[conradludgate]

The Rust allocator APIs require layout information on dealloc[0]. In the majority of cases it's a non-issue. Take Vec (like std::vector) for instance, it has pointer+length+capacity. It needs the capacity value so it knows when to realloc, so it can equivalently use it to dealloc with size information as well.

The only case I know of where this is an issue is when downsizing from Vec<T> to Box<[T]> (size optimisation for read-only arrays). Box<[T]> only stores the ptr+length, so the first step is calling shrink in order to make the capacity and length equal.

When it comes to type-erasure, it happens to work just fine. A type-erased heap pointer like Box<dyn Any> will have the size+align info stored in the static vtable. Yes it's some extra space stored, but only in the .text data and not as part of any allocations.

On this topic, I've linked a short post on allocator ideas[1] by a rust std-lib maintainer, which lists some of the other things we might add to rust's upcoming (non-global) Allocator trait

[0] https://doc.rust-lang.org/std/alloc/trait.GlobalAlloc.html#t... [1] https://shift.click/blog/allocator-trait-talk/