Improve error message when an operator is only implemented for references of a given type

[gralpli]

If an operator (for example std::ops::Sub) is only implemented for references of a given type and you try to subtract two instances of that type directly, the error message doesn't give a clue that an implementation for references exists.

use std::ops::Sub;

struct Foo {}

impl Sub<&Foo> for &Foo {
    type Output = Foo;

    fn sub(self, other: &Foo) -> Self::Output {
        Foo {}
    }
}

fn main() {
    let a = Foo {};
    let b = Foo {};

    let c = a - b; // correctly fails, because `Sub` is only implemented for references
                   //  let c = &a - &b;  works
}

error[E0369]: binary operation `-` cannot be applied to type `Foo`
  --> src/main.rs:17:15
   |
17 |     let c = a - b;
   |             - ^ - Foo
   |             |
   |             Foo
   |
   = note: an implementation of `std::ops::Sub` might be missing for `Foo`

It would be nice if the error message said something like:

= note: an implementation of `std::ops::Sub` might be missing for `Foo`
= note: an implementation of `std::ops::Sub` exists for `&Foo`

… or even suggested to try &a - &b instead.

---

I came across this when trying to subtract two IndexSets (from the indexmap crate). I saw in the documentation that there was Sub implemented, but the compiler complained that subtracting was not possible. It took me a while to figure out that i had to add two &s.

[fmease]

Triage: No significant change.

[estebank]

Current output:

error[E0369]: cannot subtract `Foo` from `Foo`
   --> src/main.rs:17:15
    |
 17 |     let c = a - b; // correctly fails, because `Sub` is only implemented for references
    |             - ^ - Foo
    |             |
    |             Foo
    |
note: an implementation of `Sub` might be missing for `Foo`
   --> src/main.rs:3:1
    |
  3 | struct Foo {}
    | ^^^^^^^^^^ must implement `Sub`
note: the trait `Sub` must be implemented
   --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/arith.rs:190:1
    |
190 | pub const trait Sub<Rhs = Self> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

We are still missing detection of the existing impl Sub for &Foo.