Show mix hex.package diff commands in hex.outdated output

lib/mix/tasks/hex.outdated.ex

@@ -33,7 +33,9 @@ defmodule Mix.Tasks.Hex.Outdated do
   at the project's current set of dependency requirements and what version
   they are locked to. When `mix deps.update` is called multiple packages may
   be updated that in turn update their own dependencies, which may cause the
-  package you want to update to not be able to update.
+  package you want to update to not be able to update. If you want to force
+  a dependency to be updated to a given version, you can directly update it
+  in your `mix.exs`.
 
   > In a project, this task must be invoked before any other tasks
   > that may load or start your application. Otherwise, you must
@@ -196,7 +198,8 @@ defmodule Mix.Tasks.Hex.Outdated do
 
       base_message = "Run `mix hex.outdated APP` to see requirements for a specific dependency."
       diff_message = maybe_diff_message(diff_links)
-      Hex.Shell.info(["\n", base_message, diff_message])
+      diff_command_message = maybe_diff_command_message(diff_links)
+      Hex.Shell.info(["\n", base_message, diff_message, diff_command_message])
 
       outdated = outdated(versions)
       any_updatable? = any_possible_to_update?(outdated)
@@ -328,6 +331,12 @@ defmodule Mix.Tasks.Hex.Outdated do
       diff_link(diff_links)
   end
 
+  defp maybe_diff_command_message([]), do: ""
+
+  defp maybe_diff_command_message(_diff_links) do
+    "\n\nTo view the diff of a specific update, run `mix hex.package diff APP FROM..TO`."
+  end
+
   defp diff_link(diff_links) do
     long_url = "https://diff.hex.pm/diffs?" <> Enum.join(diff_links, "&")
 

test/mix/tasks/hex.outdated_test.exs

@@ -668,6 +668,26 @@ defmodule Mix.Tasks.Hex.OutdatedTest do
     end)
   end
 
+  test "outdated shows a generic diff command hint when updates are available" do
+    Mix.Project.push(OutdatedDeps.MixProject)
+
+    in_tmp(fn ->
+      set_home_tmp()
+      Mix.Dep.Lock.write(%{bar: {:hex, :bar, "0.1.0"}, foo: {:hex, :foo, "0.1.0"}})
+
+      Mix.Task.run("deps.get")
+      flush()
+
+      assert catch_throw(Mix.Task.run("hex.outdated", ["--all"])) == {:exit_code, 1}
+
+      lines = flush()
+      output = Enum.map_join(lines, "\n", fn {_, _, [line]} -> line end)
+
+      assert output =~
+               "To view the diff of a specific update, run `mix hex.package diff APP FROM..TO`."
+    end)
+  end
+
   defp extract_statuses(lines) do
     Enum.flat_map(lines, fn {_, _, [line]} ->
       ~r/Up-to-date|Update not possible|Update possible/