Test that plan --diff says a left-out module before the diff, through printed()

cmd/mesh-controller/plan.go

@@ -512,6 +512,15 @@ func sortedKeysOf(m map[string]string) []string {
 	return out
 }
 
+// sayPlanDiff is `plan --diff`: what the declaration leaves out, then the diff. A module left out is not in
+// the body, so the diff alone would say "nothing would change" for a module just assigned whose settings
+// cannot compose — success-shaped silence. Said first, with why, as push and the plain plan say it
+// (novox/hq ADR 0163, rule 6).
+func sayPlanDiff(node string, declared sendable, diff func() error) error {
+	reportLeftOut(node, declared)
+	return diff()
+}
+
 // reportLeftOut says which of a machine's modules its declaration leaves out and why (novox/hq ADR
 // 0163, rule 6), one line each: the machine is told everything else, and is told it was left out.
 func reportLeftOut(node string, declared sendable) {
@@ -1239,11 +1248,9 @@ func planCommand(ctx context.Context, args []string) error {
 		if err != nil {
 			return err
 		}
-		// A module left out is not in the body, so the diff alone would say "nothing would change" for
+		return sayPlanDiff(args[0], declared, func() error {
-		// a module just assigned whose settings cannot compose — success-shaped silence. Said first, with
-		// why, as push and the plain plan say it (novox/hq ADR 0163, rule 6).
-		reportLeftOut(args[0], declared)
 			return writePlanDiff(ctx, open.inventory, args[0], body)
+		})
 	}
 	if *asJSON {
 		declared, err := declarationFor(ctx, open, args[0], plan, settings)

cmd/mesh-controller/plan_diff_left_out_test.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"os"
+	"strings"
+	"testing"
+)
+
+// 2026-10-09: nfs-server was assigned to the home server, its file asked for a setting nothing set, and
+// `plan --diff` said "nothing would change". The diff now says what is left out, and why, before the diff.
+func TestPlanDiffSaysAModuleLeftOutBeforeTheDiff(t *testing.T) {
+	declared := sendable{LeftOut: []string{"nfs-server"},
+		leftOutWhy: map[string]string{"nfs-server": `nothing sets "shares" for it`}}
+	said := printed(t, func() error {
+		return sayPlanDiff("home", declared, func() error {
+			writeDiff(os.Stdout, "home", sentDiff{}, nil)
+			return nil
+		})
+	})
+	left := strings.Index(said, "home: nfs-server left out")
+	nothing := strings.Index(said, "home: nothing would change")
+	if left < 0 || !strings.Contains(said, `nothing sets "shares" for it`) || nothing < left {
+		t.Errorf("the left-out module and why, then the diff:\n%s", said)
+	}
+}