diff --git a/src/comp/middle/visit.rs b/src/comp/middle/visit.rs
new file mode 100644
index 0000000000000000000000000000000000000000..79f74c8f3e890111fb6ca087bd073db40421497f
--- /dev/null
+++ b/src/comp/middle/visit.rs
@@ -0,0 +1,381 @@
+import front::ast::*;
+import std::option;
+import std::option::some;
+import std::option::none;
+import util::common::span;
+
+// Lots of redundant indirection and refcounting. Our typesystem doesn't do
+// circular types, so the visitor record can not hold functions that take
+// visitors. A tag breaks the cycle.
+tag vt[E] { vtor(visitor[E]); }
+fn vt[E](&vt[E] x) -> visitor[E] {
+    alt (x) { case (vtor(?v)) { ret v; } }
+}
+
+type visitor[E] =
+    @rec(fn(&_mod m, &span sp, &E e, &vt[E] v) visit_mod,
+         fn(&@native_item i, &E e, &vt[E] v)   visit_native_item,
+         fn(&@item i, &E e, &vt[E] v)          visit_item,
+         fn(&block b, &E e, &vt[E] v)          visit_block,
+         fn(&@stmt s, &E e, &vt[E] v)          visit_stmt,
+         fn(&arm a, &E e, &vt[E] v)            visit_arm,
+         fn(&@pat p, &E e, &vt[E] v)           visit_pat,
+         fn(&@decl d, &E e, &vt[E] v)          visit_decl,
+         fn(&@expr ex, &E e, &vt[E] v)         visit_expr,
+         fn(&@ty t, &E e, &vt[E] v)            visit_ty,
+         fn(&_fn f, &span sp, &ident name, 
+            &def_id d_id, &ann a,
+            &E e, &vt[E] v)                    visit_fn);
+
+fn default_visitor[E]() -> visitor[E] {
+    ret @rec(visit_mod = bind visit_mod[E](_, _, _, _),
+             visit_native_item = bind visit_native_item[E](_, _, _),
+             visit_item = bind visit_item[E](_, _, _),
+             visit_block = bind visit_block[E](_, _, _),
+             visit_stmt = bind visit_stmt[E](_, _, _),
+             visit_arm = bind visit_arm[E](_, _, _),
+             visit_pat = bind visit_pat[E](_, _, _),
+             visit_decl = bind visit_decl[E](_, _, _),
+             visit_expr = bind visit_expr[E](_, _, _),
+             visit_ty = bind visit_ty[E](_, _, _),
+             visit_fn = bind visit_fn[E](_, _, _, _, _, _, _));
+}
+
+fn visit_crate[E](&crate c, &E e, &vt[E] v) {
+    vt(v).visit_mod(c.node.module, c.span, e, v);
+}
+
+fn visit_mod[E](&_mod m, &span sp, &E e, &vt[E] v) {
+    for (@item i in m.items) {
+        vt(v).visit_item(i, e, v);
+    }
+}
+
+fn visit_item[E](&@item i, &E e, &vt[E] v) {
+    alt (i.node) {
+        case (item_const(_, ?t, ?ex, _, _)) {
+            vt(v).visit_ty(t, e, v);
+            vt(v).visit_expr(ex, e, v);
+        }
+        case (item_fn(?nm, ?f, _, ?d, ?a)) {
+            vt(v).visit_fn(f, i.span, nm, d, a, e, v);
+        }
+        case (item_mod(_, ?m, _)) {
+            vt(v).visit_mod(m, i.span, e, v);
+        }
+        case (item_native_mod(_, ?nm, _)) {
+            for (@native_item ni in nm.items) {
+                vt(v).visit_native_item(ni, e, v);
+            }
+        }
+        case (item_ty(_, ?t, _, _, _)) {
+            vt(v).visit_ty(t, e, v);
+        }
+        case (item_tag(_, ?variants, _, _, _)) {
+            for (variant vr in variants) {
+                for (variant_arg va in vr.node.args) {
+                    vt(v).visit_ty(va.ty, e, v);
+                }
+            }
+        }
+        case (item_obj(_, ?ob, _, _, _)) {
+            for (obj_field f in ob.fields) {
