diff --git a/src/librbml/lib.rs b/src/librbml/lib.rs
index f28600e5e69..25279796c03 100644
--- a/src/librbml/lib.rs
+++ b/src/librbml/lib.rs
@@ -1059,8 +1059,8 @@ pub mod writer {
             self.end_tag()
         }
 
-        fn emit_map_elt_key<F>(&mut self, _idx: uint, mut f: F) -> EncodeResult where
-            F: FnMut(&mut Encoder<'a, W>) -> EncodeResult,
+        fn emit_map_elt_key<F>(&mut self, _idx: uint, f: F) -> EncodeResult where
+            F: FnOnce(&mut Encoder<'a, W>) -> EncodeResult,
         {
 
             try!(self.start_tag(EsMapKey as uint));
diff --git a/src/libserialize/json.rs b/src/libserialize/json.rs
index bfc0143c689..8412da3b4c5 100644
--- a/src/libserialize/json.rs
+++ b/src/libserialize/json.rs
@@ -668,8 +668,8 @@ impl<'a> ::Encoder for Encoder<'a> {
         Ok(())
     }
 
-    fn emit_map_elt_key<F>(&mut self, idx: uint, mut f: F) -> EncodeResult where
-        F: FnMut(&mut Encoder<'a>) -> EncodeResult,
+    fn emit_map_elt_key<F>(&mut self, idx: uint, f: F) -> EncodeResult where
+        F: FnOnce(&mut Encoder<'a>) -> EncodeResult,
     {
         if self.is_emitting_map_key { return Err(EncoderError::BadHashmapKey); }
         if idx != 0 { try!(write!(self.writer, ",")) }
@@ -961,8 +961,8 @@ impl<'a> ::Encoder for PrettyEncoder<'a> {
         Ok(())
     }
 
-    fn emit_map_elt_key<F>(&mut self, idx: uint, mut f: F) -> EncodeResult where
-        F: FnMut(&mut PrettyEncoder<'a>) -> EncodeResult,
+    fn emit_map_elt_key<F>(&mut self, idx: uint, f: F) -> EncodeResult where
+        F: FnOnce(&mut PrettyEncoder<'a>) -> EncodeResult,
     {
         if self.is_emitting_map_key { return Err(EncoderError::BadHashmapKey); }
         if idx == 0 {
diff --git a/src/libserialize/serialize.rs b/src/libserialize/serialize.rs
index fe2d57486a8..e93d71a9dff 100644
--- a/src/libserialize/serialize.rs
+++ b/src/libserialize/serialize.rs
@@ -98,7 +98,7 @@ pub trait Encoder {
     fn emit_map<F>(&mut self, len: uint, f: F) -> Result<(), Self::Error>
         where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
     fn emit_map_elt_key<F>(&mut self, idx: uint, f: F) -> Result<(), Self::Error>
-        where F: FnMut(&mut Self) -> Result<(), Self::Error>;
+        where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
     fn emit_map_elt_val<F>(&mut self, idx: uint, f: F) -> Result<(), Self::Error>
         where F: FnOnce(&mut Self) -> Result<(), Self::Error>;
 }