CEL expressions over protobuf messages: types/1 makes a PB schema's messages and enums the types of a CEL.Env.

env =
  CEL.Env.build(
    types: PB.CEL.types(MyApp.Schema),
    container: "my.pkg",
    vars: [user: {:message, "my.pkg.User"}]
  )

CEL.eval("user.age >= 18 && user.role == Role.ADMIN", %{user: user}, env)

Message and enum types are named by PB's atoms, so a message value is {:message, :"my.pkg.User", fields} and an enum value {:enum, :"my.pkg.Role", 2}. A message variable binds to the message as PB.decode/4 returns it, and an enum variable to a value's name as an atom or its number. The well-known types are the CEL types they stand for: a google.protobuf.Timestamp field reads as a timestamp, an Int64Value one as an int or null.