+                vt(v).visit_ty(f.ty, e, v);
+            }
+            for (@method m in ob.methods) {
+                vt(v).visit_fn(m.node.meth, m.span, m.node.ident, m.node.id,
+                           m.node.ann, e, v);
+            }
+            alt (ob.dtor) {
+                case (none) {}
+                case (some(?m)) {
+                    vt(v).visit_fn(m.node.meth, m.span, m.node.ident,
+                                   m.node.id, m.node.ann, e, v);
+                }
+            }
+        }
+
+    }
+}
+
+fn visit_ty[E](&@ty t, &E e, &vt[E] v) {
+    alt (t.node) {
+        case (ty_box(?mt)) { vt(v).visit_ty(mt.ty, e, v); }
+        case (ty_vec(?mt)) { vt(v).visit_ty(mt.ty, e, v); }
+        case (ty_ptr(?mt)) { vt(v).visit_ty(mt.ty, e, v); }
+        case (ty_port(?t)) { vt(v).visit_ty(t, e, v); }
+        case (ty_chan(?t)) { vt(v).visit_ty(t, e, v); }
+        case (ty_tup(?mts)) {
+            for (mt mt in mts) {
+                vt(v).visit_ty(mt.ty, e, v);
+            }
+        }
+        case (ty_rec(?flds)) {
+            for (ty_field f in flds) {
+                vt(v).visit_ty(f.node.mt.ty, e, v);
+            }
+        }
+        case (ty_fn(_, ?args, ?out, _)) {
+            for (ty_arg a in args) {
+                vt(v).visit_ty(a.node.ty, e, v);
+            }
+            vt(v).visit_ty(out, e, v);
+        }
+        case (ty_obj(?tmeths)) {
+            for (ty_method m in tmeths) {
+                for (ty_arg a in m.node.inputs) {
+                    vt(v).visit_ty(a.node.ty, e, v);
+                }
+                vt(v).visit_ty(m.node.output, e, v);
+            }
+        }
+        case (ty_path(?p, _)) {
+            for (@ty tp in p.node.types) {
+                vt(v).visit_ty(tp, e, v);
+            }
+        }
+        case (ty_constr(?t, _)) { vt(v).visit_ty(t, e, v); }
+        case (_) {}
+    }
+}
+
+fn visit_pat[E](&@pat p, &E e, &vt[E] v) {
+    alt (p.node) {
+        case (pat_tag(?path, ?children, _)) {
+            for (@pat child in children) {
+                vt(v).visit_pat(child, e, v);
+            }
+        }
+        case (_) {}
+    }
+}
+
+fn visit_native_item[E](&@native_item ni, &E e, &vt[E] v) {
+    alt (ni.node) {
+        case (native_item_fn(_, _, ?fd, _, _, _)) {
+            visit_fn_decl(fd, e, v);
+        }
+        case (native_item_ty(_, _)) {}
+    }
+}
+
+fn visit_fn_decl[E](&fn_decl fd, &E e, &vt[E] v) {
+    for (arg a in fd.inputs) {
+        vt(v).visit_ty(a.ty, e, v);
+    }
+    vt(v).visit_ty(fd.output, e, v);
+}
+
+fn visit_fn[E](&_fn f, &span sp, &ident i, &def_id d, &ann a,
+               &E e, &vt[E] v) {
+    visit_fn_decl(f.decl, e, v);
+    vt(v).visit_block(f.body, e, v);
+}
+
+fn visit_block[E](&block b, &E e, &vt[E] v) {
+    for (@stmt s in b.node.stmts) { vt(v).visit_stmt(s, e, v); }
+    visit_expr_opt(b.node.expr, e, v);
+}
+
+fn visit_stmt[E](&@stmt s, &E e, &vt[E] v) {
+    alt (s.node) {
+        case (stmt_decl(?d, _)) { vt(v).visit_decl(d, e, v); }
+        case (stmt_expr(?ex, _)) { vt(v).visit_expr(ex, e, v); }
+        case (stmt_crate_directive(?cdir)) {}
+    }
+}
+
+fn visit_decl[E](&@decl d, &E e, &vt[E] v) {
+    alt (d.node) {
+        case (decl_local(?loc)) {
+            alt (loc.ty) {
+                case (none) {}
+                case (some(?t)) { vt(v).visit_ty(t, e, v); }
