Restore comparability of completion metadata

completion.go

@@ -61,7 +61,12 @@ type CompletionChoice struct {
 // value requests no value suggestions; hints do not constrain accepted input.
 type CompletionHint struct {
 	kind    completionHintKind
-	choices []CompletionChoice
+	choices *completionChoicesData
+}
+
+// Keep the public metadata structs comparable without exposing mutable choices.
+type completionChoicesData struct {
+	values []CompletionChoice
 }
 
 type completionHintKind uint8
@@ -78,7 +83,11 @@ const (
 // values and values containing a newline, tab, or NUL cannot be presented as
 // suggestions, but remain valid input if the command's converter accepts them.
 func CompleteChoices(choices ...CompletionChoice) CompletionHint {
-	return CompletionHint{kind: completionChoices, choices: append([]CompletionChoice(nil), choices...)}
+	hint := CompletionHint{kind: completionChoices}
+	if len(choices) != 0 {
+		hint.choices = &completionChoicesData{values: append([]CompletionChoice(nil), choices...)}
+	}
+	return hint
 }
 
 // CompleteFiles requests shell filename completion.
@@ -95,7 +104,10 @@ func bridgeCompletion(hint CompletionHint) commandmeta.CompletionHint {
 	switch hint.kind {
 	case completionChoices:
 		result.Kind = commandmeta.CompletionChoices
-		for _, choice := range hint.choices {
+		if hint.choices == nil {
+			break
+		}
+		for _, choice := range hint.choices.values {
 			result.Choices = append(result.Choices, commandmeta.CompletionChoice{Value: choice.Value, Description: choice.Description})
 		}
 	case completionFiles: