utils.rs

/// copied from rustc_lint

/// Some unicode characters *have* case, are considered upper case or lower case, but they *can't*
/// be upper cased or lower cased. For the purposes of the lint suggestion, we care about being able
/// to change the char's case.
fn char_has_case(c: char) -> bool {
    let mut l = c.to_lowercase();
    let mut u = c.to_uppercase();
    while let Some(l) = l.next() {
        match u.next() {
            Some(u) if l != u => return true,
            _ => {}
        }
    }
    u.next().is_some()
}

pub fn to_camel_case(s: &str) -> String {
    s.trim_matches('_')
        .split('_')
        .filter(|component| !component.is_empty())
        .map(|component| {
            let mut camel_cased_component = String::new();

            let mut new_word = true;
            let mut prev_is_lower_case = true;

            for c in component.chars() {
                // Preserve the case if an uppercase letter follows a lowercase letter, so that
                // `camelCase` is converted to `CamelCase`.
                if prev_is_lower_case && c.is_uppercase() {
                    new_word = true;
                }

                if new_word {
                    camel_cased_component.extend(c.to_uppercase());
                } else {
                    camel_cased_component.extend(c.to_lowercase());
                }

                prev_is_lower_case = c.is_lowercase();
                new_word = false;
            }

            camel_cased_component
        })
        .fold((String::new(), None), |(acc, prev): (String, Option<String>), next| {
            // separate two components with an underscore if their boundary cannot
            // be distinguished using an uppercase/lowercase case distinction
            let join = if let Some(prev) = prev {
                let l = prev.chars().last().unwrap();
                let f = next.chars().next().unwrap();
                !char_has_case(l) && !char_has_case(f)
            } else {
                false
            };
            (acc + if join { "_" } else { "" } + &next, Some(next))
        })
        .0
}