mem::{size,align,min_align}_of_val should accept DSTs

[ftxqxd]

Currently, mem::size_of_val and friends only support sized types, and just defer to mem::size_of etc.. Ideally, they would also support dynamically-sized types, in a way such that size_of_val(&[1, 2, 3]: &[int]) == size_of::<int>() * 3. Because this requires an actual value, they wouldn’t be able to simply defer to calling size_of etc. and would instead have to become intrinsic functions. I believe that this issue makes implementing pointer types that accept dynamically-sized type parameters very difficult (if not impossible) (e.g., #18248 (although I think that also might require a way of running destructors for unsized values)).

[aturon]

cc @japaric

[ftxqxd]

This is almost done now—size_of_val and min_align_of_val now accept DSTs, but align_of_val still doesn’t.

[eddyb]

@P1start I wonder how we should go about supporting that, as trait objects store only min_align_of.

[bluss]

This is fixed -- because align_of_val now is minimum alignment.