Handle absent gleam binary

Author: Rodrigo Álvarez

lib/mix/lib/mix/gleam.ex

@@ -14,6 +14,28 @@ defmodule Mix.Gleam do
     end
   end
 
+  def available? do
+    case version_cmd() do
+      {:ok, _} -> true
+      {:error, _} -> false
+    end
+  end
+
+  defp version_cmd do
+    try do
+      case System.cmd("gleam", ["--version"]) do
+        {output, 0} -> {:ok, output}
+        {_output, error_code} -> {:error, error_code}
+      end
+    rescue
+      error in ErlangError ->
+        case error do
+          %ErlangError{original: :enoent} -> {:error, :enoent}
+          _ -> raise error
+        end
+    end
+  end
+
   defp parse_config(config) do
     dependencies =
       config

lib/mix/lib/mix/tasks/deps.compile.ex

@@ -300,7 +300,14 @@ defmodule Mix.Tasks.Deps.Compile do
     true
   end
 
-  defp do_gleam(%Mix.Dep{opts: opts} = dep, config) do
+  defp do_gleam(%Mix.Dep{app: app, opts: opts} = dep, config) do
+    if not Mix.Gleam.available?() do
+      Mix.raise(
+        "Could not find gleam binary, which is needed to build dependency #{inspect(app)}\n" <>
+          "Please head to https://gleam.run/getting-started/installing/ and install it."
+      )
+    end
+
     lib = Path.join(Mix.Project.build_path(), "lib")
     out = opts[:build]
     package = opts[:dest]