+            }
+            alt (loc.init) {
+                case (none) {}
+                case (some(?i)) { vt(v).visit_expr(i.expr, e, v); }
+            }
+        }
+        case (decl_item(?it)) { vt(v).visit_item(it, e, v); }
+    }
+}
+
+fn visit_expr_opt[E](option::t[@expr] eo, &E e, &vt[E] v) {
+    alt (eo) {
+        case (none) {}
+        case (some(?ex)) { vt(v).visit_expr(ex, e, v);
+        }
+    }
+}
+
+fn visit_exprs[E](vec[@expr] exprs, &E e, &vt[E] v) {
+    for (@expr ex in exprs) { vt(v).visit_expr(ex, e, v); }
+}
+
+fn visit_expr[E](&@expr ex, &E e, &vt[E] v) {
+    alt (ex.node) {
+        case (expr_vec(?es, _, _)) {
+            visit_exprs(es, e, v);
+        }
+        case (expr_tup(?elts, _)) {
+            for (elt el in elts) { vt(v).visit_expr(el.expr, e, v); }
+        }
+        case (expr_rec(?flds, ?base, _)) {
+            for (field f in flds) { vt(v).visit_expr(f.node.expr, e, v); }
+            visit_expr_opt(base, e, v);
+        }
+        case (expr_call(?callee, ?args, _)) {
+            vt(v).visit_expr(callee, e, v);
+            visit_exprs(args, e, v);
+        }
+        case (expr_self_method(_, _)) { }
+        case (expr_bind(?callee, ?args, _)) {
+            vt(v).visit_expr(callee, e, v);
+            for (option::t[@expr] eo in args) { visit_expr_opt(eo, e, v); }
+        }
+        case (expr_spawn(_, _, ?callee, ?args, _)) {
+            vt(v).visit_expr(callee, e, v);
+            visit_exprs(args, e, v);
+        }
+        case (expr_binary(_, ?a, ?b, _)) {
+            vt(v).visit_expr(a, e, v);
+            vt(v).visit_expr(b, e, v);
+        }
+        case (expr_unary(_, ?a, _)) {
+            vt(v).visit_expr(a, e, v);
+        }
+        case (expr_lit(_, _)) { }
+        case (expr_cast(?x, ?t, _)) {
+            vt(v).visit_expr(x, e, v);
+            vt(v).visit_ty(t, e, v);
+        }
+        case (expr_if(?x, ?b, ?eo, _)) {
+            vt(v).visit_expr(x, e, v);
+            vt(v).visit_block(b, e, v);
+            visit_expr_opt(eo, e, v);
+        }
+        case (expr_while(?x, ?b, _)) {
+            vt(v).visit_expr(x, e, v);
+            vt(v).visit_block(b, e, v);
+        }
+        case (expr_for(?dcl, ?x, ?b, _)) {
+            vt(v).visit_decl(dcl, e, v);
+            vt(v).visit_expr(x, e, v);
+            vt(v).visit_block(b, e, v);
+        }
+        case (expr_for_each(?dcl, ?x, ?b, _)) {
+            vt(v).visit_decl(dcl, e, v);
+            vt(v).visit_expr(x, e, v);
+            vt(v).visit_block(b, e, v);
+        }
+        case (expr_do_while(?b, ?x, _)) {
+            vt(v).visit_block(b, e, v);
+            vt(v).visit_expr(x, e, v);
+        }
+        case (expr_alt(?x, ?arms, _)) {
+            vt(v).visit_expr(x, e, v);
+            for (arm a in arms) {
+                vt(v).visit_arm(a, e, v);
+            }
+        }
+        case (expr_block(?b, _)) {
+            vt(v).visit_block(b, e, v);
+        }
+        case (expr_assign(?a, ?b, _)) {
+            vt(v).visit_expr(b, e, v);
+            vt(v).visit_expr(a, e, v);
+        }
+        case (expr_move(?a, ?b, _)) {
+            vt(v).visit_expr(b, e, v);
+            vt(v).visit_expr(a, e, v);
+        }
+        case (expr_assign_op(_, ?a, ?b, _)) {
+            vt(v).visit_expr(b, e, v);
+            vt(v).visit_expr(a, e, v);
