diff --git a/src/librustc/middle/astencode.rs b/src/librustc/middle/astencode.rs
index 3a308011e75bb..e9441bfd4dfdc 100644
--- a/src/librustc/middle/astencode.rs
+++ b/src/librustc/middle/astencode.rs
@@ -355,7 +355,7 @@ impl ast::def: tr {
           ast::def_mod(did) => { ast::def_mod(did.tr(xcx)) }
           ast::def_foreign_mod(did) => { ast::def_foreign_mod(did.tr(xcx)) }
           ast::def_const(did) => { ast::def_const(did.tr(xcx)) }
-          ast::def_arg(nid, m) => { ast::def_arg(xcx.tr_id(nid), m) }
+          ast::def_arg(nid, m, b) => { ast::def_arg(xcx.tr_id(nid), m, b) }
           ast::def_local(nid, b) => { ast::def_local(xcx.tr_id(nid), b) }
           ast::def_variant(e_did, v_did) => {
             ast::def_variant(e_did.tr(xcx), v_did.tr(xcx))
diff --git a/src/librustc/middle/liveness.rs b/src/librustc/middle/liveness.rs
index da1f82a33b759..fbb0b421754f8 100644
--- a/src/librustc/middle/liveness.rs
+++ b/src/librustc/middle/liveness.rs
@@ -283,7 +283,7 @@ enum VarKind {
 fn relevant_def(def: def) -> Option<node_id> {
     match def {
       def_binding(nid, _) |
-      def_arg(nid, _) |
+      def_arg(nid, _, _) |
       def_local(nid, _) |
       def_self(nid, _) => Some(nid),
 
diff --git a/src/librustc/middle/mem_categorization.rs b/src/librustc/middle/mem_categorization.rs
index 04c57025157ab..b069c3cda98bc 100644
--- a/src/librustc/middle/mem_categorization.rs
+++ b/src/librustc/middle/mem_categorization.rs
@@ -583,18 +583,19 @@ impl &mem_categorization_ctxt {
               mutbl:m_imm, ty:expr_ty}
           }
 
-          ast::def_arg(vid, mode) => {
+          ast::def_arg(vid, mode, mutbl) => {
             // Idea: make this could be rewritten to model by-ref
             // stuff as `&const` and `&mut`?
 
             // m: mutability of the argument
             // lp: loan path, must be none for aliasable things
-            let {m,lp} = match ty::resolved_mode(self.tcx, mode) {
+            let m = if mutbl {m_mutbl} else {m_imm};
+            let lp = match ty::resolved_mode(self.tcx, mode) {
               ast::by_move | ast::by_copy => {
-                {m: m_imm, lp: Some(@lp_arg(vid))}
+                Some(@lp_arg(vid))
               }
               ast::by_ref => {
-                {m: m_imm, lp: None}
+                None
               }
               ast::by_val => {
                 // by-value is this hybrid mode where we have a
@@ -602,7 +603,7 @@ impl &mem_categorization_ctxt {
                 // considered loanable because, for example, a by-ref
                 // and and by-val argument might both actually contain
                 // the same unique ptr.
-                {m: m_imm, lp: None}
+                None
               }
             };
             @{id:id, span:span,
diff --git a/src/librustc/middle/resolve.rs b/src/librustc/middle/resolve.rs
index ffb696ec4f946..e2a2b8bd93d87 100644
--- a/src/librustc/middle/resolve.rs
+++ b/src/librustc/middle/resolve.rs
@@ -4154,9 +4154,11 @@ impl Resolver {
                     for declaration.inputs.each |argument| {
                         let binding_mode =
                             ArgumentIrrefutableMode(argument.mode);
+                        let mutability =
+                            if argument.is_mutbl {Mutable} else {Immutable};
                         self.resolve_pattern(argument.pat,
                                              binding_mode,
-                                             Immutable,
+                                             mutability,
                                              None,
                                              visitor);
 
@@ -4340,12 +4342,7 @@ impl Resolver {
     }
 
     fn resolve_local(local: @local, visitor: ResolveVisitor) {
-        let mut mutability;
-        if local.node.is_mutbl {
-            mutability = Mutable;
-        } else {
-            mutability = Immutable;
-        }
+        let mutability = if local.node.is_mutbl {Mutable} else {Immutable};
 
