There is currently no way to access a ModuleTree from a CompilationUnitTree.

This is made most evident by looking at the "USE" of ModuleTree in the API specs.

https://docs.oracle.com/en/java/javase/15/docs/api/jdk.compiler/com/sun/source/tree/class-use/ModuleTree.html

Note that JLS 7.3 defines:

CompilationUnit:
   OrdinaryCompilationUnit
   ModularCompilationUnit

OrdinaryCompilationUnit:
   [PackageDeclaration] {ImportDeclaration} {TypeDeclaration}

ModularCompilationUnit:
   {ImportDeclaration} ModuleDeclaration

https://docs.oracle.com/javase/specs/jls/se15/html/jls-7.html#jls-7.3

It is therefore appropriate to provide access to the ModuleDeclaration when that is the content.