ft: detect unused structs

lib/mix/tasks/compile.unused.ex

@@ -72,6 +72,8 @@ defmodule Mix.Tasks.Compile.Unused do
     Other allowed levels are `information`, `warning`, and `error`.
   """
 
+  alias Mix.Task.Compiler.Diagnostic
+
   @recursive true
 
   @manifest "unused.manifest"
@@ -137,15 +139,19 @@ defmodule Mix.Tasks.Compile.Unused do
     :ok = Tracer.stop()
 
     messages =
-      for {{m, f, a}, meta} <- unused do
-        %Mix.Task.Compiler.Diagnostic{
+      for {{m, f, a}, meta} = desc <- unused do
+        %Diagnostic{
           compiler_name: "unused",
-          message: "#{inspect(m)}.#{f}/#{a} is unused",
+          message: message(desc),
           severity: severity,
           position: meta.line,
-          file: meta.file
+          file: meta.file,
+          details: %{
+            mfa: {m, f, a},
+            signature: meta.signature
+          }
         }
-        |> print_diagnostic()
+        |> _tap(&print_diagnostic/1)
       end
 
     case {messages, severity, warnings_as_errors} do
@@ -185,6 +191,9 @@ defmodule Mix.Tasks.Compile.Unused do
   defp severity("warning"), do: :warning
   defp severity("error"), do: :error
 
+  defp print_diagnostic(%Diagnostic{details: %{mfa: {_, :__struct__, 1}}}),
+    do: nil
+
   defp print_diagnostic(diag) do
     file = Path.relative_to_cwd(diag.file)
 
@@ -197,8 +206,21 @@ defmodule Mix.Tasks.Compile.Unused do
       Integer.to_string(diag.position),
       "\n"
     ])
+  end
+
+  defp message({{_, :__struct__, 0}, meta}) do
+    "#{meta.signature} is unused"
+  end
+
+  defp message({{m, f, a}, _meta}) do
+    "#{inspect(m)}.#{f}/#{a} is unused"
+  end
+
+  # Elixir < 1.12 do not have tap, so we provide custom implementation
+  defp _tap(val, fun) do
+    fun.(val)
 
-    diag
+    val
   end
 
   defp level(level), do: [:bright, color(level), "#{level}: ", :reset]

lib/mix_unused/exports.ex

@@ -8,6 +8,12 @@ defmodule MixUnused.Exports do
 
   @types ~w[function macro]a
 
+  @ignored [
+    # It is created automatically by `defstruct` and it is (almost?) never used
+    # directly. Instead we will look for expansions in form of `%module{}`
+    {:__struct__, 1}
+  ]
+
   @spec fetch(module()) :: [{mfa(), metadata()}]
   def fetch(module) do
     # Check exported functions without loading modules as this could cause
@@ -18,11 +24,12 @@ defmodule MixUnused.Exports do
       callbacks = data[:attributes] |> Keyword.get(:behaviour, []) |> callbacks()
       source = data[:compile_info] |> Keyword.get(:source, "nofile") |> to_string()
 
-      for {{type, name, arity}, anno, _sig, _doc, meta} when type in @types <- docs,
+      for {{type, name, arity}, anno, [sig | _], _doc, meta} when type in @types <- docs,
           not Map.get(meta, :export, false),
+          {name, arity} not in @ignored,
           {name, arity} not in callbacks do
         line = :erl_anno.line(anno)
-        {{module, name, arity}, %{file: source, line: line}}
+        {{module, name, arity}, %{signature: sig, file: source, line: line}}
       end
     else
       _ -> []

lib/mix_unused/tracer.ex

@@ -45,6 +45,13 @@ defmodule MixUnused.Tracer do
     :ok
   end
 
+  def trace({:struct_expansion, _meta, module, _keys}, env) do
+    add_call(module, :__struct__, 0, env)
+    add_call(module, :__struct__, 1, env)
+
+    :ok
+  end
+
   def trace(_event, _env), do: :ok
 