         // Resolve the type.
         self.resolve_type(local.node.ty, visitor);
@@ -4606,7 +4603,8 @@ impl Resolver {
                                 }
                                 ArgumentIrrefutableMode(argument_mode) => {
                                     // And for function arguments, `def_arg`.
-                                    def_arg(pattern.id, argument_mode)
+                                    def_arg(pattern.id, argument_mode,
+                                            is_mutable)
                                 }
                             };
 
diff --git a/src/librustc/middle/trans/base.rs b/src/librustc/middle/trans/base.rs
index 6c042d740e1f5..c2a88a82e79b8 100644
--- a/src/librustc/middle/trans/base.rs
+++ b/src/librustc/middle/trans/base.rs
@@ -1708,6 +1708,7 @@ fn trans_enum_variant(ccx: @crate_ctxt,
     // Translate variant arguments to function arguments.
     let fn_args = vec::map(args, |varg|
         {mode: ast::expl(ast::by_copy),
+         is_mutbl: false,
          ty: varg.ty,
          pat: ast_util::ident_to_pat(ccx.tcx.sess.next_node_id(),
                                      ast_util::dummy_sp(),
@@ -1766,6 +1767,7 @@ fn trans_tuple_struct(ccx: @crate_ctxt,
     let fn_args = do fields.map |field| {
         {
             mode: ast::expl(ast::by_copy),
+            is_mutbl: false,
             ty: field.node.ty,
             pat: ast_util::ident_to_pat(ccx.tcx.sess.next_node_id(),
                                         ast_util::dummy_sp(),
diff --git a/src/librustc/middle/trans/expr.rs b/src/librustc/middle/trans/expr.rs
index d3a1772a6b0f8..69356b72c424a 100644
--- a/src/librustc/middle/trans/expr.rs
+++ b/src/librustc/middle/trans/expr.rs
@@ -830,7 +830,7 @@ fn trans_local_var(bcx: block,
                 }
             }
         }
-        ast::def_arg(nid, _) => {
+        ast::def_arg(nid, _, _) => {
             take_local(bcx, bcx.fcx.llargs, nid, expr_id_opt)
         }
         ast::def_local(nid, _) | ast::def_binding(nid, _) => {
diff --git a/src/librustc/middle/typeck/check/mod.rs b/src/librustc/middle/typeck/check/mod.rs
index df7444ee6e94a..d94f45693ed7f 100644
--- a/src/librustc/middle/typeck/check/mod.rs
+++ b/src/librustc/middle/typeck/check/mod.rs
@@ -2740,7 +2740,7 @@ fn ty_param_bounds_and_ty_for_def(fcx: @fn_ctxt, sp: span, defn: ast::def) ->
     ty_param_bounds_and_ty {
 
     match defn {
-      ast::def_arg(nid, _) | ast::def_local(nid, _) |
+      ast::def_arg(nid, _, _) | ast::def_local(nid, _) |
       ast::def_self(nid, _) | ast::def_binding(nid, _) => {
         assert (fcx.inh.locals.contains_key(nid));
         let typ = ty::mk_var(fcx.ccx.tcx, lookup_local(fcx, sp, nid));
diff --git a/src/librustc/middle/typeck/check/regionck.rs b/src/librustc/middle/typeck/check/regionck.rs
index 797a21c38e20b..2be94eb801ff5 100644
--- a/src/librustc/middle/typeck/check/regionck.rs
+++ b/src/librustc/middle/typeck/check/regionck.rs
@@ -49,8 +49,8 @@ type rvt = visit::vt<@rcx>;
 fn encl_region_of_def(fcx: @fn_ctxt, def: ast::def) -> ty::Region {
     let tcx = fcx.tcx();
     match def {
-        def_local(node_id, _) | def_arg(node_id, _) | def_self(node_id, _) |
-        def_binding(node_id, _) =>
+        def_local(node_id, _) | def_arg(node_id, _, _) |
+        def_self(node_id, _) | def_binding(node_id, _) =>
             return encl_region(tcx, node_id),
         def_upvar(_, subdef, closure_id, body_id) => {
             match ty_fn_proto(fcx.node_ty(closure_id)) {
diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index c7c8c77c70fa3..aa686a36c89e2 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -127,7 +127,7 @@ enum def {
     def_mod(def_id),
     def_foreign_mod(def_id),
     def_const(def_id),
-    def_arg(node_id, mode),
+    def_arg(node_id, mode, bool /* is_mutbl */),
     def_local(node_id, bool /* is_mutbl */),
     def_variant(def_id /* enum */, def_id /* variant */),
     def_ty(def_id),
@@ -191,9 +191,10 @@ impl def : cmp::Eq {
                     _ => false
                 }
             }
-            def_arg(e0a, e1a) => {
+            def_arg(e0a, e1a, e2a) => {
                 match (*other) {
-                    def_arg(e0b, e1b) => e0a == e0b && e1a == e1b,
+                    def_arg(e0b, e1b, e2b) =>
+                        e0a == e0b && e1a == e1b && e2a == e2b,
                     _ => false
                 }
             }
@@ -1138,7 +1139,7 @@ impl Ty : to_bytes::IterBytes {
 
 #[auto_encode]
 #[auto_decode]
-type arg = {mode: mode, ty: @Ty, pat: @pat, id: node_id};
+type arg = {mode: mode, is_mutbl: bool, ty: @Ty, pat: @pat, id: node_id};
 
