StrExt::splitn requires a DoubleEndedSearcher whereas StrExt::split does not

[rnijveld]

Wanting to limit your splits to a certain number suddenly requires the pattern to implement DoubleEndedSearcher, so the following code fragment using strings as splits fails:

fn main() {
   let v1: Vec<_> = "test::thing::test".split("::").collect();
   let v2: Vec<_> = "test::thing::test".splitn(1, "::").collect();
}

With:

<anon>:3:54: 3:63 error: type `core::str::SplitN<'_, &str>` does not implement any method in scope named `collect`
<anon>:3    let v2: Vec<_> = "test:thing:test".splitn(1, ":").collect();
                                                              ^~~~~~~~~

I would expect both of them to have pretty much the same requirements as for the pattern argument.

[sfackler]

At the very least, the trait bound should be placed on splitn as well so you get a compilation error in the right place.

[shepmaster]

@sfackler some conversation on the PR indicates that we aren't going to add the trait bounds on the method.