diff --git a/src/comp/metadata/tydecode.rs b/src/comp/metadata/tydecode.rs
index 250522dda4f..0e1a01b0e15 100644
--- a/src/comp/metadata/tydecode.rs
+++ b/src/comp/metadata/tydecode.rs
@@ -216,7 +216,7 @@ fn parse_ty(st: @pstate, sd: str_def) -> ty::t {
         ret ty::mk_param(st.tcx, parse_int(st) as uint, k);
       }
       '@' { ret ty::mk_box(st.tcx, parse_mt(st, sd)); }
-      '~' { ret ty::mk_uniq(st.tcx, parse_ty(st, sd)); }
+      '~' { ret ty::mk_uniq(st.tcx, parse_mt(st, sd)); }
       '*' { ret ty::mk_ptr(st.tcx, parse_mt(st, sd)); }
       'I' { ret ty::mk_vec(st.tcx, parse_mt(st, sd)); }
       'R' {
diff --git a/src/comp/metadata/tyencode.rs b/src/comp/metadata/tyencode.rs
index 99f171907de..7c3c396ec35 100644
--- a/src/comp/metadata/tyencode.rs
+++ b/src/comp/metadata/tyencode.rs
@@ -123,7 +123,7 @@ fn enc_sty(w: io::writer, cx: @ctxt, st: ty::sty) {
         w.write_char(']');
       }
       ty::ty_box(mt) { w.write_char('@'); enc_mt(w, cx, mt); }
-      ty::ty_uniq(t) { w.write_char('~'); enc_ty(w, cx, t); }
+      ty::ty_uniq(mt) { w.write_char('~'); enc_mt(w, cx, mt); }
       ty::ty_ptr(mt) { w.write_char('*'); enc_mt(w, cx, mt); }
       ty::ty_vec(mt) { w.write_char('I'); enc_mt(w, cx, mt); }
       ty::ty_rec(fields) {
diff --git a/src/comp/middle/alias.rs b/src/comp/middle/alias.rs
index f1788fda27e..2353aebb50f 100644
--- a/src/comp/middle/alias.rs
+++ b/src/comp/middle/alias.rs
@@ -559,10 +559,9 @@ fn ty_can_unsafely_include(cx: ctx, needle: unsafe_ty, haystack: ty::t,
             }
             ret false;
           }
-          ty::ty_box(mt) | ty::ty_ptr(mt) {
+          ty::ty_box(mt) | ty::ty_ptr(mt) | ty::ty_uniq(mt) {
             ret helper(tcx, needle, mt.ty, get_mut(mut, mt));
           }
-          ty::ty_uniq(t) { ret helper(tcx, needle, t, false); }
           ty::ty_rec(fields) {
             for f: ty::field in fields {
                 if helper(tcx, needle, f.mt.ty, get_mut(mut, f.mt)) {
@@ -619,7 +618,7 @@ fn copy_is_expensive(tcx: ty::ctxt, ty: ty::t) -> bool {
           ty::ty_fn(_, _, _, _, _) | ty::ty_native_fn(_, _, _) |
           ty::ty_obj(_) { 4u }
           ty::ty_str. | ty::ty_vec(_) | ty::ty_param(_, _) { 50u }
-          ty::ty_uniq(t) { 1u + score_ty(tcx, t) }
+          ty::ty_uniq(mt) { 1u + score_ty(tcx, mt.ty) }
           ty::ty_tag(_, ts) | ty::ty_tup(ts) {
             let sum = 0u;
             for t in ts { sum += score_ty(tcx, t); }
diff --git a/src/comp/middle/mut.rs b/src/comp/middle/mut.rs
index 8974509a8a9..6ac17db4209 100644
--- a/src/comp/middle/mut.rs
+++ b/src/comp/middle/mut.rs
@@ -23,7 +23,7 @@ fn expr_root(tcx: ty::ctxt, ex: @expr, autoderef: bool) ->
                 t = mt.ty;
               }
               ty::ty_uniq(mt) {
-                ds += [@{mut: false, kind: unbox, outer_t: t}];
