> it also breaks many formerly-well-defined programs that use `malloc_usable_size` Does it? According to the documentation for malloc_reusable_size, it can be

> it also breaks many formerly-well-defined programs that use `malloc_usable_size` Does it? According to the documentation for malloc_reusable_size, it can be used to find out the actual size of an allocation, but you still have to call realloc before writing to bytes beyond the size passed to malloc. "This function is intended to only be used for diagnostics and statistics; writing to the excess memory without first calling realloc(3) to resize the allocation is not supported."