yak(credo-strict-tuned-config-both-apps-x02d): credo strict, tuned config (both apps)

app/.credo.exs

@@ -4,6 +4,13 @@
 # If you find anything wrong or unclear in this file, please report an
 # issue on GitHub: https://github.com/rrrene/credo/issues
 #
+# The gate for this app is `mix credo --strict` on the command line (the
+# `credo` target in Makefile, a prerequisite of `check`). The command line, not
+# this file, decides: `--strict` overrides the inert `strict: false` key below.
+# Editing `strict` here to silence a finding would change nothing - fix the
+# finding, or write down why the check is wrong for this codebase (see
+# `disabled:`).
+#
 %{
   #
   # You can have as many configs as you like in the `configs:` field.
@@ -126,7 +133,6 @@
           #
           {Credo.Check.Refactor.Apply, []},
           {Credo.Check.Refactor.CondStatements, []},
-          {Credo.Check.Refactor.CyclomaticComplexity, []},
           {Credo.Check.Refactor.FilterCount, []},
           {Credo.Check.Refactor.FilterFilter, []},
           {Credo.Check.Refactor.FunctionArity, []},
@@ -174,6 +180,18 @@
           {Firehose.Checks.NoConnShadowing, []}
         ],
         disabled: [
+          #
+          # Our one deliberate deviation from credo's defaults. CRAP (`ex_crap`,
+          # threshold 30, wired into the Lens pass in Makefile) is this app's
+          # complexity sensor, and it scores complexity against real coverage: a
+          # wide `case` that the tests exercise is not a problem - that was the
+          # itch behind ExCrap. A second, coverage-blind complexity sensor would
+          # just argue with the first, so complexity questions are settled by
+          # the CRAP target. Re-enabling this check without retiring the CRAP
+          # target is a policy change, not a cleanup.
+          #
+          {Credo.Check.Refactor.CyclomaticComplexity, []},
+
           #
           # Checks scheduled for next check update (opt-in for now)
           {Credo.Check.Refactor.UtcNowTruncate, []},

app/test/credo_config_test.exs

@@ -0,0 +1,80 @@
+defmodule Firehose.CredoConfigTest do
+  @moduledoc """
+  Pins the *policy* in `app/.credo.exs`, not just the file's presence.
+
+  The gate is `mix credo --strict` (the `credo` target in `app/Makefile`, a
+  prerequisite of `check`), so the config decides what every future commit has
+  to satisfy. Plain `ExUnit.Case`: this reads and evaluates a file, it does not
+  touch the database or boot the app's supervision tree.
+
+  The assertions are deliberately coarse - "this check sits in that list",
+  "the disable says why", "the custom check is still loaded" - so they survive
+  credo's own boilerplate churn but not a policy change.
+  """
+  use ExUnit.Case, async: true
+
+  @config_path ".credo.exs"
+  @complexity_check "Credo.Check.Refactor.CyclomaticComplexity"
+  @custom_check "Firehose.Checks.NoConnShadowing"
+  @custom_check_load_path "lib_dev/firehose/checks/"
+
+  test "cyclomatic complexity is disabled in favour of CRAP" do
+    checks = default_config()[:checks]
+
+    assert @complexity_check in check_modules(checks[:disabled])
+    refute @complexity_check in check_modules(checks[:enabled])
+  end
+
+  test "the disable carries a rationale comment" do
+    rationale = rationale_above(@complexity_check)
+
+    refute rationale == [], "no comment block above the #{@complexity_check} entry"
+    assert Enum.join(rationale, "\n") =~ ~r/crap/i
+  end
+
+  # Regression guard: `mix credo.gen.config` would silently drop both of these,
+  # and the custom check would stop running without one word of warning.
+  test "the custom check and its load path survive" do
+    config = default_config()
+
+    assert @custom_check in check_modules(config[:checks][:enabled])
+    assert @custom_check_load_path in config[:requires]
+  end
+
+  defp default_config do
+    {config, _bindings} = Code.eval_file(@config_path)
+    Enum.find(config.configs, &(&1.name == "default"))
+  end
+
+  defp check_modules(checks) do
+    Enum.map(checks, fn
+      {module, _opts} -> module_name(module)
+      module -> module_name(module)
+    end)
+  end
+
+  # `Atom.to_string/1` keeps the "Elixir." prefix that `inspect/1` drops; the
+  # assertions read better (and diff better) without it.
+  defp module_name(module) do
+    module |> Atom.to_string() |> String.replace_prefix("Elixir.", "")
+  end
+
+  # The contiguous `#` block immediately above the check's own line. The entry
+  # line is the one that is not itself a comment: a comment mentioning the
+  # check by name must not be mistaken for the entry.
+  defp rationale_above(check_name) do
+    lines = @config_path |> File.read!() |> String.split("\n")
+
+    index =
+      Enum.find_index(lines, fn line ->
+        String.contains?(line, check_name) and not comment?(line)
+      end)
+
+    lines
+    |> Enum.take(index)
+    |> Enum.reverse()
+    |> Enum.take_while(&comment?/1)
+  end
+
+  defp comment?(line), do: line |> String.trim() |> String.starts_with?("#")
+end