Lint against calling to_string() on &&str, &&String, etc. #4586
Description
ToString is specialized for string types to avoid using expensive formatting facility. This specialization can be missed by calling to_string on redundantly-referenced strings like &&str or &&String which may easily occur when working with iterators, for example:
// Generic implementation for `T: Display` is used (slow)
["foo", "bar"].iter().map(|s| s.to_string());
// OK, the specialized impl is used
["foo", "bar"].iter().map(|&s| s.to_string());