diff --git a/src/libserialize/json.rs b/src/libserialize/json.rs
index d42132440163c768fd8d8e9d3fb64d3477594327..87b9304dde2c8e4a79b65fa379af47a2b2bcb010 100644
--- a/src/libserialize/json.rs
+++ b/src/libserialize/json.rs
@@ -365,6 +365,9 @@ fn description(&self) -> &str { "encoder error" }
 }
 
 impl From<fmt::Error> for EncoderError {
+    /// Converts a `fmt::Error` into `EncoderError`
+    ///
+    /// This conversion does not allocate memory.
     fn from(err: fmt::Error) -> EncoderError { EncoderError::FmtError(err) }
 }