std.ArrayListUnamanged fix compile error when @sizeOf(T) == 0

[pmovzxdq]

Fixes #24886

[andrewrk]

I think there's a better way to implement this, and arguably a unit test is the more valuable deliverable here anyway.

[pmovzxdq]

I have some questions:

var list: ArrayList(void) = .empty;
try testing.expect(list.items.len == 0);
try testing.expect(list.capacity == 0); // ???

I am not sure if this is right. I think it makes more sense if list.capacity is initialized to math.maxInt(usize):

var list: ArrayList(void) = .empty;
try testing.expect(list.items.len == 0);
try testing.expect(list.capacity == math.maxInt(usize));

initCapacity() currently is like the second example:

var list = try ArrayList(void).initCapacity(testing.allocator, 200);
defer list.deinit(testing.allocator);

try testing.expect(list.items.len == 0);
try testing.expect(list.capacity == math.maxInt(usize));

Which one should be used?

You said that there is a better way to implement this.
I actually like the current fix because:

• init_capacity is only used by growCapacity().
• It makes sense, because init_capacity is equal to the number of T that can fit in a std.atomic.cache_line. An unlimited amount (math.maxInt(usize)) of 0 bytes (like @sizeOf(void) or @sizeOf(enum { one })) can fit into any amount of memory.

I am open to any suggestions.