Brigadier: providing argument contexts (future work)

[Cypher121]

Issue

It would be useful to provide additional contexts for builder lambdas in the Brigadier DSL. Those could, for instance, hold additional extension functions that cannot currently be implemented due to requiring too many receivers, or, in case of optional arguments, safely accessing the argument's exact builder type only on the branch containing the argument:

//This shorthand is not possible as the only argument for unary minus is the ArgumentConstructor from `integer("test")`
+integer("test") {
...
}

optional(player("target")) { target ->
    //there is no valid class to hold this function
    ifPresent {
        requires { ... } // possible by instead checking target != null
        suggests { ... } // requires checking target != null and then performing an unsafe cast on `this`
    }
}

Solutions

This could be resolved in two ways:

1. Providing functions with a custom implementation of ArgumentBuilder as their receiver. Requires manually delegating to existing implementations as ArgumentBuilder is not an interface and has a self upper bound.
2. Providing context arguments in addition to the ArgumentBuilder. This requires context arguments to become a stable Kotlin feature.
3. Providing the context as a second argument next to the argument accessor. Causes a lot of visual noise, can lead to unexpected behavior if accessed from a child builder.

Prior work

Possible implementation of 2 can be found at this fork branch. Note context arguments in CommandArgument.register.

Compatibility

Adding a context changes lambda types from Function1 to Function2, causing an ABI break, but maintaining source compatibility. It may be possible to keep both versions and go through a deprecation cycle with the non-contextual ones, though that depends on the compiler defaulting to the non-deprecated context version.