Add Warning Check to Evaluate Total Field amount in Structs

lib/credo/check/warning/struct_field_amount.ex

@@ -0,0 +1,73 @@
+defmodule Credo.Check.Warning.StructFieldAmount do
+  @moduledoc false
+
+  use Credo.Check,
+    id: "EX????",
+    base_priority: :normal,
+    category: :warning,
+    explanations: [
+      check: """
+      Structs in Elixir are implemented as compile-time maps, which have a predefined amount of fields.
+      When structs have 32 or more fields, their internal representation in the Erlang Virtual Machines
+      changes, potentially leading to bloating and higher memory usage.
+      """
+    ]
+
+  alias Credo.Code.Name
+  @doc false
+  @impl true
+  def run(%SourceFile{} = source_file, params) do
+    issue_meta = IssueMeta.for(source_file, params)
+    Credo.Code.prewalk(source_file, &traverse(&1, &2, issue_meta))
+  end
+
+  defp traverse(
+         {:defmodule, _, [{:__aliases__, _, _aliases} | _] = ast},
+         issues,
+         issue_meta
+       ) do
+    case Macro.prewalk(ast, [], &find_structs_with_32_fields/2) do
+      {ast, []} ->
+        {ast, issues}
+
+      {ast, structs} ->
+        issues =
+          Enum.reduce(structs, issues, fn {curr, meta}, acc ->
+            [issue_for(issue_meta, meta, curr) | acc]
+          end)
+
+        {ast, issues}
+    end
+  end
+
+  defp traverse(ast, issues, _issue_meta) do
+    {ast, issues}
+  end
+
+  defp find_structs_with_32_fields(
+         [
+           {:__aliases__, meta, aliases},
+           [do: {:defstruct, _, [fields]}]
+         ],
+         acc
+       ) do
+    if length(fields) >= 32 do
+      {[], [{Name.full(aliases), meta} | acc]}
+    else
+      {[], acc}
+    end
+  end
+
+  defp find_structs_with_32_fields(ast, acc) do
+    {ast, acc}
+  end
+
+  defp issue_for(issue_meta, meta, struct) do
+    format_issue(issue_meta,
+      message: "Struct %#{struct}{} found to have more than 32 fields.",
+      trigger: "#{struct} do",
+      line_no: meta[:line],
+      column: meta[:column]
+    )
+  end
+end

test/credo/check/warning/struct_field_amount_test.exs

@@ -0,0 +1,82 @@
+defmodule Credo.Check.Warning.StructFieldAmountTest do
+  use Credo.Test.Case
+
+  @described_check Credo.Check.Warning.StructFieldAmount
+
+  @large_struct_module """
+  defmodule MyApp.LargeStruct do
+    defstruct field_1: "field_1",
+              field_2: "field_2",
+              field_3: "field_3",
+              field_4: "field_4",
+              field_5: "field_5",
+              field_6: "field_6",
+              field_7: "field_7",
+              field_8: "field_8",
+              field_9: "field_9",
+              field_10: "field_10",
+              field_11: "field_11",
+              field_12: "field_12",
+              field_13: "field_13",
+              field_14: "field_14",
+              field_15: "field_15",
+              field_16: "field_16",
+              field_17: "field_17",
+              field_18: "field_18",
+              field_19: "field_19",
+              field_20: "field_20",
+              field_21: "field_21",
+              field_22: "field_22",
+              field_23: "field_23",
+              field_24: "field_24",
+              field_25: "field_25",
+              field_26: "field_26",
+              field_27: "field_27",
+              field_28: "field_28",
+              field_29: "field_29",
+              field_30: "field_30",
+              field_31: "field_31",
+              field_32: "field_32"
+  end
+  """
+
+  @small_struct_module """
+  defmodule MyApp.SmallStruct do
+    defstruct [field_1: "field_1"]
+  end
+  """
+
+  #
+  # cases NOT raising issues
+  #
+
+  test "it should NOT report an issue if the struct has fewer than 32 fields" do
+    [
+      @small_struct_module
+    ]
+    |> to_source_files()
+    |> run_check(@described_check)
+    |> refute_issues()
+  end
+
+  #
+  # cases raising issues
+  #
+
+  test "it should report an issue if a struct has 32 or more fields" do
+    [
+      @large_struct_module
+    ]
+    |> to_source_files()
+    |> run_check(@described_check)
+    |> assert_issue(fn issue ->
+      assert %{
+               line_no: 1,
+               message: "Struct %MyApp.LargeStruct{} found to have more than 32 fields."
+             } = issue
+
+      assert issue.trigger == "MyApp.LargeStruct do"
+      assert issue.column == 11
+    end)
+  end
+end