Allow releases as an anonymous function

Author: José Valim

lib/mix/lib/mix/release.ex

@@ -158,7 +158,8 @@ defmodule Mix.Release do
   end
 
   defp find_release(name, config) do
-    {name, opts} = lookup_release(name, config) || infer_release(config)
+    {name, opts_fun_or_list} = lookup_release(name, config) || infer_release(config)
+    opts = if is_function(opts_fun_or_list, 0), do: opts_fun_or_list.(), else: opts_fun_or_list
     {apps, opts} = Keyword.pop(opts, :applications, [])
 
     if apps == [] and Mix.Project.umbrella?(config) do

lib/mix/lib/mix/tasks/release.ex

@@ -346,7 +346,7 @@ defmodule Mix.Tasks.Release do
 
   ### Options
 
-  The following options can be set inside your mix.exs on each release definition:
+  The following options can be set inside your `mix.exs` on each release definition:
 
     * `:applications` - a keyword list that configures and adds new applications
       to the release. The key is the application name and the value is one of:
@@ -438,6 +438,15 @@ defmodule Mix.Tasks.Release do
     * `:steps` - a list of steps to execute when assembling the release. See
       the "Steps" section for more information.
 
+  Note each release definition can be given as an anonymous function. This
+  is useful if some release attributes are expensive to compute:
+
+      releases: [
+        demo: fn ->
+          [version: @version <> "+" <> git_ref()]
+        end
+      ]
+
   Besides the options above, it is possible to customize the generated
   release with custom files, by tweaking the release steps or by running
   custom options and commands on boot. We will detail both approaches next.

lib/mix/test/mix/release_test.exs

@@ -74,6 +74,12 @@ defmodule Mix.ReleaseTest do
       assert release.applications.kernel[:otp_app?]
     end
 
+    test "allows release to be given as an anonymous function" do
+      release = from_config!(:foo, config(releases: [foo: fn -> [version: "0.2.0"] end]), [])
+      assert release.name == :foo
+      assert release.version == "0.2.0"
+    end
+
     test "uses chosen release via the CLI" do
       release =
         from_config!(