A shipped routing-only extension for testing composition with foreign macro routing.

Registering routes for its macros is how a library ships the knowledge its DSL relies on (Mutare.CallRouting). Testing that your mutator honours routing from an independent module therefore needs a routed macro that module owns — this fixture is that module, so a plugin suite doesn't have to author a no-op routing provider of its own. It defines two inert pass-through macros and registers their argument routing:

Both expand to their (first) argument, so fixtures using them still compile and run (Mutare.Test.assert_metamutant_compiles/3, Mutare.Test.compile_metamutant/3). Enable the routes by threading the module through the Mutare.Test helpers' opts — routing applies only when the extension is enabled, so the same source doubles as the unrouted contrast:

import Mutare.Test
alias Mutare.Test.RoutingExtension

source = """
defmodule M do
  import Mutare.Test.RoutingExtension
  def f(x), do: opaque(x + 1)
end
"""

# Unrouted, the argument mutates; routed, the foreign :raw suppresses it.
assert {:arithmetic, "x + 1", "x - 1"} in diffs(source, [:arithmetic])
assert diffs(source, [:arithmetic], extensions: [RoutingExtension]) == []

This is an extension — no name/0, no mutations — so it goes under :extensions (or a declarative :call_routes entry naming its macros, to test the config channel). It deliberately covers only the end-user treatments: the adapter-grade ones (:interpolated, {:keyword, …}, :hosted) assert facts about a real DSL's semantics that no generic fixture can stand in for — model those on the Mutare.CallRouting and Mutare.Mutator.MacroHost contracts against the library you're describing.