuitePlugins::only(new InlineTestPlugin()),
),

If the user adds #[TestInline] but the test never runs, check the suite plugin list first.

Pitfalls

• Don't put #[TestInline] on methods that touch globals, the filesystem, time, or randomness.
• Don't try to label cases with a third argument — case identity is the attribute position. Re-order, don't relabel.
• A failing #[TestInline] reports the method's FQN + the attribute index as the test identifier. Order matters for diff readability.