How do you display a Toast using Kotlin on Android?

It can be an extension function for Context:

fun Context.toast(message: CharSequence) = 
    Toast.makeText(this, message, Toast.LENGTH_SHORT).show()

You can place this anywhere in your project, where exactly is up to you. For example, you can define a file mypackage.util.ContextExtensions.kt and put it there as a top level function.

Whenever you have access to a Context instance, you can import this function and use it:

import mypackage.util.ContextExtensions.toast

fun myFun(context: Context) {
    context.toast("Hello world!")
}

This is one line solution in Kotlin:

Toast.makeText(this@MainActivity, "Its a toast!", Toast.LENGTH_SHORT).show()