Recursive types ok in Vec but not Arrays?

How come the following is ok:

struct My {
    a: Vec<My>,
}

But not this:

struct My2 {
    a: [My2; 10],
}

I understand that My2 does not have a know size and therefore I need to wrap inside a Box like so:

struct My3 {
    a: [Box<My3>; 10],
}

But should not the same problem exist for Vec and needing me to use:

struct My {
    a: Vec<Box<My>>,
}
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That’s because the Vec itself is like a resizeable Box. The Vec object has a fixed size of 3x usize, and the content is stored on the heap.

You can also use Box<[My; 10]> (it’s one box of 10 structs, your box example is one struct of 10 boxes).

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