Function same_key 

fn same_key(one: &Value, other: &Value) -> bool

Whether two keys about to be written are one key in the file.

Value’s equality is CBOR’s everywhere but on NaN, where it follows IEEE 754 and answers false for a value compared with itself. Two NaN keys encode to the same bytes, so a map carrying both is as invalid as any other map with a repeated key, and equality alone would wave exactly that one case through.