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The docs for size_of say the following:

If T is Sized, all of those types [pointer types such as &T] have the same size as usize.

Why that qualifier? Would a pointer not have a definitive size regardless of any property of the type being pointed to?

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Unsized object references have extra data in addition to the pointer to the object. In the case of slice references (&[T]), they contain a size in order to indicate how long this slice is. And in the case of references to traits (trait objects), they contain a pointer to a vtable in order to enable dynamic dispatch.

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now that it's written down in front of me, it seems painfully obvious... I should have thought more carefully what kinds of unsized types there are. Thanks!

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