+        }
+        case (expr_send(?a, ?b, _)) {
+            vt(v).visit_expr(a, e, v);
+            vt(v).visit_expr(b, e, v);
+        }
+        case (expr_recv(?a, ?b, _)) {
+            vt(v).visit_expr(a, e, v);
+            vt(v).visit_expr(b, e, v);
+        }
+        case (expr_field(?x, _, _)) {
+            vt(v).visit_expr(x, e, v);
+        }
+        case (expr_index(?a, ?b, _)) {
+            vt(v).visit_expr(a, e, v);
+            vt(v).visit_expr(b, e, v);
+        }
+        case (expr_path(?p, _)) {
+            for (@ty tp in p.node.types) {
+                vt(v).visit_ty(tp, e, v);
+            }
+        }
+        case (expr_ext(_, _, _, ?expansion, _)) {
+            vt(v).visit_expr(expansion, e, v);
+        }
+        case (expr_fail(_, _)) { }
+        case (expr_break(_)) { }
+        case (expr_cont(_)) { }
+        case (expr_ret(?eo, _)) {
+            visit_expr_opt(eo, e, v);
+        }
+        case (expr_put(?eo, _)) {
+            visit_expr_opt(eo, e, v);
+        }
+        case (expr_be(?x, _)) {
+            vt(v).visit_expr(x, e, v);
+        }
+        case (expr_log(_,?x, _)) {
+            vt(v).visit_expr(x, e, v);
+        }
+        case (expr_check(?x, _)) {
+            vt(v).visit_expr(x, e, v);
+        }
+        case (expr_assert(?x, _)) {
+            vt(v).visit_expr(x, e, v);
+        }
+        case (expr_port(_)) { }
+        case (expr_chan(?x, _)) {
+            vt(v).visit_expr(x, e, v);
+        }
+
+        case (expr_anon_obj(?anon_obj,_,_,_)) { 
+            alt (anon_obj.fields) {
+                case (none) { }
+                case (some(?fields)) {
+                    for (obj_field f in fields) {
+                        vt(v).visit_ty(f.ty, e, v);
+                    }
+                }
+            }
+            alt (anon_obj.with_obj) {
+                case (none) { }
+                case (some(?ex)) {
+                    vt(v).visit_expr(ex, e, v);
+                }
+            }
+            for (@method m in anon_obj.methods) {
+                vt(v).visit_fn(m.node.meth, m.span, m.node.ident, 
+                           m.node.id, m.node.ann, e, v);
+            }
+        }
+    }
+}
+
+fn visit_arm[E](&arm a, &E e, &vt[E] v) {
+    vt(v).visit_pat(a.pat, e, v);
+    vt(v).visit_block(a.block, e, v);
+}
+
+// Local Variables:
+// mode: rust
+// fill-column: 78;
+// indent-tabs-mode: nil
+// c-basic-offset: 4
+// buffer-file-coding-system: utf-8-unix
+// compile-command: "make -k -C $RBUILD 2>&1 | sed -e 's/\\/x\\//x:\\//g'";
+// End:
diff --git a/src/comp/rustc.rc b/src/comp/rustc.rc
index e082222fd46923f89c4331396723695a4efb56c2..1b323182cf0cddfaa7cc29b77560a2c352c3eef2 100644
--- a/src/comp/rustc.rc
+++ b/src/comp/rustc.rc
@@ -16,6 +16,7 @@ mod middle {
     mod trans;
     mod ty;
     mod walk;
+    mod visit;
     mod metadata;
     mod resolve;
     mod typeck;
diff --git a/src/test/compile-fail/unsafe-for.rs b/src/test/compile-fail/unsafe-for.rs
new file mode 100644
index 0000000000000000000000000000000000000000..bc050173bc0df32c193948cc29d0c333541b9265
--- /dev/null
+++ b/src/test/compile-fail/unsafe-for.rs
@@ -0,0 +1,9 @@
+// error-pattern:invalidate alias x
+
+fn main() {
+    let vec[mutable int] v = [mutable 1, 2, 3];
+    for (int x in v) {
+        v.(0) = 10;
+        log x;
+    }
+}