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

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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
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Steve Klabnik 2017-01-23 16:40:49 -05:00 committed by GitHub
commit e113038a36
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src/doc/book/trait-objects.md
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@ -263,10 +263,7 @@ any resources of the vtable’s type: for `u8` it is trivial, but for `String` i
will free the memory. This is necessary for owning trait objects like
`Box<Foo>`, which need to clean-up both the `Box` allocation as well as the
internal type when they go out of scope. The `size` and `align` fields store
the size of the erased type, and its alignment requirements; these are
essentially unused at the moment since the information is embedded in the
destructor, but will be used in the future, as trait objects are progressively
made more flexible.
the size of the erased type, and its alignment requirements.
Suppose we’ve got some values that implement `Foo`. The explicit form of
construction and use of `Foo` trait objects might look a bit like (ignoring the