Better error messages for &T == T

[iqualfragile]

Something along the lines of "you are trying to compare a refference to to Type, try dereferencing first" would be nice, right now it just has the usual PartialEq not implemented.

[estebank]

Would the following output (as proposed in #40565) be enough to make the situation a bit more clear?

error[E0277]: the trait bound `&T: std::cmp::PartialEq<T>>` is not satisfied
  --> $DIR/file.rs:14:5
   |
14 |     &t == t;
   |     ^^^^^^^ can't compare `&T` with `T`

[iqualfragile]

that would certainly improve things, but in this case the error message could be even more specific and thereby helpful by providing the information about deref-ing, so i would keep it as a separate issue, i think

[estebank]

Current output for:

fn main() {
    let x: usize = 3;
    let y: &usize = &3;
    if x == y {
        println!("asdf");
    }
    if y == x {
        println!("asdf");
    }
    if 3 == &3 {
        println!("asdf");
    }
    if &3 == 3 {
        println!("asdf");
    }
}

error[E0308]: mismatched types
 --> src/main.rs:4:13
  |
4 |     if x == y {
  |             ^
  |             |
  |             expected usize, found &usize
  |             help: consider dereferencing the borrow: `*y`
  |
  = note: expected type `usize`
             found type `&usize`

error[E0277]: the trait bound `&usize: std::cmp::PartialEq<usize>` is not satisfied
 --> src/main.rs:7:10
  |
7 |     if y == x {
  |          ^^ can't compare `&usize` with `usize`
  |
  = help: the trait `std::cmp::PartialEq<usize>` is not implemented for `&usize`

error[E0277]: the trait bound `{integer}: std::cmp::PartialEq<&{integer}>` is not satisfied
  --> src/main.rs:10:10
   |
10 |     if 3 == &3 {
   |          ^^ can't compare `{integer}` with `&{integer}`
   |
   = help: the trait `std::cmp::PartialEq<&{integer}>` is not implemented for `{integer}`

error[E0277]: the trait bound `&{integer}: std::cmp::PartialEq<{integer}>` is not satisfied
  --> src/main.rs:13:11
   |
13 |     if &3 == 3 {
   |           ^^ can't compare `&{integer}` with `{integer}`
   |
   = help: the trait `std::cmp::PartialEq<{integer}>` is not implemented for `&{integer}`

There's room for improvement.

[estebank]

Current output

error[E0308]: mismatched types
 --> src/main.rs:4:13
  |
4 |     if x == y {
  |             ^
  |             |
  |             expected usize, found &usize
  |             help: consider dereferencing the borrow: `*y`
  |
  = note: expected type `usize`
             found type `&usize`

error[E0277]: can't compare `&usize` with `usize`
 --> src/main.rs:7:10
  |
7 |     if y == x {
  |          ^^ no implementation for `&usize == usize`
  |
  = help: the trait `std::cmp::PartialEq<usize>` is not implemented for `&usize`

error[E0277]: can't compare `{integer}` with `&{integer}`
  --> src/main.rs:10:10
   |
10 |     if 3 == &3 {
   |          ^^ no implementation for `{integer} == &{integer}`
   |
   = help: the trait `std::cmp::PartialEq<&{integer}>` is not implemented for `{integer}`

error[E0277]: can't compare `&{integer}` with `{integer}`
  --> src/main.rs:13:11
   |
13 |     if &3 == 3 {
   |           ^^ no implementation for `&{integer} == {integer}`
   |
   = help: the trait `std::cmp::PartialEq<{integer}>` is not implemented for `&{integer}`