Indeed, and more — there are code bases (ours, FRRouting) that do the opposite: expect the allocator to track sizes and invoke malloc_usable_size() to do our ow

Indeed, and more — there are code bases (ours, FRRouting) that do the opposite: expect the allocator to track sizes and invoke malloc_usable_size() to do our own (per-type-of-allocation grouped) accounting (to subtract on realloc/free; for adding it's not needed of course since you have the size already.)