yak(boundary-repo-as-sibling-boundary-web-repo-forbidden-tgrc): boundary: Repo as sibling boundary, web->Repo forbidden

app/mix.exs

@@ -11,7 +11,7 @@ defmodule Firehose.MixProject do
       aliases: aliases(),
       deps: deps(),
       test_coverage: test_coverage(),
-      compilers: [:phoenix_live_view] ++ Mix.compilers(),
+      compilers: extra_compilers(Mix.env()) ++ [:phoenix_live_view] ++ Mix.compilers(),
       listeners: [Phoenix.CodeReloader]
     ]
   end
@@ -44,6 +44,14 @@ defmodule Firehose.MixProject do
   defp elixirc_paths(:test), do: ["lib", "test/support"]
   defp elixirc_paths(_), do: ["lib"]
 
+  # The boundary compiler must be listed *before* `:elixir`: it enforces by
+  # registering an `after_compiler(:elixir, ...)` callback, so a later position
+  # would look wired and report nothing. It stays out of `:prod` on boundary's
+  # own advice (a violation must not be able to fail a release build); `:dev`
+  # and `:test` are where code is written.
+  defp extra_compilers(env) when env in [:dev, :test], do: [:boundary]
+  defp extra_compilers(_), do: []
+
   # Specifies your project dependencies.
   #
   # Type `mix help deps` for examples and options.
@@ -81,6 +89,11 @@ defmodule Firehose.MixProject do
       {:microprints,
        git: "https://gitea.apps.sustainabledelivery.com/QWAN/microprints-phoenix.git",
        branch: "main"},
+      # Compile-time boundary enforcement (Firehose / FirehoseWeb / Firehose.Repo).
+      # Declared directly even though `ex_crap` already pulls it in transitively:
+      # dropping a test-only metric tool must not silently take the boundary
+      # compiler with it.
+      {:boundary, "~> 0.11", runtime: false},
       {:credo, "~> 1.7", only: [:dev, :test], runtime: false},
       {:ex_crap, "~> 0.1.1", only: [:dev, :test], runtime: false},
       {:muex, "~> 0.12", only: [:dev, :test], runtime: false},