+                ds += [@{mut: mt.mut != imm, kind: unbox, outer_t: t}];
               }
               ty::ty_res(_, inner, tps) {
                 ds += [@{mut: false, kind: unbox, outer_t: t}];
diff --git a/src/comp/middle/shape.rs b/src/comp/middle/shape.rs
index 5a213e31ca5..2f4718223c4 100644
--- a/src/comp/middle/shape.rs
+++ b/src/comp/middle/shape.rs
@@ -370,9 +370,9 @@ fn shape_of(ccx: @crate_ctxt, t: ty::t, ty_param_map: [uint]) -> [u8] {
         s += [shape_box];
         add_substr(s, shape_of(ccx, mt.ty, ty_param_map));
       }
-      ty::ty_uniq(subt) {
+      ty::ty_uniq(mt) {
         s += [shape_uniq];
-        add_substr(s, shape_of(ccx, subt, ty_param_map));
+        add_substr(s, shape_of(ccx, mt.ty, ty_param_map));
       }
       ty::ty_vec(mt) {
         s += [shape_vec];
diff --git a/src/comp/middle/trans.rs b/src/comp/middle/trans.rs
index bb3c399dda4..3f29c757aaf 100644
--- a/src/comp/middle/trans.rs
+++ b/src/comp/middle/trans.rs
@@ -162,9 +162,10 @@ fn type_of_inner(cx: @crate_ctxt, sp: span, t: ty::t)
         let mt_ty = mt.ty;
         check non_ty_var(cx, mt_ty);
         T_ptr(T_box(type_of_inner(cx, sp, mt_ty))) }
-      ty::ty_uniq(t) {
-        check non_ty_var(cx, t);
-        T_ptr(type_of_inner(cx, sp, t)) }
+      ty::ty_uniq(mt) {
+        let mt_ty = mt.ty;
+        check non_ty_var(cx, mt_ty);
+        T_ptr(type_of_inner(cx, sp, mt_ty)) }
       ty::ty_vec(mt) {
         let mt_ty = mt.ty;
         if ty::type_has_dynamic_size(cx.tcx, mt_ty) {
@@ -478,7 +479,9 @@ fn simplify_type(ccx: @crate_ctxt, typ: ty::t) -> ty::t {
     fn simplifier(ccx: @crate_ctxt, typ: ty::t) -> ty::t {
         alt ty::struct(ccx.tcx, typ) {
           ty::ty_box(_) { ret ty::mk_imm_box(ccx.tcx, ty::mk_nil(ccx.tcx)); }
-          ty::ty_uniq(_) { ret ty::mk_uniq(ccx.tcx, ty::mk_nil(ccx.tcx)); }
+          ty::ty_uniq(_) {
+            ret ty::mk_imm_uniq(ccx.tcx, ty::mk_nil(ccx.tcx));
+          }
           ty::ty_fn(_, _, _, _, _) {
             ret ty::mk_tup(ccx.tcx,
                            [ty::mk_imm_box(ccx.tcx, ty::mk_nil(ccx.tcx)),
@@ -1313,7 +1316,7 @@ fn make_free_glue(bcx: @block_ctxt, v0: ValueRef, t: ty::t) {
                 trans_non_gc_free(bcx, v)
             } else { bcx }
           }
-          ty::ty_uniq(content_t) {
+          ty::ty_uniq(content_mt) {
             let free_cx = new_sub_block_ctxt(bcx, "uniq_free");
             let next_cx = new_sub_block_ctxt(bcx, "uniq_free_next");
             let vptr = Load(bcx, v0);
@@ -1321,7 +1324,7 @@ fn make_free_glue(bcx: @block_ctxt, v0: ValueRef, t: ty::t) {
             CondBr(bcx, null_test, next_cx.llbb, free_cx.llbb);
 
             let bcx = free_cx;
-            let bcx = drop_ty(bcx, vptr, content_t);
+            let bcx = drop_ty(bcx, vptr, content_mt.ty);
             let bcx = trans_shared_free(bcx, vptr);
