Rust: Closure in a struct

struct-closure.rs

trait Separator {
    fn is_sep(&self, c: char) -> bool;
}

struct ClosureSeparator<F: Fn(char) -> bool>(F);

impl<F> Separator for ClosureSeparator<F> where  F: Fn(char) -> bool {

    #[inline]
    fn is_sep(&self, c: char) -> bool {
        self.0(c)
    }

}


trait AsSeparator {
    type Inner: Separator;

    fn as_separator(self) -> Self::Inner;
}

impl<F> AsSeparator for F where F: Fn(char) -> bool + 'static {

    type Inner = ClosureSeparator<F>;

    #[inline]
    fn as_separator(self) -> ClosureSeparator<F> {
        ClosureSeparator(self)
    }
}

fn seps<S: AsSeparator>(build: S) {

    let s = build.as_separator();
    println!("is_sep(' ') = {}", s.is_sep(' '));
    println!("is_sep('a') = {}", s.is_sep('a'));

}

fn main() {

    seps(|c: char| c.is_whitespace());
    seps(|c: char| c.is_alphanumeric());

}