Use try_borrow for serializing RefCell

serde/src/ser/impls.rs

@@ -459,7 +459,10 @@ where
     where
         S: Serializer,
     {
-        self.borrow().serialize(serializer)
+        match self.try_borrow() {
+            Ok(value) => value.serialize(serializer),
+            Err(_) => Err(S::Error::custom("already mutably borrowed")),
+        }
     }
 }
 

test_suite/tests/test_ser.rs

@@ -9,6 +9,7 @@
 #[macro_use]
 extern crate serde_derive;
 
+use std::cell::RefCell;
 use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 use std::ffi::CString;
 use std::mem;
@@ -563,6 +564,13 @@ fn test_cannot_serialize_paths() {
     assert_ser_tokens_error(&path_buf, &[], "path contains invalid UTF-8 characters");
 }
 
+#[test]
+fn test_cannot_serialize_mutably_borrowed_ref_cell() {
+    let ref_cell = RefCell::new(42);
+    let _reference = ref_cell.borrow_mut();
+    assert_ser_tokens_error(&ref_cell, &[], "already mutably borrowed");
+}
+
 #[test]
 fn test_enum_skipped() {
     assert_ser_tokens_error(