compute the offset of dst fields by checking the vtable

src/lvalue.rs

@@ -214,7 +214,16 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
                     _ => bug!("field access on non-product type: {:?}", base_layout),
                 };
 
-                let ptr = base_ptr.offset(offset.bytes());
+                let offset = match base_extra {
+                    LvalueExtra::Vtable(tab) => {
+                        let (_, align) = self.size_and_align_of_dst(base_ty, Value::ByValPair(PrimVal::Ptr(base_ptr), PrimVal::Ptr(tab)))?;
+                        // magical formula taken from rustc
+                        (offset.bytes() + (align - 1)) & (-(align as i64) as u64)
+                    }
+                    _ => offset.bytes(),
+                };
+
+                let ptr = base_ptr.offset(offset);
 
                 if packed {
                     let size = self.type_size(field_ty)?.expect("packed struct must be sized");

src/terminator/intrinsic.rs

@@ -405,7 +405,7 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
         Ok(())
     }
 
-    fn size_and_align_of_dst(
+    pub fn size_and_align_of_dst(
         &self,
         ty: ty::Ty<'tcx>,
         value: Value,

tests/run-pass/dst-field-align.rs

@@ -0,0 +1,30 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![allow(dead_code)]
+
+struct Foo<T: ?Sized> {
+    a: u16,
+    b: T
+}
+
+trait Bar {
+    fn get(&self) -> usize;
+}
+
+impl Bar for usize {
+    fn get(&self) -> usize { *self }
+}
+
+fn main() {
+    let f : Foo<usize> = Foo { a: 0, b: 11 };
+    let f : &Foo<Bar> = &f;
+    assert_eq!(f.b.get(), 11);
+}