fix: stop the missing-tar warning claiming a verdict it cannot reach

It said the release "is never packed" and that "nothing can install this
version", then acknowledged the counterexample in a trailing sentence without
retracting either claim. All the check can see is one atom missing from the
list as given; whether an archive appears is not visible to it, because a
function step later in the list can pack one itself or add :tar to the steps
still to run - %Mix.Release{} carries those remaining steps precisely so a
step can.

That combination is the worst available: a definite diagnosis on the error
channel, sending an operator to investigate a packaging failure that may not
exist, with the retraction buried after it. The warning now states the
condition it observed and what follows unless something else packs the
archive, and asserts nothing else.

The test for it asserts the absence of both retracted claims as well as the
presence of the conditional, so a message that dropped the consequence
altogether fails too.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ausimian

lib/castle.ex

@@ -131,17 +131,28 @@ defmodule Castle do
   # refuses, and `Forecastle.steps/1` hands such a list back untouched for that
   # refusal to happen. A second implementation of Mix's rule could only drift
   # from it.
+  # **The warning states the condition it observed and a conditional
+  # consequence, and asserts no verdict.** What this can see is one atom's
+  # absence from the list as given. What it cannot see is whether an archive
+  # appears anyway: a function step later in the list can pack one itself, or
+  # add `:tar` to the steps still to run, and `%Mix.Release{}` carries those
+  # remaining steps precisely so a step can. An earlier version of this said the
+  # release "is never packed" and that "nothing can install this version" - then
+  # acknowledged the counterexample in a trailing sentence without retracting
+  # either claim, which is the worst of both: a definite diagnosis on the error
+  # channel sending an operator to investigate a packaging failure that may not
+  # exist. Say what was seen, say what follows *unless* something else packs it,
+  # and stop.
   defp warn_missing_tar(steps) do
     if :assemble in steps and :tar not in steps do
       Mix.shell().error(
-        "warning: Castle.customize/1 was given a :steps list with no :tar step, " <>
-          "so this release is assembled but never packed into <name>-<vsn>.tar.gz. " <>
-          "That tarball is what is copied into a deployment's releases directory " <>
-          "for bin/castle unpack to read, so nothing can install this version onto " <>
-          "a running system. Add :tar after :assemble if this version is meant to " <>
-          "be installed anywhere. A deployment that is only ever upgraded from " <>
-          "needs no tarball of its own, and a step of your own may be packing one, " <>
-          "in which case there is nothing here to act on."
+        "warning: Castle.customize/1 was given a :steps list with no :tar step. " <>
+          "Unless a step of your own packs one, this release will not produce the " <>
+          "<name>-<vsn>.tar.gz that is copied into a deployment's releases " <>
+          "directory for bin/castle unpack to read - and bin/castle unpack is how " <>
+          "a version is installed onto a running system. Add :tar after :assemble " <>
+          "if this version is meant to be installed anywhere. A deployment that is " <>
+          "only ever upgraded *from* needs no tarball of its own."
       )
     end
   end

test/castle/customize_test.exs

@@ -101,6 +101,29 @@ defmodule Castle.CustomizeTest do
       assert message =~ "bin/castle unpack"
     end
 
+    test "claims no verdict about whether an archive appears" do
+      # All this can see is one atom missing from the list as given. A function
+      # step later in the list can pack an archive itself, or add `:tar` to the
+      # steps still to run - `%Mix.Release{}` carries those precisely so a step
+      # can - so the absence of the atom is not the absence of a tarball.
+      #
+      # Asserted as the absence of the assertions, because that is the defect: an
+      # earlier version said the release "is never packed" and that "nothing can
+      # install this version", then acknowledged the counterexample in a trailing
+      # sentence without retracting either. A definite diagnosis on the error
+      # channel sends an operator after a packaging failure that may not exist.
+      Castle.customize(steps: [:assemble])
+
+      assert_received {:mix_shell, :error, [message]}
+
+      refute message =~ "never packed"
+      refute message =~ "nothing can install"
+
+      # And the conditional is present rather than merely the claims being gone,
+      # so a message that dropped the consequence entirely fails too.
+      assert message =~ "Unless a step of your own packs one"
+    end
+
     test "says nothing when the list has one" do
       Castle.customize(steps: [:assemble, :tar])