Expressions Setup

First of all make sure you are on the latest beta. You can switch to it as explained in the README for your platform.

Mixin Config

Expressions require you to add the following entry to your mixin config:

{
	...
	"minMixinExtrasVersion": "<the version>",
	...
}

This will be used to gate behavioural changes as the feature evolves. Note that during its beta period, to reduce the permanent maintenance effort, some changes may not be gated, and you accept that if such changes affect your mixins you will need to update them. Such cases should, however, be rare and niche, and I will communicate them clearly.

MCDev

Expressions have incredible support in the latest releases of the Minecraft Development IntelliJ IDEA Plugin. Make sure both your IDE and the plugin are up-to-date.

Slim Jar

If for some reason you are particularly concerned about the file size of your mod, and are therefore worried about including MixinExtras, you should consider including the new slim jar, whose net size is about 50% reduced. This comes at the cost of debugging being much harder, and so you should not use the slim jar in dev. For example, on Fabric:

dependencies {
	implementation(annotationProcessor("io.github.llamalad7:mixinextras-fabric:<version>"))
	include("io.github.llamalad7:mixinextras-fabric:<version>:slim")
}