Magento 2: Run `setup:di:compile` **only** a Specific Module?

Short Answer: No. That is not possible with tools shipped with Magento now.

Explanation: We have to distinguish code generation and compilation. Compiler does both. For both operations we need to read configuration of all other magento modules, otherwise their output will be incorrect or not full.

Compilation for one module does not make sense, as magento will not work with compiled configuration for one module only.

Generation for one module might make sense, but we will have to read configuration and constructor signatures from all other installed modules anyway. Without it not all code might be generated. So such operation would not be much faster than full compilation. We might think of it, but I think the bug you referenced will be fixed before the command is implemented.

Magento's bin/magento executable is just a Symfony Console component. This means you can find the individual file reponsible for a single command. The naming is always the same so for setup:di:compile you can find the DiCompileCommand.php (setup/src/Magento/Setup/Console/Command/DiCompileCommand.php).

It'll always run the exeucte() method so that's where we need to look. In this function, you'll see an $operations var is set, which is filled by the getOperationsConfiguration() method. This method basically tells the DiCompileCommand what to compile.

The first thing I tried is returning just the application code generator part, like so;

private function getOperationsConfiguration(
    array $compiledPathsList
) {
    $excludePatterns = [];
    foreach ($this->excludedPathsList as $excludedPaths) {
        $excludePatterns = array_merge($excludedPaths, $excludePatterns);
    }

    return [
        OperationFactory::APPLICATION_CODE_GENERATOR => [
            'paths' => [
                $compiledPathsList['application'],
                $compiledPathsList['library'],
                $compiledPathsList['generated_helpers'],
            ],
            'filePatterns' => ['php' => '/\.php$/'],
            'excludePatterns' => $excludePatterns,
        ]
    ];
}

This went pretty well, compilation time was cut down dramatically;

Compilation was started.
Application code generator... 1/1 [============================] 100% 45 secs 308.8 MiB
Generated code and dependency injection configuration successfully.

As opposed to;

Compilation was started.
Interception cache generation... 7/7 [============================] 100% 3 mins 377.0 MiBB8 MiB
Generated code and dependency injection configuration successfully.

Of course, this was to be expected since we cut out a number of things. But you didn't specify which files you want to have generated. Not all files can be generated on a per-module basis, since for example, the Interception classes can be dependent on multiple modules and would therefore give you a limited output of functionality if you'd only run this for one module.

You can find the responsible generators here;

setup/src/Magento/Setup/Module/Di/App/Task/Operation/

Each class has a foreach in it, which will allow you to add an if/else with a continue to skip certain modules/paths. Maybe it would be useful to check out the Symfony Console Component Input Arguments documentation on how to feed arguments into the command.

You can recognize by,

Package -> Module -> registration.php

<?php
    use \Magento\Framework\Component\ComponentRegistrar;
    ComponentRegistrar::register(ComponentRegistrar::MODULE, 'Package_Module', __DIR__);

And than run,

php -f bin/magento module:enable --clear-static-content Package_Module

registration.php will recognize the module and compile your custom module.

I hope this will help you.