Idiomatic way to free item memory in a zig ArrayList([] const u8)

I have an ArrayList([]const u8). I pass a pointer to it into functions that append to it. They append the results of calls to std.fmt.allocPrint(). To free everything up, the top-level function deinits the ArrayList after it frees all the items:

var typeLines = std.ArrayList([]const u8).init(allocator);
defer typeLines.deinit();
defer for (typeLines.items) |line| {
    allocator.free(line);
};

This works. But I have some cases now where the called functions should append constant strings. So I can't simply loop through the items and free them all.

I've thought about checking each item's type to see if I need to free it, or maybe keeping a separate ArrayList of just the items that need to be freed. What's the idiomatic way to identify which items need to be freed?

I ended up putting the []const u8 slice in a struct with a bool flag:

const Line = struct {
    text: []const u8,
    freeIt: bool,
};

I used the Line struct in the ArrayList:

var typeLines = std.ArrayList(Line).init(allocator);
defer typeLines.deinit();
defer for (typeLines.items) |line| {
    if (line.freeIt) {
        allocator.free(line.text);
    }
};

This lets the called functions control whether or not the caller frees their additions to the ArrayList:

var s = try std.fmt.allocPrint(allocator, "\npub const {s} = struct {{\n", .{typeName});
try typeLines.append(Line{ .text = s, .freeIt = true });

try typeLines.append(Line{ .text = "\n};\n", .freeIt = false });

I think an ArenaAllocator would be the best solution here.

It essentially stores all allocations in bigger buffers and can free these all at once. So you don't need to free elements individually, and you don't need to check if you allocated them.

const arena = std.heap.ArenaAllocator.init(allocator);
defer arena.deinit();

var typeLines = std.ArrayList(Line).init(allocator);
defer typeLines.deinit();

...
var s = try std.fmt.allocPrint(arena.allocator(), "\npub const {s} = struct {{\n", .{typeName});
try typeLines.append(s);

try typeLines.append("\n};\n");