             Store(bcx, C_null(val_ty(vptr)), v0);
             Br(bcx, next_cx.llbb);
diff --git a/src/comp/middle/ty.rs b/src/comp/middle/ty.rs
index 56f9ab3d7f9..0ed7fd66935 100644
--- a/src/comp/middle/ty.rs
+++ b/src/comp/middle/ty.rs
@@ -65,6 +65,7 @@ export mk_ctxt;
 export mk_float;
 export mk_fn;
 export mk_imm_box;
+export mk_imm_uniq;
 export mk_mut_ptr;
 export mk_int;
 export mk_str;
@@ -251,7 +252,7 @@ tag sty {
     ty_str;
     ty_tag(def_id, [t]);
     ty_box(mt);
-    ty_uniq(t);
+    ty_uniq(mt);
     ty_vec(mt);
     ty_ptr(mt);
     ty_rec([field]);
@@ -448,7 +449,7 @@ fn mk_raw_ty(cx: ctxt, st: sty, _in_cname: option::t<str>) -> @raw_t {
         for tt: t in tys { derive_flags_t(cx, has_params, has_vars, tt); }
       }
       ty_box(m) { derive_flags_mt(cx, has_params, has_vars, m); }
-      ty_uniq(tt) { derive_flags_t(cx, has_params, has_vars, tt); }
+      ty_uniq(m) { derive_flags_mt(cx, has_params, has_vars, m); }
       ty_vec(m) { derive_flags_mt(cx, has_params, has_vars, m); }
       ty_ptr(m) { derive_flags_mt(cx, has_params, has_vars, m); }
       ty_rec(flds) {
@@ -534,7 +535,11 @@ fn mk_tag(cx: ctxt, did: ast::def_id, tys: [t]) -> t {
 
 fn mk_box(cx: ctxt, tm: mt) -> t { ret gen_ty(cx, ty_box(tm)); }
 
-fn mk_uniq(cx: ctxt, typ: t) -> t { ret gen_ty(cx, ty_uniq(typ)); }
+fn mk_uniq(cx: ctxt, tm: mt) -> t { ret gen_ty(cx, ty_uniq(tm)); }
+
+fn mk_imm_uniq(cx: ctxt, ty: t) -> t {
+    ret mk_uniq(cx, {ty: ty, mut: ast::imm});
+}
 
 fn mk_ptr(cx: ctxt, tm: mt) -> t { ret gen_ty(cx, ty_ptr(tm)); }
 
@@ -643,7 +648,7 @@ fn walk_ty(cx: ctxt, walker: ty_walk, ty: t) {
       ty_constr(sub, _) { walk_ty(cx, walker, sub); }
       ty_var(_) {/* no-op */ }
       ty_param(_, _) {/* no-op */ }
-      ty_uniq(sub) { walk_ty(cx, walker, sub); }
+      ty_uniq(tm) { walk_ty(cx, walker, tm.ty); }
     }
     walker(ty);
 }
@@ -678,7 +683,9 @@ fn fold_ty(cx: ctxt, fld: fold_mode, ty_0: t) -> t {
       ty_box(tm) {
         ty = mk_box(cx, {ty: fold_ty(cx, fld, tm.ty), mut: tm.mut});
       }
-      ty_uniq(subty) { ty = mk_uniq(cx, fold_ty(cx, fld, subty)); }
+      ty_uniq(tm) {
+        ty = mk_uniq(cx, {ty: fold_ty(cx, fld, tm.ty), mut: tm.mut});
+      }
       ty_ptr(tm) {
         ty = mk_ptr(cx, {ty: fold_ty(cx, fld, tm.ty), mut: tm.mut});
       }
@@ -1420,7 +1427,7 @@ fn hash_type_structure(st: sty) -> uint {
         for c: @type_constr in cs { h += h << 5u + hash_type_constr(h, c); }
         ret h;
       }
-      ty_uniq(t) { let h = 37u; h += h << 5u + hash_ty(t); ret h; }
+      ty_uniq(mt) { let h = 37u; h += h << 5u + hash_ty(mt.ty); ret h; }
     }
 }
 
@@ -2184,13 +2191,20 @@ mod unify {
               _ { ret ures_err(terr_mismatch); }
             }
           }
-          ty::ty_uniq(expected_sub) {
+          ty::ty_uniq(expected_mt) {