   @spec add_call(module(), atom(), arity(), Macro.Env.t()) :: :ok

test/fixtures/clean/lib/simple_server.ex

@@ -5,7 +5,7 @@ defmodule SimpleServer do
 
   def init(_), do: {:ok, []}
 
-  def handle_call(_msg, _ref, state), do: {:reply, :ok, state}
+  def handle_call(%SimpleStruct{}, _ref, state), do: {:reply, :ok, state}
 
   def handle_cast(_msg, state), do: {:noreply, state}
 end

test/fixtures/clean/lib/simple_struct.ex

@@ -0,0 +1,3 @@
+defmodule SimpleStruct do
+  defstruct [:foo]
+end

test/fixtures/unclean/lib/bar.ex

@@ -0,0 +1,3 @@
+defmodule Bar do
+  defstruct [:foo]
+end

test/mix_unused/tracer_test.exs

@@ -151,4 +151,14 @@ defmodule MixUnused.TracerTest do
   test "contains infromation about remote calls using dynamic module attributes" do
     assert {Remote, :foo, 0} in @subject.get_calls()
   end
+
+  @code (quote do
+           def test do
+             %MapSet{}
+           end
+         end)
+  test "struct expansion add macros for struct" do
+    assert {MapSet, :__struct__, 0} in @subject.get_calls()
+    assert {MapSet, :__struct__, 1} in @subject.get_calls()
+  end
 end

test/mix_unused_test.exs

@@ -38,9 +38,7 @@ defmodule MixUnusedTest do
     test "exit if severity is warning and `--warnings-as-errors is used`" do
       in_fixture("umbrella", fn ->
         assert {:shutdown, 1}
-                 catch_exit(
-                   run(:umbrella, "compile", ~w[--severity warning --warnings-as-errors])
-                 )
+        catch_exit(run(:umbrella, "compile", ~w[--severity warning --warnings-as-errors]))
       end)
     end
   end
@@ -54,15 +52,14 @@ defmodule MixUnusedTest do
 
     test "exit if severity is set to error" do
       in_fixture("clean", fn ->
-        assert {{:ok, []}, _} =
-                   run(:clean, "compile", ~w[--severity error])
+        assert {{:ok, []}, _} = run(:clean, "compile", ~w[--severity error])
       end)
     end
 
     test "exit if severity is warning and `--warnings-as-errors is used`" do
       in_fixture("clean", fn ->
         assert {{:ok, []}, _} =
-                   run(:clean, "compile", ~w[--severity warning --warnings-as-errors])
+                 run(:clean, "compile", ~w[--severity warning --warnings-as-errors])
       end)
     end
   end
@@ -86,6 +83,15 @@ defmodule MixUnusedTest do
       end)
     end
 
+    test "unused struct is reported" do
+      in_fixture("unclean", fn ->
+        assert {{:ok, diagnostics}, output} = run(:unclean, "compile")
+
+        assert output =~ "%Bar{} is unused"
+        assert find_diagnostics_for(diagnostics, Bar, :__struct__, 0)
+      end)
+    end
+
     test "exit if severity is set to error" do
       in_fixture("unclean", fn ->
         assert {:shutdown, 1} = catch_exit(run(:unclean, "compile", ~w[--severity error]))
@@ -111,9 +117,7 @@ defmodule MixUnusedTest do
     test "exit if severity is warning and `--warnings-as-errors is used`" do
       in_fixture("unclean", fn ->
         assert {:shutdown, 1}
-                 catch_exit(
-                   run(:unclean, "compile", ~w[--severity warning --warnings-as-errors])
-                 )
+        catch_exit(run(:unclean, "compile", ~w[--severity warning --warnings-as-errors]))
       end)
     end
   end
@@ -157,6 +161,6 @@ defmodule MixUnusedTest do
   end
 
   defp find_diagnostics_for(diagnostics, m, f, a) do
-    Enum.find(diagnostics, &(&1.message =~ "#{inspect(m)}.#{f}/#{a}"))
+    Enum.find(diagnostics, &(&1.details.mfa == {m, f, a}))
   end
 end