Source
   pub fn resize(
    ctx: *anyopaque,
    buf: []u8,
    alignment: mem.Alignment,
    new_size: usize,
    return_address: usize,
) bool {
    const self: *FixedBufferAllocator = @ptrCast(@alignCast(ctx));
    _ = alignment;
    _ = return_address;
    assert(@inComptime() or self.ownsSlice(buf));
    if (!self.isLastAllocation(buf)) {
        if (new_size > buf.len) return false;
        return true;
    }
    if (new_size <= buf.len) {
        const sub = buf.len - new_size;
        self.end_index -= sub;
        return true;
    }
    const add = new_size - buf.len;
    if (add + self.end_index > self.buffer.len) return false;
    self.end_index += add;
    return true;
}