Add Mix.installed?/0 (#11374)

Author: wojtekmach

lib/mix/lib/mix.ex

@@ -747,6 +747,14 @@ defmodule Mix do
     end
   end
 
+  @doc """
+  Returns whether `Mix.install/2` was called in the current node.
+  """
+  @doc since: "1.13.0"
+  def installed? do
+    Mix.State.get(:installed) != nil
+  end
+
   defp stop_apps([]), do: :ok
 
   defp stop_apps(apps) do

lib/mix/test/mix_test.exs

@@ -137,6 +137,16 @@ defmodule MixTest do
       Application.delete_env(:unknown_app, :foo, persistent: true)
     end
 
+    test "install?", %{tmp_dir: tmp_dir} do
+      refute Mix.installed?()
+
+      Mix.install([
+        {:install_test, path: Path.join(tmp_dir, "install_test")}
+      ])
+
+      assert Mix.installed?()
+    end
+
     defp test_project(%{tmp_dir: tmp_dir}) do
       path = :code.get_path()