Pass information from one compiler component to another without mutation

Question

I am building a compiler plugin that has two components

1. Permission Accumulator: Load the function definitions and some extra meta data about them into a structure like a Map[String, (...)] where String keys represents the function name and the tuple contains the meta information + the definition in scope.

2. Function Transformer: Recursively traverse the function bodies to check if the metadata of the caller aligns with the callee. More specifically caller.metadata ⊆ callee.metada

This kind of preloading is a rather common thing in compilers (Zinc, Unison etc. all have similar tricks they pull). The first component needs to pass this information it has accumulated to the second component.

Unfortunately the current implementation uses a mutable.Map in the Plugin class and initiates the phases with a reference to this mutable Map. While given the fact that this code won't be surfaced to the end user and some amount of mutation could be tolerated, if someone (including myself) were to add another component/phase that touched this Map in the future, things can go very wrong, resulting in a situation that is painful to debug.

Question: I am wondering if there is a way to instantiate one component, extract some information from it, use that info to init the second component and run it.

Current Implementation:

import scala.collection.mutable.{ Map => MMap }

class Contentable(override val global: Global) extends Plugin {
   val functions: MMap[String, (String, List[String])] = MMap()
   val components = new PermissionAccumulator(global, functions) :: new FunctionRewriter(global, functions) :: Nil
}

The first component mutates the Map as such:

functions += (dd.name.toString -> ((md5HashString(dd.rhs.toString()), roles)))

What I have tried:

Original plan was to encapsulate the mutation inside the first component and do something like secondComponent(global, firstComponent.functions) but because Scala class create a copy of their arguments when an instance is created, the changes to this Map is not reflected in the second component

Note: I have no problem turning these component to phases if that makes a difference.