vtable: fix UB detected as miri errors

[ogoffart]

We can't create references for things that aren't represented by the reference. Even if we never dereference that reference.

Also add a miri test in the CI that runs the test on crate which are error free.

[tronical]

I’m curious: what’s the intermediate reference that’s created where the types don’t match?

[tronical]

I obviously trust you on this, I understand also the premise of the change, but I don’t understand yet where we create the wrong reference, temporarily. (Asking for a future $me writing pointer code)

[ogoffart]

What we create is a VRcInner<VTable, T> where T is some specific type that implement the trait for the VTable. So we should be careful not to take reference to VRcInner<VTable, Dyn> or VRcInner<VTable, Layout>, which are used to get the metadata of dynamic VRc or destroyed VRc, respectively, but are not "real"

[tronical]

Thanks, makes sense