Enclose a string written by a closure in opening and closing chars, or a specified number of parentheses.

enclose.rs

/// Encloses the string written by `enclosed` in the opening an closing
/// characters provided.
fn enclose<F>(s: &mut String, open: char, close: char, enclosed: F)
where
    F: FnOnce(&mut String),
{
    s.push(open);

    enclosed(s);

    s.push(close);
}

/// The string written by the `enclosed` function will be enclosed in
/// `parens_count` pairs of parentheses. A count of `0` is allowed.
fn parenthesize<F>(s: &mut String, parens_count: usize, enclosed: F)
where
    F: FnOnce(&mut String),
{
    match parens_count {
        0 => {
            enclosed(s);
            return;
        }
        1 => {
            enclose(s, '(', ')', enclosed);
            return;
        }
        _ => {}
    }

    let mut enclosed: Box<dyn FnOnce(&mut String)> = Box::new(enclosed);
    for _ in 0..parens_count {
        enclosed = Box::new(|s| enclose(s, '(', ')', enclosed));
    }

    enclosed(s);
}