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Rollup merge of #100208 - RalfJung:dyn-upcast-nop, r=petrochenkov

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make NOP dyn casts not require anything about the vtable

As suggested [on Zulip](292151439). This matches what the codegen backends already do, and what Miri did do until https://github.com/rust-lang/rust/pull/99420 when I made it super extra paranoid.
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Dylan DPC 2022-08-19 12:26:41 +05:30 • committed by GitHub
commit c4707ff8ef
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6 changed files with 46 additions and 21 deletions
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7
compiler/rustc_const_eval/src/interpret/cast.rs
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@ -298,7 +298,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
self.write_immediate(val, dest)
}
(&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => {
let (old_data, old_vptr) = self.read_immediate(src)?.to_scalar_pair()?;
let val = self.read_immediate(src)?;
if data_a.principal() == data_b.principal() {
// A NOP cast that doesn't actually change anything, should be allowed even with mismatching vtables.
return self.write_immediate(*val, dest);
}
let (old_data, old_vptr) = val.to_scalar_pair()?;
let old_vptr = old_vptr.to_pointer(self)?;
let (ty, old_trait) = self.get_ptr_vtable(old_vptr)?;
if old_trait != data_a.principal() {