Lint againt the use of `ref`

The ref keyword seems to be causing some confusion
https://users.rust-lang.org/t/ref-keyword-versus/18818/26
https://github.com/rust-lang/rust-by-example/issues/390
https://www.reddit.com/r/rust/comments/2tq33x/ref_keyword/

There was also some effort in removing it from compiler suggestions (rust-lang/rust#52423).

As far as I know, you can replace any use of it, by borrowing the variable, that it is matched against.

For example

if let Some(ref x) = Some("Hello") {
    println!("{}", x);
}

if let Some(x) = &Some("Hello") {
    println!("{}", x);
}

or

match Some("Hello") {
    Some(ref x) =>  {
        println!("{}", x);
   }
   None => ()
}

match &Some("Hello") {
    Some(x) =>  {
        println!("{}", x);
   }
   None => ()
}

Some real world uses

https://github.com/TedDriggs/darling/blob/3c3d9a8224a164a5b8604a784381849caa32f034/core/src/error/kind.rs#L63
https://github.com/TedDriggs/darling/blob/b16d131a011f8b1f6cf99113d78edd4b119b5d97/core/src/codegen/variant_data.rs#L72
https://github.com/rust-random/rand/blob/249ebfc4352d8f3e1ebcb2a884f047b31ba32981/rand_distr/src/gamma.rs#L156

Edit(2020-04-11):

Relevant discussion on the rust internals forum:

https://internals.rust-lang.org/t/is-ref-more-powerful-than-match-ergonomics/12111

Start by reviewing the linked Rust Book page, forum discussions, and compiler-suggestions issue to understand the proposed lint and its edge cases. Inspect the three real-world Rust examples in darling and rand, then define the supported scope and verify that the lint gives useful results without rejecting necessary uses. Done means the behavior and policy are agreed upon and covered by appropriate lint tests.