useless_vec doesn't suggest a necessary &

[isaacg1]

In some cases, the useless_vec warning will suggest changing a vec![...] to a &[...]. In this case, a * or a & may be required. This is not mentioned in the help text or in the extended help text at https://rust-lang-nursery.github.io/rust-clippy/master/index.html#useless_vec

A test case reproducing this:

fn main() {
    for a in vec![0] {
        foo(a)
    }
}

fn foo(a: i64) {
    println!("{}", a)
}

Clippy will suggest changing vec![0] to &[0]. A & before the a in for a in is also needed, or similar. While the compiler will suggest this change, it'd be better to make this clear up front.

[kehrazy]

possibly this can be decorated further with a suggestion, seeing as the current implementation is, indeed, correct - the slice seems to be preferable here.

span_lint_and_sugg(
    cx,
    USELESS_VEC,
    span,
    "useless use of `vec!`",
    "you can use a slice directly",
    "consider making the value a reference",
    snippet,
    applicability,
);