Lint uses of `Vec` where arrays would do fine

[clarfonthey]

For example:

fn something_with_slice<T>(s: &[T]) { ... }

let vals = vec![1, 2];
something_with_slice(&vals);

Bonus points: suggest using ArrayVec when the size is statically guaranteed to be less than a certain amount.

[oli-obk]

Don't we already have this? https://rust-lang-nursery.github.io/rust-clippy/master/index.html#useless_vec

Doesn't trigger in your case. So just an improvement to the existing lint required

[matthiaskrgr]

It should also catch cases like vec![.....].iter() where we could use a simple array:

let x: i32 = vec![1,2,3].iter().sum();

[y21]

I've been experimenting with the idea of improving the already existing useless_vec lint a little bit and came up with this idea:

• If a vec![_] macro invocation is found and the parent node is a local (i.e. let ve = vec![_]), then for each use of this local:
• Check if:
  • (changing it to an array would not be too big for the stack, though this check is already covered by the lint)
  • the use expression adjusts to a slice (e.g. calling ve.chunks(16), ve adjusts to &[_])
  • the parent expression is an index expression (e.g. ve[5])
  • the parent expression is a method call and the method name is one of ["len", "as_ptr", "as_mut_ptr", "as_slice", "is_empty"]
    • as_slice would need some special handling, we would need to also suggest removing that call
• If every use of the local passes at least one of these checks, the vec can (hopefully) be safely replaced with an array or a slice

This already catches a lot of cases in the tests alone (61) and it's going to take a while to go through all of them.

(Also, interestingly, the case @matthiaskrgr mentioned does not get caught, but changing it very slightly to immediately borrow the vec makes clippy catch it: playground)