diff --git a/src/boot/me/effect.ml b/src/boot/me/effect.ml
index be0c5af222028..c55e1d12b30d7 100644
--- a/src/boot/me/effect.ml
+++ b/src/boot/me/effect.ml
@@ -51,7 +51,7 @@ let mutability_checking_visitor
       if (is_mutable or is_init)
       then ()
       else err (Some s.id)
-        "writing to non-mutable slot of type %a in statement %a"
+        "writing to immutable type %a in statement %a"
         Ast.sprintf_ty dst_ty Ast.sprintf_stmt s
   in
     (* FIXME (issue #75): enforce the no-write-alias-to-immutable-slot
diff --git a/src/boot/me/type.ml b/src/boot/me/type.ml
index 9d74959a7ac69..7943e88bb80f0 100644
--- a/src/boot/me/type.ml
+++ b/src/boot/me/type.ml
@@ -45,7 +45,7 @@ let check_stmt (cx:Semant.ctxt) : (fn_ctx -> Ast.stmt -> unit) =
 
   let get_slot_ty (slot:Ast.slot) : Ast.ty =
     match slot.Ast.slot_ty with
-        Some ty -> ty 
+        Some ty -> ty
       | None -> Common.bug () "get_slot_ty: no type in slot"
   in
 
@@ -62,7 +62,11 @@ let check_stmt (cx:Semant.ctxt) : (fn_ctx -> Ast.stmt -> unit) =
   in
 
   let maybe_mutable (mutability:Ast.mutability) (ty:Ast.ty) : Ast.ty =
-    if mutability = Ast.MUT_mutable then Ast.TY_mutable ty else ty
+    let res =
+      if mutability = Ast.MUT_mutable then Ast.TY_mutable ty else ty
+    in
+      log cx "maybe_mutable: %a -> %a" Ast.sprintf_ty ty Ast.sprintf_ty res;
+      res
   in
 
   (*
@@ -229,7 +233,7 @@ let check_stmt (cx:Semant.ctxt) : (fn_ctx -> Ast.stmt -> unit) =
             ()
             "internal_check_slot: supplied defn wasn't a slot at all"
     in
-    match infer, slot.Ast.slot_ty with 
+    match infer, slot.Ast.slot_ty with
         Some expected, Some actual ->
           demand expected actual;
           actual
@@ -301,6 +305,10 @@ let check_stmt (cx:Semant.ctxt) : (fn_ctx -> Ast.stmt -> unit) =
         | `Module items -> Ast.sprintf_mod_items chan items
     in
 
+    let _ = log cx "base lval %a, base type %a"
+      Ast.sprintf_lval base sprintf_itype ()
+    in
+
     let rec typecheck base_ity =
       match base_ity, comp with
           `Type (Ast.TY_rec ty_rec), Ast.COMP_named (Ast.COMP_ident id) ->
@@ -455,14 +463,14 @@ let check_stmt (cx:Semant.ctxt) : (fn_ctx -> Ast.stmt -> unit) =
       : (Ast.ty * int) =
     let yield_ty ty =
       let (ty, n_boxes) = if deref then unbox ty else (ty, 0) in
-      (maybe_mutable mut ty, n_boxes)
+        (maybe_mutable mut ty, n_boxes)
     in
     match infer, internal_check_lval infer lval with
       | None, LTYPE_mono ty -> yield_ty ty
       | Some expected, LTYPE_mono actual ->
           demand expected actual;
           yield_ty actual
-      | None, (LTYPE_poly _ as lty) -> 
+      | None, (LTYPE_poly _ as lty) ->
           Common.err
             None
             "not enough context to automatically instantiate the polymorphic \
@@ -487,9 +495,21 @@ let check_stmt (cx:Semant.ctxt) : (fn_ctx -> Ast.stmt -> unit) =
      * Get the real one. *)
     let lval_id = Semant.lval_base_id lval in
     let lval = Hashtbl.find cx.Semant.ctxt_all_lvals lval_id in
+    let _ = log cx "generic_check_lval %a mut=%s deref=%s infer=%s"
+      Ast.sprintf_lval lval
+      (if mut = Ast.MUT_mutable then "mutable" else "immutable")
+      (if deref then "true" else "false")
+      (match infer with
+           None -> "<none>"
+         | Some t -> Fmt.fmt_to_str Ast.fmt_ty t)
+    in
     let (lval_ty, n_boxes) =
       internal_check_outer_lval ~mut:mut ~deref:deref infer lval
     in
+    let _ = log cx "checked lval %a with type %a"
+      Ast.sprintf_lval lval
+      Ast.sprintf_ty lval_ty
+    in
 
     if Hashtbl.mem cx.Semant.ctxt_all_lval_types lval_id then
       assert ((Hashtbl.find cx.Semant.ctxt_all_lval_types lval_id) = lval_ty)
@@ -514,8 +534,8 @@ let check_stmt (cx:Semant.ctxt) : (fn_ctx -> Ast.stmt -> unit) =
         implemented; please add explicit dereference operators";
     Hashtbl.replace cx.Semant.ctxt_auto_deref_lval lval_id (n_boxes > 0);
 
-    (* Before demoting the lval to a value, strip off mutability. *)
-    fundamental_ty lval_ty
+      (* Before demoting the lval to a value, strip off mutability. *)
+      fundamental_ty lval_ty
 
