Ensure we aren't adding extra newlines to failure output

lib/credo/cli/command/diff/output/default.ex

@@ -328,20 +328,12 @@ defmodule Credo.CLI.Command.Diff.Output.Default do
       " ",
       first_line
     ]
-    |> UI.puts()
+    |> UI.write_line()
 
     other_lines
     |> Enum.each(&print_issue_message(&1, issue, outer_color, message_color))
   end
 
-  defp print_issue_message(
-         "",
-         _issue,
-         _outer_color,
-         _message_color
-       ) do
-  end
-
   defp print_issue_message(
          message,
          issue,
@@ -358,7 +350,7 @@ defmodule Credo.CLI.Command.Diff.Output.Default do
       " ",
       message
     ]
-    |> UI.puts()
+    |> UI.write_line()
   end
 
   defp print_issue_line(

lib/credo/cli/command/suggest/output/default.ex

@@ -263,15 +263,12 @@ defmodule Credo.CLI.Command.Suggest.Output.Default do
       " ",
       first_line
     ]
-    |> UI.puts()
+    |> UI.write_line()
 
     other_lines
     |> Enum.each(&print_issue_message(&1, outer_color, message_color))
   end
 
-  defp print_issue_message("", _outer_color, _message_color) do
-  end
-
   defp print_issue_message(message, outer_color, message_color) do
     [
       UI.edge(outer_color),
@@ -282,7 +279,7 @@ defmodule Credo.CLI.Command.Suggest.Output.Default do
       " ",
       message
     ]
-    |> UI.puts()
+    |> UI.write_line()
   end
 
   defp print_issue_line(

lib/credo/cli/output/shell.ex

@@ -28,6 +28,11 @@ defmodule Credo.CLI.Output.Shell do
     GenServer.call(__MODULE__, {:suppress_output, false})
   end
 
+  @doc "Like `puts/1`, but does not append a trailing newline."
+  def write(value) do
+    GenServer.call(__MODULE__, {:write, value})
+  end
+
   @doc "Like `puts/1`, but writes to `:stderr`."
   def warn(value) do
     GenServer.call(__MODULE__, {:warn, value})
@@ -77,6 +82,32 @@ defmodule Credo.CLI.Output.Shell do
     {:reply, nil, current_state}
   end
 
+  def handle_call({:write, _value}, _from, %{suppress_output: true} = current_state) do
+    {:reply, nil, current_state}
+  end
+
+  def handle_call(
+        {:write, value},
+        _from,
+        %{use_colors: true, suppress_output: false} = current_state
+      ) do
+    do_write(value)
+
+    {:reply, nil, current_state}
+  end
+
+  def handle_call(
+        {:write, value},
+        _from,
+        %{use_colors: false, suppress_output: false} = current_state
+      ) do
+    value
+    |> remove_colors()
+    |> do_write()
+
+    {:reply, nil, current_state}
+  end
+
   def handle_call({:warn, _value}, _from, %{suppress_output: true} = current_state) do
     {:reply, nil, current_state}
   end
@@ -114,6 +145,10 @@ defmodule Credo.CLI.Output.Shell do
     Bunt.puts(value)
   end
 
+  defp do_write(value) do
+    Bunt.write(value)
+  end
+
   defp do_warn(value) do
     Bunt.warn(value)
   end

lib/credo/cli/output/ui.ex

@@ -23,6 +23,8 @@ defmodule Credo.CLI.Output.UI do
     def puts(_), do: nil
     def puts(_, color) when is_atom(color), do: nil
 
+    def write(_), do: nil
+
     def warn(_), do: nil
   else
     defdelegate puts, to: @shell_service
@@ -32,9 +34,26 @@ defmodule Credo.CLI.Output.UI do
       @shell_service.puts([color, v])
     end
 
+    defdelegate write(v), to: @shell_service
+
     defdelegate warn(v), to: @shell_service
   end
 
+  def write_line(value) do
+    value
+    |> with_trailing_newline()
+    |> write()
+  end
+
+  @doc false
+  def with_trailing_newline(value) do
+    if ends_with_newline?(value) do
+      value
+    else
+      List.wrap(value) ++ ["\n"]
+    end
+  end
+
   def edge(color, indent \\ 2) when is_integer(indent) do
     [:reset, color, @edge |> String.pad_trailing(indent)]
   end
@@ -97,4 +116,16 @@ defmodule Credo.CLI.Output.UI do
         String.slice(line, 0, chars_to_display) <> ellipsis
     end
   end
+
+  defp ends_with_newline?(value) do
+    value
+    |> List.wrap()
+    |> List.flatten()
+    |> Enum.reject(&is_atom/1)
+    |> List.last()
+    |> case do
+      nil -> false
+      value -> value |> to_string() |> String.ends_with?("\n")
+    end
+  end
 end

test/credo/cli/output/ui_test.exs

@@ -33,6 +33,49 @@ defmodule Credo.CLI.Output.UITest do
     assert expected == lines
   end
 
+  test "it should keep each line break on its own element so callers can write them" do
+    lines =
+      "First line of the message.\nSecond line of the message."
+      |> UI.wrap_at(80)
+
+    expected = [
+      "First line of the message.\n",
+      "Second line of the message."
+    ]
+
+    assert expected == lines
+  end
+
+  test "it should keep a blank line as its own element" do
+    lines =
+      "First paragraph.\n\nSecond paragraph."
+      |> UI.wrap_at(80)
+
+    expected = [
+      "First paragraph.\n",
+      "\n",
+      "Second paragraph."
+    ]
+
+    assert expected == lines
+  end
+
+  test "with_trailing_newline should append a newline to plain content" do
+    assert ["Plain wrapped line", "\n"] == UI.with_trailing_newline("Plain wrapped line")
+  end
+
+  test "with_trailing_newline should not double a newline already present in the final chunk" do
+    line = ["First line of the message.\n"]
+
+    assert line == UI.with_trailing_newline(line)
+  end
+
+  test "with_trailing_newline should keep a blank line as a single line break" do
+    line = ["\n"]
+
+    assert line == UI.with_trailing_newline(line)
+  end
+
   test "it should be able to break up a line including unicode characters" do
     lines =
       "あいうえ"