Enhance match implied borrow

[colinfang]

struct Foo {
    bar: Option<String>
}

fn f(foo: &Foo) -> i32 {
    match &foo.bar {
        None => 1,
        Some(s) => 2,
    }
}

The above works because of the implied borrow introduced in v1.26

However, if we don't put & before foo.bar, the ref is mandatory.

fn f(foo: &Foo) -> i32 {
    match foo.bar {
        None => 1,
        Some(ref s) => 2,
    }

Is it a good idea in this case that the compiler also implicitly adds ref for us, since we cannot move foo.bar anyway.

[estebank]

The diagnostic does suggest the appropriate code (borrowing foo.bar), but leaving open as a match ergonomics enhancement request:

error[E0507]: cannot move out of borrowed content
  --> src/lib.rs:8:11
   |
8  |     match foo.bar {
   |           ^^^^^^^
   |           |
   |           cannot move out of borrowed content
   |           help: consider borrowing here: `&foo.bar`
9  |         None => 1,
10 |         Some(s) => 2,
   |              - data moved here
   |
note: move occurs because `s` has type `std::string::String`, which does not implement the `Copy` trait
  --> src/lib.rs:10:14
   |
10 |         Some(s) => 2,
   |              ^