pub fn ArrayList(comptime T: type) type {
        return struct {
            const Self = @This();

            raw_items: []T,
            len: usize,
            allocator: *Allocator,

            // ...

            pub fn append(self: *Self, item: T) !void {
                const new_item_ptr = try self.addOne();
                new_item_ptr.* = item;
            }
            
            pub fn addOne(self: *Self) !*T {
                const new_length = self.len + 1;
                try self.ensureCapacity(new_length);
                return self.addOneAssumeCapacity();
            }
            
            // ...
        };
    }


    test "std.ArrayList.basic" {
        var bytes: [1024]u8 = undefined;
        const allocator = &std.heap.FixedBufferAllocator.init(bytes[0..]).allocator;

        var list = ArrayList(i32).init(allocator);

        try list.append(1);
        try list.append(2);
        try list.append(3);

        assert(list.len == 3);
    }

[1]: https://github.com/ziglang/zig/blob/e3d8cae35a5d194386eacd9a...

[2]: https://ziglang.org/documentation/master/#Error-Set-Type