Skip version-manager hint when .tool-versions is unreadable

The Mix task should not suggest mise/asdf when failure is an I/O error
from File.read. Document read failures in Toolchain.verify/2 and add a
regression test using a directory named .tool-versions.

Co-authored-by: Dominic Letz <dominicletz@users.noreply.github.com>

Author: cursoragent

lib/desktop/toolchain.ex

@@ -5,7 +5,8 @@ defmodule Desktop.Toolchain do
   Verifies that the running Erlang/OTP and Elixir versions match `.tool-versions` in
   `project_root` when that file declares `erlang` and/or `elixir` entries.
 
-  Returns `{:ok, :no_tool_versions}` if the file is missing, or `{:ok, :verified}` on success.
+  Returns `{:ok, :no_tool_versions}` if the file is missing, `{:ok, :verified}` on success,
+  or `{:error, [message]}` when `.tool-versions` cannot be read or versions do not match.
 
   `opts` may override runtime versions for testing:
 

lib/mix/tasks/desktop.check_toolchain.ex

@@ -32,12 +32,14 @@ defmodule Mix.Tasks.Desktop.CheckToolchain do
       {:error, messages} ->
         for msg <- messages, do: Mix.shell().error(msg)
 
-        Mix.shell().error("""
-        Activate the versions in .tool-versions for this project, for example:
-          mise install && mise exec -- mix desktop.check_toolchain
-        or:
-          asdf install && asdf exec mix desktop.check_toolchain
-        """)
+        unless toolchain_hint_irrelevant?(messages) do
+          Mix.shell().error("""
+          Activate the versions in .tool-versions for this project, for example:
+            mise install && mise exec -- mix desktop.check_toolchain
+          or:
+            asdf install && asdf exec mix desktop.check_toolchain
+          """)
+        end
 
         System.halt(1)
     end
@@ -54,4 +56,8 @@ defmodule Mix.Tasks.Desktop.CheckToolchain do
   defp mix_project? do
     Mix.Project.get() != nil
   end
+
+  defp toolchain_hint_irrelevant?(messages) do
+    Enum.any?(messages, &match?("Could not read .tool-versions:" <> _, &1))
+  end
 end

test/desktop/toolchain_test.exs

@@ -39,4 +39,11 @@ defmodule Desktop.ToolchainTest do
 
     assert msg =~ "Elixir mismatch"
   end
+
+  test "error when .tool-versions cannot be read", %{root: root} do
+    File.mkdir!(Path.join(root, ".tool-versions"))
+
+    assert {:error, [msg]} = Desktop.Toolchain.verify(root)
+    assert msg =~ "Could not read .tool-versions"
+  end
 end