mix help app:APP: compile current Mix project (if any) (#14958)

Prior to this patch, it was impossible to get help for the current app:

    nimble_csv$ mix help app:nimble_csv
    Application nimble_csv does not exist or is not loaded

It also helps with projects with depenendencies when they haven't been
compiled yet:

    req$ mix deps.get && mix help app:finch # mix deps.compile haven't been run yet
    Application finch does not exist or is not loaded

Now it works:

    nimble_csv$ mix help app:nimble_csv
    Compiling 1 file (.ex)
    Generated nimble_csv app
    # NimbleCSV

    NimbleCSV is a small and fast parsing and dumping library.

    # NimbleCSV.RFC4180

    A CSV parser that uses comma as separator and double-quotes as escape according
    to RFC4180.

    # NimbleCSV.Spreadsheet

    A parser with spreadsheet friendly settings.

Author: wojtekmach

lib/mix/lib/mix/tasks/help.ex

@@ -112,14 +112,19 @@ defmodule Mix.Tasks.Help do
 
   def run(["app:" <> app]) do
     loadpaths!()
+
     app = String.to_atom(app)
 
     # If the application is not available, attempt to load it from Erlang/Elixir
     if is_nil(Application.spec(app, :vsn)) do
       try do
         Mix.ensure_application!(app)
       rescue
-        _ -> :ok
+        _ ->
+          # Otherwise, it may be a dep or the current project not yet compiled
+          if Mix.Project.get() do
+            Mix.Task.run("compile")
+          end
       end
     end
 

lib/mix/test/mix/tasks/help_test.exs

@@ -222,6 +222,41 @@ defmodule Mix.Tasks.HelpTest do
     end)
   end
 
+  defmodule ExampleProject do
+    def project do
+      [
+        app: :sample,
+        version: "0.1.0"
+      ]
+    end
+  end
+
+  test "help app:APP for current project", context do
+    in_tmp(context.test, fn ->
+      Mix.Project.push(ExampleProject)
+      File.mkdir_p!("lib")
+
+      File.write!("lib/example.ex", ~s'''
+      defmodule Example do
+        @moduledoc """
+        This is an example module.
+        """
+      end
+      ''')
+
+      output =
+        capture_io(fn ->
+          Mix.Tasks.Help.run(["app:sample"])
+        end)
+
+      assert output =~ """
+             # Example
+
+             This is an example module.
+             """
+    end)
+  end
+
   test "help Elixir MODULE", context do
     in_tmp(context.test, fn ->
       output =