Rollup merge of #39191 - cesarb:book/trait-objects-vtable-size-and-align, r=steveklabnik
Rollup merge of #39191 - cesarb:book/trait-objects-vtable-size-and-align, r=steveklabnik

book: size and align in trait object vtables are used

The book currently claims that the `size` and `align` fields in the
trait object vtable are not used, but this is false. These two fields
are used by the stable `mem::size_of_val` and `mem::align_of_val`
functions.

See the `ty::TyDynamic` case of the `glue::size_and_align_of_dst`
function in librustc_trans, which is used to implement both intrinsics
in the unsized case.

r? @steveklabnik

GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust