fix Stacked Borrows interaction with dyn*

src/helpers.rs

@@ -478,6 +478,10 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 } else if matches!(v.layout.fields, FieldsShape::Union(..)) {
                     // A (non-frozen) union. We fall back to whatever the type says.
                     (self.unsafe_cell_action)(v)
+                } else if matches!(v.layout.ty.kind(), ty::Dynamic(_, _, ty::DynStar)) {
+                    // This needs to read the vtable pointer to proceed type-driven, but we don't
+                    // want to reentrantly read from memory here.
+                    (self.unsafe_cell_action)(v)
                 } else {
                     // We want to not actually read from memory for this visit. So, before
                     // walking this value, we have to make sure it is not a

tests/pass/dyn-star.rs

@@ -1,5 +1,3 @@
-// Dyn* handling leads to some funky reentrancy in Stacked Borrows, for some reason
-//@compile-flags: -Zmiri-disable-stacked-borrows
 #![feature(dyn_star)]
 #![allow(incomplete_features)]