Presentation Compiler is now loaded with correct classloader

[rochala]

Fixes #6494
Fixes scala/scala3#20560

We've found that classloader used to load presentation compiler is defined as follows:

package scala.meta.internal.metals

/**
 * ClassLoader that is used to reflectively invoke presentation compiler APIs.
 *
 * The presentation compiler APIs are compiled against exact Scala versions of the compiler
 * while Metals only runs in a single Scala version. In order to communicate between Metals and the
 * reflectively loaded compiler, this classloader shares a subset of Java classes that appear in
 * method signatures of the `PresentationCompiler` class.
 */
class PresentationCompilerClassLoader(parent: ClassLoader)
    extends ClassLoader(null) {
  override def findClass(name: String): Class[_] = {
    val isShared =
      name.startsWith("org.eclipse.lsp4j") ||
        name.startsWith("com.google.gson") ||
        name.startsWith("scala.meta.pc") ||
        name.startsWith("javax")
    if (isShared) {
      parent.loadClass(name)
    } else {
      throw new ClassNotFoundException(name)
    }
  }
}

and it is used as a parent classloader:

  private def newPresentationCompilerClassLoader(
      mtags: MtagsBinaries.Artifacts
  ): URLClassLoader = {
    val allJars = mtags.jars.iterator
    val allURLs = allJars.map(_.toUri.toURL).toArray
    // Share classloader for a subset of types.
    val parent =
      new PresentationCompilerClassLoader(this.getClass.getClassLoader)
    new URLClassLoader(allURLs, parent)
  }

This approach won't work on java 9+
More information here: https://github.com/scala/scala3/pull/11658/files

[tgodzik]

Great catch!