Auto merge of #41559 - GuillaumeGomez:partial-eq-msg, r=estebank

Add better error message when == operator is badly used

Part of #40660.

With the following code:

```rust
fn foo<T: PartialEq>(a: &T, b: T) {
    a == b;
}

fn main() {
    foo(&1, 1);
}
```

It prints:

```
error[E0277]: the trait bound `&T: std::cmp::PartialEq<T>` is not satisfied
 --> test.rs:2:5
  |
2 |     a == b;
  |     ^^^^^^ can't compare `&T` with `T`
  |
  = help: the trait `std::cmp::PartialEq<T>` is not implemented for `&T`
  = help: consider adding a `where &T: std::cmp::PartialEq<T>` bound

error: aborting due to previous error
```

src/test/ui/impl-trait/equality.stderr

@@ -11,9 +11,9 @@ error[E0277]: the trait bound `u32: std::ops::Add<impl Foo>` is not satisfied
   --> $DIR/equality.rs:34:9
    |
 34 |         n + sum_to(n - 1)
-   |         ^^^^^^^^^^^^^^^^^ the trait `std::ops::Add<impl Foo>` is not implemented for `u32`
+   |         ^^^^^^^^^^^^^^^^^ no implementation for `u32 + impl Foo`
    |
-   = note: no implementation for `u32 + impl Foo`
+   = help: the trait `std::ops::Add<impl Foo>` is not implemented for `u32`
 
 error[E0308]: mismatched types
   --> $DIR/equality.rs:53:18

src/test/ui/mismatched_types/binops.stderr

@@ -2,57 +2,57 @@ error[E0277]: the trait bound `{integer}: std::ops::Add<std::option::Option<{int
   --> $DIR/binops.rs:12:5
    |
 12 |     1 + Some(1);
-   |     ^^^^^^^^^^^ the trait `std::ops::Add<std::option::Option<{integer}>>` is not implemented for `{integer}`
+   |     ^^^^^^^^^^^ no implementation for `{integer} + std::option::Option<{integer}>`
    |
-   = note: no implementation for `{integer} + std::option::Option<{integer}>`
+   = help: the trait `std::ops::Add<std::option::Option<{integer}>>` is not implemented for `{integer}`
 
 error[E0277]: the trait bound `usize: std::ops::Sub<std::option::Option<{integer}>>` is not satisfied
   --> $DIR/binops.rs:13:5
    |
 13 |     2 as usize - Some(1);
-   |     ^^^^^^^^^^^^^^^^^^^^ the trait `std::ops::Sub<std::option::Option<{integer}>>` is not implemented for `usize`
+   |     ^^^^^^^^^^^^^^^^^^^^ no implementation for `usize - std::option::Option<{integer}>`
    |
-   = note: no implementation for `usize - std::option::Option<{integer}>`
+   = help: the trait `std::ops::Sub<std::option::Option<{integer}>>` is not implemented for `usize`
 
 error[E0277]: the trait bound `{integer}: std::ops::Mul<()>` is not satisfied
   --> $DIR/binops.rs:14:5
    |
 14 |     3 * ();
-   |     ^^^^^^ the trait `std::ops::Mul<()>` is not implemented for `{integer}`
+   |     ^^^^^^ no implementation for `{integer} * ()`
    |
-   = note: no implementation for `{integer} * ()`
+   = help: the trait `std::ops::Mul<()>` is not implemented for `{integer}`
 
 error[E0277]: the trait bound `{integer}: std::ops::Div<&str>` is not satisfied
   --> $DIR/binops.rs:15:5
    |
 15 |     4 / "";
-   |     ^^^^^^ the trait `std::ops::Div<&str>` is not implemented for `{integer}`
+   |     ^^^^^^ no implementation for `{integer} / &str`
    |
-   = note: no implementation for `{integer} / &str`
+   = help: the trait `std::ops::Div<&str>` is not implemented for `{integer}`
 
