Resolve associated constants.

Fixes #130.

src/eval_context.rs

@@ -814,6 +814,7 @@ pub(super) fn eval_operand(&mut self, op: &mir::Operand<'tcx>) -> EvalResult<'tc
                             // function items are zero sized
                             Value::ByRef(self.memory.allocate(0, 0)?)
                         } else {
+                            let (def_id, substs) = self.resolve_associated_const(def_id, substs);
                             let cid = GlobalId { def_id, substs, promoted: None };
                             self.read_lvalue(Lvalue::Global(cid))
                         }

src/step.rs

@@ -149,6 +149,7 @@ struct ConstantExtractor<'a, 'b: 'a, 'tcx: 'b> {
 
 impl<'a, 'b, 'tcx> ConstantExtractor<'a, 'b, 'tcx> {
     fn global_item(&mut self, def_id: DefId, substs: &'tcx subst::Substs<'tcx>, span: Span, immutable: bool) {
+        let (def_id, substs) = self.ecx.resolve_associated_const(def_id, substs);
         let cid = GlobalId { def_id, substs, promoted: None };
         if self.ecx.globals.contains_key(&cid) {
             return;

src/traits.rs

@@ -209,4 +209,24 @@ fn normalize_and_test_predicates(&mut self, predicates: Vec<ty::Predicate<'tcx>>
             fulfill_cx.select_all_or_error(&infcx).is_ok()
         })
     }
+
+    pub(crate) fn resolve_associated_const(
+        &self,
+        def_id: DefId,
+        substs: &'tcx Substs<'tcx>,
+    ) -> (DefId, &'tcx Substs<'tcx>) {
+        if let Some(trait_id) = self.tcx.trait_of_item(def_id) {
+            let trait_ref = ty::Binder(ty::TraitRef::new(trait_id, substs));
+            let vtable = self.fulfill_obligation(trait_ref);
+            if let traits::VtableImpl(vtable_impl) = vtable {
+                let name = self.tcx.item_name(def_id);
+                let assoc_const_opt = self.tcx.associated_items(vtable_impl.impl_def_id)
+                    .find(|item| item.kind == ty::AssociatedKind::Const && item.name == name);
+                if let Some(assoc_const) = assoc_const_opt {
+                    return (assoc_const.def_id, vtable_impl.substs);
+                }
+            }
+        }
+        (def_id, substs)
+    }
 }