 #[auto_encode]
 #[auto_decode]
diff --git a/src/libsyntax/ast_util.rs b/src/libsyntax/ast_util.rs
index eaa61b304cd16..1e8880d189fd9 100644
--- a/src/libsyntax/ast_util.rs
+++ b/src/libsyntax/ast_util.rs
@@ -81,7 +81,7 @@ pure fn def_id_of_def(d: def) -> def_id {
       def_use(id) | def_struct(id) => {
         id
       }
-      def_arg(id, _) | def_local(id, _) | def_self(id, _) | def_self_ty(id)
+      def_arg(id, _, _) | def_local(id, _) | def_self(id, _) | def_self_ty(id)
       | def_upvar(id, _, _, _) | def_binding(id, _) | def_region(id)
       | def_typaram_binder(id) | def_label(id) => {
         local_def(id)
diff --git a/src/libsyntax/ext/auto_encode.rs b/src/libsyntax/ext/auto_encode.rs
index b8fe00e246c7c..af9f4ca2c7c4c 100644
--- a/src/libsyntax/ext/auto_encode.rs
+++ b/src/libsyntax/ext/auto_encode.rs
@@ -540,6 +540,7 @@ fn mk_ser_method(
 
     let ser_inputs = ~[{
         mode: ast::infer(cx.next_id()),
+        is_mutbl: false,
         ty: ty_s,
         pat: @{id: cx.next_id(),
                node: ast::pat_ident(
@@ -600,6 +601,7 @@ fn mk_deser_method(
 
     let deser_inputs = ~[{
         mode: ast::infer(cx.next_id()),
+        is_mutbl: false,
         ty: ty_d,
         pat: @{id: cx.next_id(),
                node: ast::pat_ident(
@@ -1120,6 +1122,7 @@ fn mk_enum_deser_body(
             {
                 inputs: ~[{
                     mode: ast::infer(cx.next_id()),
+                    is_mutbl: false,
                     ty: @{
                         id: cx.next_id(),
                         node: ast::ty_infer,
diff --git a/src/libsyntax/ext/build.rs b/src/libsyntax/ext/build.rs
index d63f14b57749a..a7cb102f090d4 100644
--- a/src/libsyntax/ext/build.rs
+++ b/src/libsyntax/ext/build.rs
@@ -287,6 +287,7 @@ fn mk_arg(cx: ext_ctxt,
     let arg_pat = mk_pat_ident(cx, span, ident);
     {
         mode: ast::infer(cx.next_id()),
+        is_mutbl: false,
         ty: ty,
         pat: arg_pat,
         id: cx.next_id()
diff --git a/src/libsyntax/ext/pipes/ast_builder.rs b/src/libsyntax/ext/pipes/ast_builder.rs
index 7a87d909fe677..eafde9cbb0825 100644
--- a/src/libsyntax/ext/pipes/ast_builder.rs
+++ b/src/libsyntax/ext/pipes/ast_builder.rs
@@ -178,6 +178,7 @@ impl ext_ctxt: ext_ctxt_ast_builder {
 
     fn arg(name: ident, ty: @ast::Ty) -> ast::arg {
         {mode: ast::infer(self.next_id()),
+         is_mutbl: false,
          ty: ty,
          pat: @{id: self.next_id(),
                 node: ast::pat_ident(
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index 6d32c73e1f703..37bea4547590b 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -116,6 +116,7 @@ fn fold_attribute_(at: attribute, fld: ast_fold) ->
 //used in noop_fold_foreign_item and noop_fold_fn_decl
 fn fold_arg_(a: arg, fld: ast_fold) -> arg {
     return {mode: a.mode,
+            is_mutbl: a.is_mutbl,
             ty: fld.fold_ty(a.ty),
             pat: fld.fold_pat(a.pat),
             id: fld.new_id(a.id)};
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index f85ca3fefef88..c0a6b3e401df6 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -684,8 +684,10 @@ impl Parser {
     // identifier names.
     fn parse_arg_general(require_name: bool) -> arg {
         let mut m;
+        let mut is_mutbl = false;
         let pat = if require_name || self.is_named_argument() {
             m = self.parse_arg_mode();
+            is_mutbl = self.eat_keyword(~"mut");
             let pat = self.parse_pat(false);
             self.expect(token::COLON);
             pat
@@ -698,7 +700,7 @@ impl Parser {
 
         let t = self.parse_ty(false);
 
-        {mode: m, ty: t, pat: pat, id: self.get_id()}
+        {mode: m, is_mutbl: is_mutbl, ty: t, pat: pat, id: self.get_id()}
     }
 
     fn parse_arg() -> arg_or_capture_item {
@@ -720,7 +722,8 @@ impl Parser {
                   node: ty_infer,
                   span: mk_sp(p.span.lo, p.span.hi)}
             };
-            either::Left({mode: m, ty: t, pat: pat, id: p.get_id()})
+            either::Left({mode: m, is_mutbl: false, ty: t, pat: pat,
+                          id: p.get_id()})
         }
     }