Even with the hash set solution, it would be nice to not copy the string contents, which unfortunately is needed if the hash set contains lowercased versions of

Even with the hash set solution, it would be nice to not copy the string contents, which unfortunately is needed if the hash set contains lowercased versions of the strings. On the other hand, converting all strings to lowercase then comparing them isn't a great way to case-insensitively compare strings anyway, as Unicode doesn't guarantee that will work. The UniCase crate defines a wrapper around strings with a case-insensitive Eq implementation, so this works: use std::collections::HashSet; use unicase::UniCase; fn main () { let strings = vec![ "apple", "cAt", "cat", "Dog", "apple", "dog", "Cat", "Apple", "dOg", "banana", "cat", "dog", "apple", ]; let mut set = HashSet::new(); let strings: Vec<_> = strings .iter() .filter(|&string| set.insert(UniCase::new(string))) .collect(); println!("{:?}", strings); } https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=b8109c4f270266724b14d426ade5bcc8 https://play.rust-lang.org/?version=stable&mode=debug&editio...