NodeKind/String: Don't panic on unknown

NodeKind.String() will currently panic if called with an unknown
NodeKind.

This change verifies that the name of the NodeKind is known before
looking it up in kindNames. If the name is unknown, a generic
`NodeKind(N)` string representation is used.

ast/ast.go

@@ -26,7 +26,10 @@ const (
 type NodeKind int
 
 func (k NodeKind) String() string {
+	if int(k) < len(kindNames) {
 		return kindNames[k]
+	}
+	return fmt.Sprintf("NodeKind(%v)", int(k))
 }
 
 var kindMax NodeKind

ast/ast_test.go

@@ -73,3 +73,31 @@ func node(n Node, children ...Node) Node {
 	}
 	return n
 }
+
+func TestNodeKindString(t *testing.T) {
+	tests := []struct {
+		name string
+		give NodeKind
+		want string
+	}{
+		{
+			name: "known",
+			give: KindLink,
+			want: "Link",
+		},
+		{
+			name: "unknown",
+			give: NodeKind(500),
+			want: "NodeKind(500)",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := tt.give.String()
+			if tt.want != got {
+				t.Errorf("String() expected %q, got %q", tt.want, got)
+			}
+		})
+	}
+}