fix(implementation-sweep): name every missing required flag, in flag order

cmd/internal-tools/implementation-sweep/main_test.go

@@ -143,6 +143,33 @@ func TestParseArguments_Rejections(t *testing.T) {
 	}
 }
 
+// Three flags missing is one rerun, not three: the operator is told about all
+// of them at once, in flag order, whatever order the check happened to walk.
+func TestParseArguments_NamesEveryMissingRequiredFlagInFlagOrder(t *testing.T) {
+	_, err := parseArguments(
+		[]string{"--spec", "s", "--max-steps", "10"},
+		io.Discard,
+	)
+	if err == nil {
+		t.Fatal("got no error, want every missing flag named")
+	}
+	message := err.Error()
+	previous := -1
+	for _, name := range []string{"--implementations", "--seeds", "--output"} {
+		at := strings.Index(message, name)
+		if at < 0 {
+			t.Fatalf("got %q, want %s named", message, name)
+		}
+		if at < previous {
+			t.Errorf("got %q, want the flags named in flag order", message)
+		}
+		previous = at
+	}
+	if strings.Contains(message, "--spec") {
+		t.Errorf("got %q, want the supplied --spec left out", message)
+	}
+}
+
 // The seed reaches two independent things, the campaign's own seed and the
 // scaffold's failure stream, and a replay reproduces neither unless they carry
 // the same number.