trait Join: AsRef<str> {
    fn write_join<I: AsRef<str>, W: Write, T: IntoIterator<Item=I>>(&self, writer: W, coll: T) -> io::Result<usize>;
    fn join<I: AsRef<str>, T: IntoIterator<Item=I>>(&self, coll: T) -> String {
        let mut s = Vec::new();
        // Safety: will only panic if we OOM, in which case we're screwed anyway
        self.write_join(&mut s, coll).expect("This shouldn't fail");
        // Safety: all inputs to write_join are `AsRef<str>` so they will all be valid utf-8 and
        // therefore this can't fail
        String::from_utf8(s).unwrap()
    }
}
impl<A: AsRef<str>> Join for A {
    fn write_join<I: AsRef<str>, W: Write, T: IntoIterator<Item=I>>(&self, mut writer: W, coll: T) -> io::Result<usize> {
        let mut iter = coll.into_iter();
        let mut written = 0;
        if let Some(first) = iter.next() {
            let bytes = first.as_ref().as_bytes();
            writer.write_all(bytes)?;
            written += bytes.len();
        }
        for s in iter {
            let t1 = self.as_ref().as_bytes();
            writer.write_all(t1)?;
            written += t1.len();

            let t2 = s.as_ref().as_bytes();
            writer.write_all(t2)?;
            written += t2.len();
        }
        writer.flush()?;
        io::Result::Ok(written)
    }
}


#[cfg(test)]
mod tests {
    #[test]
    fn write_join_to_file() {
        let v = vec![];
        let mut cur = Cursor::new(v);
        "=".write_join(&mut cur, &["a", "b", "c"]).expect("Couldn't write joined string");
        let result = cur.into_inner();
        assert_eq!(&result[..], "a=b=c".as_bytes());
    }

    #[test]
    fn join_with_heap_string() {
        let equal = String::from("=");
        let result = equal.join(&["a", "b", "c"]);
        assert_eq!("a=b=c", &result[..]);
    }

    #[test]
    fn join_with_borrowed_string() {
        let equal = String::from("=");
        let eq = &equal[..];
        let result = eq.join(&["a", "b", "c"]);
        assert_eq!("a=b=c", &result[..]);
    }

    #[test]
    fn join_with_cow_string() {
        let equal = String::from("=");
        let eq: Cow<'_, str> = Cow::Borrowed(&equal[..]);
        let result = eq.join(&["a", "b", "c"]);
        assert_eq!("a=b=c", &result[..]);
    }

    #[test]
    fn join_hashset() {
        let set = {
            let mut set = HashSet::new();
            set.insert("a");
            set.insert("b");
            set.insert("c");
            set
        };
        let result = "=".join(&set);
        assert!(vec!["a=b=c", "a=c=b", "b=a=c", "b=c=a", "c=a=b", "c=b=a"].contains(&&result[..]));
    }

    use std::io::Cursor;
    use std::borrow::Cow;
    use std::collections::HashSet;
    use super::Join;
}

use std::io::{self, Write};