Function negotiate 

pub fn negotiate<L: Locale>(accept_language: &str) -> L

The locale of L that best matches an Accept-Language header value, or the default.

The ranges in accept_language are tried from the highest q to the lowest, and in the order written where two are equal. A range matches a locale whose tag is the same, ignoring case, or the same after the last -subtag of the range is removed one at a time, so ja-JP matches ja. A locale whose tag starts with the range and a - matches too, so en matches en-US. *, a header with no match, and an empty header give the first locale of Locale::ALL.

assert_eq!(negotiate::<Lang>("ja-JP,ja;q=0.9,en;q=0.8"), Lang::Ja);
assert_eq!(negotiate::<Lang>("fr, en;q=0.5"), Lang::En);
assert_eq!(negotiate::<Lang>("fr"), Lang::En);

Panics

Panics if Locale::ALL is empty.