add a Vtable kind of symbolic allocations

Author: RalfJung

src/common.rs

@@ -183,6 +183,13 @@ impl<'gcc, 'tcx> ConstMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
             }
             Scalar::Ptr(ptr, _size) => {
                 let (alloc_id, offset) = ptr.into_parts();
+                // For vtables, get the underlying data allocation.
+                let alloc_id = match self.tcx.global_alloc(alloc_id) {
+                    GlobalAlloc::Vtable(ty, trait_ref) => {
+                        self.tcx.vtable_allocation((ty, trait_ref))
+                    }
+                    _ => alloc_id,
+                };
                 let base_addr =
                     match self.tcx.global_alloc(alloc_id) {
                         GlobalAlloc::Memory(alloc) => {
@@ -201,6 +208,7 @@ impl<'gcc, 'tcx> ConstMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
                         GlobalAlloc::Function(fn_instance) => {
                             self.get_fn_addr(fn_instance)
                         },
+                        GlobalAlloc::Vtable(..) => panic!("vtables are already handled"),
                         GlobalAlloc::Static(def_id) => {
                             assert!(self.tcx.is_static(def_id));
                             self.get_static(def_id).get_address(None)