             alt struct(cx.tcx, actual) {
-              ty::ty_uniq(actual_sub) {
-                let result = unify_step(cx, expected_sub, actual_sub);
+              ty::ty_uniq(actual_mt) {
+                let mut = expected_mt.mut;
+                // FIXME (409) Write a test then uncomment
+                /*alt unify_mut(expected_mt.mut, actual_mt.mut) {
+                  none. { ret ures_err(terr_box_mutability); }
+                  some(m) { mut = m; }
+                }*/
+                let result = unify_step(cx, expected_mt.ty, actual_mt.ty);
                 alt result {
-                  ures_ok(result_sub) {
-                    ret ures_ok(mk_uniq(cx.tcx, result_sub));
+                  ures_ok(result_mt) {
+                    let mt = {ty: result_mt, mut: mut};
+                    ret ures_ok(mk_uniq(cx.tcx, mt));
                   }
                   _ { ret result; }
                 }
diff --git a/src/comp/middle/typeck.rs b/src/comp/middle/typeck.rs
index 758883a1aa0..88035da6c62 100644
--- a/src/comp/middle/typeck.rs
+++ b/src/comp/middle/typeck.rs
@@ -286,7 +286,7 @@ fn ast_ty_to_ty(tcx: ty::ctxt, getter: ty_getter, ast_ty: @ast::ty) -> ty::t {
         typ = ty::mk_box(tcx, ast_mt_to_mt(tcx, getter, mt));
       }
       ast::ty_uniq(mt) {
-        typ = ty::mk_uniq(tcx, ast_ty_to_ty(tcx, getter, mt.ty));
+        typ = ty::mk_uniq(tcx, ast_mt_to_mt(tcx, getter, mt));
       }
       ast::ty_vec(mt) {
         typ = ty::mk_vec(tcx, ast_mt_to_mt(tcx, getter, mt));
@@ -1720,12 +1720,12 @@ fn check_expr_with_unifier(fcx: @fn_ctxt, expr: @ast::expr, unify: unifier,
         alt unop {
           ast::box(mut) { oper_t = ty::mk_box(tcx, {ty: oper_t, mut: mut}); }
           ast::uniq(mut) {
-            oper_t = ty::mk_uniq(tcx, oper_t);
+            oper_t = ty::mk_uniq(tcx, {ty: oper_t, mut: mut});
           }
           ast::deref. {
             alt structure_of(fcx, expr.span, oper_t) {
               ty::ty_box(inner) { oper_t = inner.ty; }
-              ty::ty_uniq(inner) { oper_t = inner; }
+              ty::ty_uniq(inner) { oper_t = inner.ty; }
               ty::ty_res(_, inner, _) { oper_t = inner; }
               ty::ty_tag(id, tps) {
                 let variants = ty::tag_variants(tcx, id);
diff --git a/src/comp/util/ppaux.rs b/src/comp/util/ppaux.rs
index 44cf92ea1b9..11c50affd28 100644
--- a/src/comp/util/ppaux.rs
+++ b/src/comp/util/ppaux.rs
@@ -101,7 +101,7 @@ fn ty_to_str(cx: ctxt, typ: t) -> str {
           ty_char. { "char" }
           ty_str. { "str" }
           ty_box(tm) { "@" + mt_to_str(cx, tm) }
-          ty_uniq(t) { "~" + ty_to_str(cx, t) }
+          ty_uniq(tm) { "~" + mt_to_str(cx, tm) }
           ty_vec(tm) { "[" + mt_to_str(cx, tm) + "]" }
           ty_type. { "type" }
           ty_rec(elems) {