Function heapsize::heap_size_of

pub unsafe fn heap_size_of<T>(ptr: *const T) -> usize

Get the size of a heap block.

Ideally Rust would expose a function like this in std::rt::heap.

unsafe because the caller must ensure that the pointer is from jemalloc. FIXME: This probably interacts badly with custom allocators: https://doc.rust-lang.org/book/custom-allocators.html