Replace values in a string using flat_map

Question

I'm trying to write a simple function to escape HTML text (yes I know there are crates). Something like this:

fn escape(s: &str) -> String {
    s.chars().flat_map(|c| {
        match c {
            '<' => ['&', 'l', 't'].iter(),
            '>' => ['&', 'g', 't'].iter(),
            '"' => ['&', 'q', 'u', 'o', 't'].iter(),
            '\'' => ['&', 'a', 'p', 'o', 's'].iter(),
            '&' => ['&', 'a', 'm', 'p'].iter(),
            c => std::iter::once(c),
        }
    }).collect()
}

But it doesn't work, because the return types are different for the different arms:

expected struct `std::slice::Iter`, found struct `std::iter::Once`

I've tried many many things, but everything I try has some small problem:

• You can't do "&lt".chars() and c.chars() because char doesn't implement chars().
• You can't do [c].iter() because c is on the stack and iter() only keeps a reference to it - I couldn't work out how to move stuff into the iterator (I guess you probably can't).

Is there some sensible solution (that doesn't involve .to_string()).