   (* Note that this function should be avoided when possible, because it
    * cannot perform type inference. In general you should prefer
@@ -538,11 +558,12 @@ let check_stmt (cx:Semant.ctxt) : (fn_ctx -> Ast.stmt -> unit) =
   in
 
   let infer_lval
-      ?mut:(mut=Ast.MUT_mutable)
+      ?mut:(mut=Ast.MUT_immutable)
       (ty:Ast.ty)
       (lval:Ast.lval)
       : unit =
-    ignore (generic_check_lval ?mut:mut ~deref:false (Some ty) lval)
+    ignore (generic_check_lval ?mut:mut ~deref:false
+              (Some (Ast.TY_mutable ty)) lval)
   in
 
   (*
@@ -574,7 +595,7 @@ let check_stmt (cx:Semant.ctxt) : (fn_ctx -> Ast.stmt -> unit) =
       | Ast.EXPR_binary (binop, lhs, rhs) ->
           let operand_ty = check_atom ~deref:true lhs in
           demand operand_ty (check_atom ~deref:true rhs);
-          check_binop binop operand_ty 
+          check_binop binop operand_ty
       | Ast.EXPR_unary (Ast.UNOP_not, atom) ->
           demand Ast.TY_bool (check_atom ~deref:true atom);
           Ast.TY_bool
@@ -596,7 +617,7 @@ let check_stmt (cx:Semant.ctxt) : (fn_ctx -> Ast.stmt -> unit) =
   let check_fn (callee:Ast.lval) (args:Ast.atom array) : Ast.ty =
     let arg_tys = Array.map check_atom args in
     let callee_ty = check_lval callee in
-    demand_fn (Array.map (fun ty -> Some ty) arg_tys) callee_ty 
+    demand_fn (Array.map (fun ty -> Some ty) arg_tys) callee_ty
   in
 
   let rec check_pat (expected:Ast.ty) (pat:Ast.pat) : unit =
diff --git a/src/test/compile-fail/writing-through-read-alias.rs b/src/test/compile-fail/writing-through-read-alias.rs
index b3d21521abf49..2a8ec11eaf3a7 100644
--- a/src/test/compile-fail/writing-through-read-alias.rs
+++ b/src/test/compile-fail/writing-through-read-alias.rs
@@ -1,6 +1,6 @@
 // -*- rust -*-
 
-// error-pattern: writing to non-mutable slot
+// error-pattern: writing to immutable type
 
 type point = rec(int x, int y, int z);
 
diff --git a/src/test/compile-fail/writing-to-immutable-obj.rs b/src/test/compile-fail/writing-to-immutable-obj.rs
new file mode 100644
index 0000000000000..ffa2cebe2fd80
--- /dev/null
+++ b/src/test/compile-fail/writing-to-immutable-obj.rs
@@ -0,0 +1,8 @@
+// error-pattern: writing to immutable type
+obj objy(int x) {
+    fn foo() -> () {
+        x = 5;
+    }
+}
+fn main() {
+}
diff --git a/src/test/compile-fail/writing-to-immutable-rec.rs b/src/test/compile-fail/writing-to-immutable-rec.rs
new file mode 100644
index 0000000000000..42206fe4124ef
--- /dev/null
+++ b/src/test/compile-fail/writing-to-immutable-rec.rs
@@ -0,0 +1,5 @@
+// error-pattern: writing to immutable type
+fn main() {
+  let rec(int x) r = rec(x=1);
+  r.x = 6;
+}
diff --git a/src/test/compile-fail/writing-to-immutable-tup.rs b/src/test/compile-fail/writing-to-immutable-tup.rs
new file mode 100644
index 0000000000000..bd974dcc786aa
--- /dev/null
+++ b/src/test/compile-fail/writing-to-immutable-tup.rs
@@ -0,0 +1,5 @@
+// error-pattern: writing to immutable type
+fn main() {
+  let tup(int) t = tup(1);
+  t._0 = 5;
+}
diff --git a/src/test/compile-fail/writing-to-immutable-vec.rs b/src/test/compile-fail/writing-to-immutable-vec.rs
new file mode 100644
index 0000000000000..341037c346c2a
--- /dev/null
+++ b/src/test/compile-fail/writing-to-immutable-vec.rs
@@ -0,0 +1,5 @@
+// error-pattern: writing to immutable type
+fn main() {
+  let vec[int] v = vec(1, 2, 3);
+  v.(1) = 4;
+}
\ No newline at end of file
diff --git a/src/test/run-pass/foreach-nested-2.rs b/src/test/run-pass/foreach-nested-2.rs
index 35487e7d92ee1..d8d67c141b0c1 100644
--- a/src/test/run-pass/foreach-nested-2.rs
+++ b/src/test/run-pass/foreach-nested-2.rs
@@ -14,7 +14,8 @@ iter range(int start, int stop) -> int {
 }
 
 fn main() {
-    let vec[int] a = vec(-1, -1, -1, -1, -1, -1, -1, -1);
+    let vec[mutable int] a =
+      vec[mutable](-1, -1, -1, -1, -1, -1, -1, -1);
     let int p = 0;
 
     for each (int i in two()) {
diff --git a/src/test/run-pass/foreach-nested.rs b/src/test/run-pass/foreach-nested.rs
index 848adb26e7087..6287477ab93f2 100644
--- a/src/test/run-pass/foreach-nested.rs
+++ b/src/test/run-pass/foreach-nested.rs
@@ -6,7 +6,7 @@ iter two() -> int {
 }
 
 fn main() {
-    let vec[int] a = vec(-1, -1, -1, -1);
+    let vec[mutable int] a = vec[mutable](-1, -1, -1, -1);
     let int p = 0;
 
     for each (int i in two()) {