Support PHX_NEW_CACHE_DIR for cached builds

installer/lib/mix/tasks/phx.new.ex

@@ -113,6 +113,28 @@ defmodule Mix.Tasks.Phx.New do
 
   You can read more about umbrella projects using the
   official [Elixir guide](https://hexdocs.pm/elixir/dependencies-and-umbrella-projects.html#umbrella-projects)
+
+  ## PHX_NEW_CACHE_DIR
+  In rare cases, it may be useful to copy the build from a previously
+  cached build. To do this, set the `PHX_NEW_CACHE_DIR` environment
+  variable before running `mix phx.new`. For example, you could generate a
+  cache by running:
+
+  ```shell
+  mix phx.new mycache --no-install && cd mycache \
+    && mix deps.get && mix deps.compile && mix assets.setup \
+    && rm -rf assets config lib priv test mix.exs README.md
+  ```
+
+  Your cached build directory
+  should contain:
+
+      _build
+      deps
+      mix.lock
+
+  The entire cache directory will be copied to the new project, replacing
+  any existing files where conflicts exist.
   """
   use Mix.Task
   alias Phx.New.{Generator, Project, Single, Umbrella, Web, Ecto}
@@ -185,7 +207,8 @@ defmodule Mix.Tasks.Phx.New do
     |> Generator.put_binding()
     |> validate_project(path)
     |> generator.generate()
-    |> prompt_to_install_deps(generator, path)
+    |> maybe_copy_cached_build(path)
+    |> maybe_prompt_to_install_deps(generator, path)
   end
 
   defp validate_project(%Project{opts: opts} = project, path) do
@@ -197,6 +220,15 @@ defmodule Mix.Tasks.Phx.New do
     project
   end
 
+  defp maybe_prompt_to_install_deps(%Project{} = project, generator, path_key) do
+    # we can skip the install deps setup, even with --install, because we already copied deps
+    if project.cached_build_path do
+      project
+    else
+      prompt_to_install_deps(project, generator, path_key)
+    end
+  end
+
   defp prompt_to_install_deps(%Project{} = project, generator, path_key) do
     path = Map.fetch!(project, path_key)
 
@@ -404,4 +436,24 @@ defmodule Mix.Tasks.Phx.New do
       )
     end
   end
+
+  defp maybe_copy_cached_build(%Project{} = project, path_key) do
+    project_path = Map.fetch!(project, path_key)
+
+    case System.fetch_env("PHX_NEW_CACHE_DIR") do
+      {:ok, cache_dir} ->
+        copy_cached_build(%{project_path: project_path, cache_dir: cache_dir})
+        %Project{project | cached_build_path: cache_dir}
+
+      :error ->
+        project
+    end
+  end
+
+  defp copy_cached_build(%{project_path: project_path, cache_dir: cache_dir}) do
+    if File.exists?(cache_dir) do
+      Mix.shell().info("Copying cached build from #{cache_dir}")
+      System.cmd("cp", ["-Rp", Path.join(cache_dir, "."), project_path])
+    end
+  end
 end

installer/lib/phx_new/project.ex

@@ -16,6 +16,7 @@ defmodule Phx.New.Project do
             opts: :unset,
             in_umbrella?: false,
             binding: [],
+            cached_build_path: nil,
             generators: [timestamp_type: :utc_datetime]
 
   def new(project_path, opts) do

installer/test/phx_new_test.exs

@@ -829,4 +829,43 @@ defmodule Mix.Tasks.Phx.NewTest do
       end)
     end)
   end
+
+  describe "PHX_CACHE_DIR" do
+    @phx_new_cache_dir System.get_env("PHX_NEW_CACHE_DIR")
+    test "new with PHX_NEW_CACHE_DIR" do
+      System.put_env("PHX_NEW_CACHE_DIR", __DIR__)
+      cache_files = File.ls!(__DIR__)
+      in_tmp("new with cache dir", fn ->
+        Mix.Tasks.Phx.New.run([@app_name])
+        project_files = File.ls!(Path.join(File.cwd!(), @app_name))
+        assert "mix.exs" in project_files
+        for file <- cache_files do
+          assert file in project_files, "#{file} not copied to new project"
+        end
+      end)
+    after
+      if @phx_new_cache_dir do
+        System.put_env("PHX_NEW_CACHE_DIR", @phx_new_cache_dir)
+      else
+        System.delete_env("PHX_NEW_CACHE_DIR")
+      end
+    end
+
+    test "new with PHX_NEW_CACHE_DIR that doesn't exist" do
+      cache_dir = Path.join(__DIR__, "does-not-exist")
+      System.put_env("PHX_NEW_CACHE_DIR", cache_dir)
+      refute File.exists?(cache_dir)
+      in_tmp("new with cache dir", fn ->
+        Mix.Tasks.Phx.New.run([@app_name])
+        project_files = File.ls!(Path.join(File.cwd!(), @app_name))
+        assert "mix.exs" in project_files
+      end)
+    after
+      if @phx_new_cache_dir do
+        System.put_env("PHX_NEW_CACHE_DIR", @phx_new_cache_dir)
+      else
+        System.delete_env("PHX_NEW_CACHE_DIR")
+      end
+    end
+  end
 end