don't UB on dangling ptr deref, instead check inbounds on projections

compiler/rustc_const_eval/src/interpret/machine.rs

@@ -436,6 +436,7 @@ pub trait Machine<'mir, 'tcx: 'mir>: Sized {
         place: &PlaceTy<'tcx, Self::Provenance>,
     ) -> InterpResult<'tcx> {
         // Without an aliasing model, all we can do is put `Uninit` into the place.
+        // Conveniently this also ensures that the place actually points to suitable memory.
         ecx.write_uninit(place)
     }