scalac Plugins with Pants

The Scala compiler, scalac, has a plugin mechanism.

Plugins allow you to hook into the compiler while it's running, to perform various tasks such as custom error checking, code analysis and so on.

Pants supports both building and using scalac plugins.

Building scalac plugins

scalac plugins are specified using a scalac_plugin target:

scalac_plugin(
  name = 'plugin',
  plugin = 'simple_scalac_plugin',
  sources = ['SimpleScalacPlugin.scala'],
  dependencies = [],
  classname = 'org.pantsbuild.example.scalac.plugin.SimpleScalacPlugin',
  scope='compile',
)

A scalac plugin target has the same fields as a scala_library target, plus two extra:

  • classname: The name of the Plugin implementation class. Required.
  • plugin: The logical name of the plugin, as returned by the Plugin class's getName() method. If unspecified, this field defaults to the target name.

Building a plugin target will, in addition to compiling the code, generate the appropriate metadata into scalac-plugin.xm, so that scalac can load the plugin by name at runtime.

A plugin may be published for later consumption in any repo. It can also be consumed (with some restrictions) from source, in the same repo.

Using scalac plugins

Plugins can be integrated in one of two ways:

  • Global plugins: specified in pants.ini and used on all Scala code.
  • Per-target plugins: specified on a Scala target and used only when compiling that target.

Global plugins

Global plugins are specified using the scalac_plugins key in the compile.zinc section of pants.ini:

[compile.zinc]
scalac_plugins: ['simple_scalac_plugin']

Plugins can optionally take arguments, specified like this:

[compile.zinc]
scalac_plugins: ['simple_scalac_plugin]
scalac_plugin_args: {
    'simple_scalac_plugin': ['arg1', 'arg2']
  }

Per-target plugins

These are specified like this:

scala_library(
  ...
  scalac_plugins: ['simple_scalac_plugin'],
  scalac_plugin_args: {
      'simple_scalac_plugin': ['arg1', 'arg2']
    }
)

Depending on plugins

In order to load a plugin, it has to be on scalac's classpath. This can be achieved in one of two ways: - Have targets that must be compiled with a plugin depend (directly or indirectly) either on the scalac_plugin target, or on a jar_library pointing to a published version of the plugin. - Have a scalac-plugin-dep target in BUILD.tools:

jar_library(name='scala-plugin-dep',
            jars = [jar(org='com.foo', name='foo_plugin', rev='1.2.3')],

Note that, as always with BUILD.tools, plugin locations specified via scala-plugin-dep must be published jars. They cannot be local scalac_plugin targets.

Usually, it will make more sense to use scala-plugin-dep with global plugins, to avoid laborious repetition of that dependency, and to use target dependencies for per-target plugins, to keep the dependencies selective.

Depending directly on the scalac_plugin has the added advantage of allowing plugin changes to be picked up when compiling the target that uses the plugin, with no need for an intermediate publishing step.

Note that, to avoid a chicken-and-egg problem, an in-repo plugin will not be used when compiling its own scalac_plugin target, or any of that target's dependencies. To use a plugin on its own code, you must publish it and consume the published plugin via scala-plugin-dep.

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