 error[E0277]: the trait bound `{integer}: std::cmp::PartialEq<std::string::String>` is not satisfied
   --> $DIR/binops.rs:16:5
    |
 16 |     5 < String::new();
-   |     ^^^^^^^^^^^^^^^^^ the trait `std::cmp::PartialEq<std::string::String>` is not implemented for `{integer}`
+   |     ^^^^^^^^^^^^^^^^^ can't compare `{integer}` with `std::string::String`
    |
-   = note: can't compare `{integer}` with `std::string::String`
+   = help: the trait `std::cmp::PartialEq<std::string::String>` is not implemented for `{integer}`
 
 error[E0277]: the trait bound `{integer}: std::cmp::PartialOrd<std::string::String>` is not satisfied
   --> $DIR/binops.rs:16:5
    |
 16 |     5 < String::new();
-   |     ^^^^^^^^^^^^^^^^^ the trait `std::cmp::PartialOrd<std::string::String>` is not implemented for `{integer}`
+   |     ^^^^^^^^^^^^^^^^^ can't compare `{integer}` with `std::string::String`
    |
-   = note: can't compare `{integer}` with `std::string::String`
+   = help: the trait `std::cmp::PartialOrd<std::string::String>` is not implemented for `{integer}`
 
 error[E0277]: the trait bound `{integer}: std::cmp::PartialEq<std::result::Result<{integer}, _>>` is not satisfied
   --> $DIR/binops.rs:17:5
    |
 17 |     6 == Ok(1);
-   |     ^^^^^^^^^^ the trait `std::cmp::PartialEq<std::result::Result<{integer}, _>>` is not implemented for `{integer}`
+   |     ^^^^^^^^^^ can't compare `{integer}` with `std::result::Result<{integer}, _>`
    |
-   = note: can't compare `{integer}` with `std::result::Result<{integer}, _>`
+   = help: the trait `std::cmp::PartialEq<std::result::Result<{integer}, _>>` is not implemented for `{integer}`
 
 error: aborting due to 7 previous errors
 

src/test/ui/mismatched_types/cast-rfc0401.stderr

@@ -210,18 +210,18 @@ error[E0277]: the trait bound `[u8]: std::marker::Sized` is not satisfied
   --> $DIR/cast-rfc0401.rs:63:13
    |
 63 |     let _ = fat_v as *const Foo;
-   |             ^^^^^ the trait `std::marker::Sized` is not implemented for `[u8]`
+   |             ^^^^^ `[u8]` does not have a constant size known at compile-time
    |
-   = note: `[u8]` does not have a constant size known at compile-time
+   = help: the trait `std::marker::Sized` is not implemented for `[u8]`
    = note: required for the cast to the object type `Foo`
 
 error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
   --> $DIR/cast-rfc0401.rs:72:13
    |
 72 |     let _ = a as *const Foo;
-   |             ^ the trait `std::marker::Sized` is not implemented for `str`
+   |             ^ `str` does not have a constant size known at compile-time
    |
-   = note: `str` does not have a constant size known at compile-time
+   = help: the trait `std::marker::Sized` is not implemented for `str`
    = note: required for the cast to the object type `Foo`
 
 error: casting `&{float}` as `f32` is invalid

src/test/ui/resolve/issue-5035-2.stderr

@@ -2,9 +2,9 @@ error[E0277]: the trait bound `I + 'static: std::marker::Sized` is not satisfied
   --> $DIR/issue-5035-2.rs:14:8
    |
 14 | fn foo(_x: K) {} //~ ERROR: `I + 'static: std::marker::Sized` is not satisfied
-   |        ^^ the trait `std::marker::Sized` is not implemented for `I + 'static`
+   |        ^^ `I + 'static` does not have a constant size known at compile-time
    |
-   = note: `I + 'static` does not have a constant size known at compile-time
+   = help: the trait `std::marker::Sized` is not implemented for `I + 'static`
    = note: all local variables must have a statically known size
 
 error: aborting due to previous error

src/test/ui/span/multiline-span-simple.stderr

@@ -7,9 +7,9 @@ error[E0277]: the trait bound `u32: std::ops::Add<()>` is not satisfied
 25 | |         bar(x,
 26 | |
 27 | |             y),
-   | |______________^ the trait `std::ops::Add<()>` is not implemented for `u32`
+   | |______________^ no implementation for `u32 + ()`
    |
-   = note: no implementation for `u32 + ()`
+   = help: the trait `std::ops::Add<()>` is not implemented for `u32`
 
 error: aborting due to previous error