Add :formatter to ExUnit.CaptureLog options

CHANGELOG.md

@@ -46,6 +46,10 @@ The type system was made possible thanks to a partnership between [CNRS](https:/
   * [Kernel] Warn on unused requires
   * [Regex] Add `Regex.import/1` to import regexes defined with `/E`
 
+#### ExUnit
+
+  * [ExUnit.CaptureLog] Add `:formatter` option for custom log formatting
+
 #### Mix
 
   * [mix test] Add `mix test --dry-run`

lib/ex_unit/lib/ex_unit/capture_log.ex

@@ -44,16 +44,17 @@ defmodule ExUnit.CaptureLog do
   @compile {:no_warn_undefined, Logger}
 
   @type capture_log_opts :: [
-          level: Logger.level() | nil
+          {:level, Logger.level() | nil}
+          | {:formatter, {module(), term()} | nil}
         ]
 
   @doc """
   Captures Logger messages generated when evaluating `fun`.
 
   Returns the binary which is the captured output. The captured log
-  messages will be formatted using `Logger.default_formatter/1`. Any
-  option, besides the `:level`, will be forwarded as an override to
-  the default formatter.
+  messages will be formatted using `Logger.default_formatter/1` by
+  default. Any option, besides `:level` or `formatter`, will be
+  forwarded as an override to the default formatter.
 
   This function mutes the default logger handler and captures any log
   messages sent to Logger from the calling processes. It is possible
@@ -75,6 +76,11 @@ defmodule ExUnit.CaptureLog do
   configured in this function, no message will be captured.
   The behaviour is undetermined if async tests change Logger level.
 
+  It is possible to use an alternative log formatter with `:formatter`,
+  which must be provided as `{module, config}`. If `:formatter` is not
+  provided, the formatter configuration from `Logger.default_formatter/1`
+  will be used.
+
   To get the result of the evaluation along with the captured log,
   use `with_log/2`.
   """

lib/ex_unit/lib/ex_unit/capture_server.ex

@@ -75,7 +75,10 @@ defmodule ExUnit.CaptureServer do
     refs = Map.put(config.log_captures, ref, true)
 
     {level, opts} = Keyword.pop(opts, :level)
-    {formatter_mod, formatter_config} = Logger.default_formatter(opts)
+
+    {formatter_mod, formatter_config} =
+      Keyword.get_lazy(opts, :formatter, fn -> Logger.default_formatter(opts) end)
+
     true = :ets.insert(@ets, {ref, string_io, level || :all, formatter_mod, formatter_config})
 
     if map_size(refs) == 1 do

lib/ex_unit/test/ex_unit/capture_log_test.exs

@@ -143,6 +143,44 @@ defmodule ExUnit.CaptureLogTest do
     end
   end
 
+  defmodule CustomFormatter do
+    def new do
+      {_, opts} =
+        Logger.Formatter.new(
+          format: "$metadata| $message",
+          metadata: [:id],
+          colors: [enabled: false]
+        )
+
+      {__MODULE__, opts}
+    end
+
+    def format(event, config) do
+      event
+      |> wrap()
+      |> Logger.Formatter.format(config)
+    end
+
+    defp wrap(event) do
+      msg =
+        event
+        |> Logger.Formatter.format_event(:infinity)
+        |> to_string()
+
+      %{event | msg: {:string, ["[CUSTOM]", msg, "[/CUSTOM]"]}}
+    end
+  end
+
+  test "uses the formatter from the `:formatter` option" do
+    log =
+      capture_log([formatter: CustomFormatter.new()], fn ->
+        Logger.info("hello", id: 123)
+        2 + 2
+      end)
+
+    assert log == "id=123 | [CUSTOM]hello[/CUSTOM]"
+  end
+
   defp wait_capture_removal() do
     if ExUnit.CaptureServer in Enum.map(:logger.get_handler_config(), & &1.id) do
       Process.sleep(20)