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
```

10 files changed, 30 insertions, 23 deletions

diff --git a/src/test/compile-fail/E0277-2.rs b/src/test/compile-fail/E0277-2.rs
index 211c0e6f890..816d3529554 100644
--- a/src/test/compile-fail/E0277-2.rs
+++ b/src/test/compile-fail/E0277-2.rs
@@ -25,7 +25,6 @@ fn is_send<T: Send>() { }
 fn main() {
     is_send::<Foo>();
     //~^ ERROR the trait bound `*const u8: std::marker::Send` is not satisfied in `Foo`
-    //~| NOTE within `Foo`, the trait `std::marker::Send` is not implemented for `*const u8`
     //~| NOTE: `*const u8` cannot be sent between threads safely
     //~| NOTE: required because it appears within the type `Baz`
     //~| NOTE: required because it appears within the type `Bar`
diff --git a/src/test/compile-fail/E0277.rs b/src/test/compile-fail/E0277.rs
index e31fea1e458..8b34936419d 100644
--- a/src/test/compile-fail/E0277.rs
+++ b/src/test/compile-fail/E0277.rs
@@ -20,7 +20,6 @@ fn some_func<T: Foo>(foo: T) {
 
 fn f(p: Path) { }
 //~^ ERROR the trait bound `[u8]: std::marker::Sized` is not satisfied in `std::path::Path`
-//~| NOTE within `std::path::Path`, the trait `std::marker::Sized` is not implemented for `[u8]`
 //~| NOTE `[u8]` does not have a constant size known at compile-time
 //~| NOTE required because it appears within the type `std::path::Path`
 //~| NOTE all local variables must have a statically known size
diff --git a/src/test/compile-fail/const-unsized.rs b/src/test/compile-fail/const-unsized.rs
index 23cff4ac6ad..4b212814ded 100644
--- a/src/test/compile-fail/const-unsized.rs
+++ b/src/test/compile-fail/const-unsized.rs
@@ -12,25 +12,21 @@ use std::fmt::Debug;
 
 const CONST_0: Debug+Sync = *(&0 as &(Debug+Sync));
 //~^ ERROR `std::fmt::Debug + std::marker::Sync + 'static: std::marker::Sized` is not satisfied
-//~| NOTE the trait `std::marker::Sized` is not implemented for `std::fmt::Debug + std::marker::Syn
 //~| NOTE does not have a constant size known at compile-time
 //~| NOTE constant expressions must have a statically known size
 
 const CONST_FOO: str = *"foo";
 //~^ ERROR `str: std::marker::Sized` is not satisfied
-//~| NOTE the trait `std::marker::Sized` is not implemented for `str`
 //~| NOTE does not have a constant size known at compile-time
 //~| NOTE constant expressions must have a statically known size
 
 static STATIC_1: Debug+Sync = *(&1 as &(Debug+Sync));
 //~^ ERROR `std::fmt::Debug + std::marker::Sync + 'static: std::marker::Sized` is not satisfied
-//~| NOTE the trait `std::marker::Sized` is not implemented for `std::fmt::Debug + std::marker::Syn
 //~| NOTE does not have a constant size known at compile-time
 //~| NOTE constant expressions must have a statically known size
 
 static STATIC_BAR: str = *"bar";
 //~^ ERROR `str: std::marker::Sized` is not satisfied
-//~| NOTE the trait `std::marker::Sized` is not implemented for `str`
 //~| NOTE does not have a constant size known at compile-time
 //~| NOTE constant expressions must have a statically known size
 
diff --git a/src/test/compile-fail/impl-trait/auto-trait-leak.rs b/src/test/compile-fail/impl-trait/auto-trait-leak.rs
index 13e53cab172..8a5033e7647 100644
--- a/src/test/compile-fail/impl-trait/auto-trait-leak.rs
+++ b/src/test/compile-fail/impl-trait/auto-trait-leak.rs
@@ -26,7 +26,6 @@ fn send<T: Send>(_: T) {}
 fn main() {
     send(before());
     //~^ ERROR the trait bound `std::rc::Rc<std::cell::Cell<i32>>: std::marker::Send` is not satisfied
-    //~| NOTE the trait `std::marker::Send` is not implemented for `std::rc::Rc<std::cell::Cell<i32>>`
     //~| NOTE `std::rc::Rc<std::cell::Cell<i32>>` cannot be sent between threads safely
     //~| NOTE required because it appears within the type `[closure
     //~| NOTE required because it appears within the type `impl std::ops::Fn<(i32,)>`
@@ -34,7 +33,6 @@ fn main() {
 
     send(after());
     //~^ ERROR the trait bound `std::rc::Rc<std::cell::Cell<i32>>: std::marker::Send` is not satisfied
-    //~| NOTE the trait `std::marker::Send` is not implemented for `std::rc::Rc<std::cell::Cell<i32>>`
     //~| NOTE `std::rc::Rc<std::cell::Cell<i32>>` cannot be sent between threads safely
     //~| NOTE required because it appears within the type `[closure
     //~| NOTE required because it appears within the type `impl std::ops::Fn<(i32,)>`
diff --git a/src/test/compile-fail/on-unimplemented/multiple-impls.rs b/src/test/compile-fail/on-unimplemented/multiple-impls.rs
index 0ad9f21e098..0df8c41ffe1 100644
--- a/src/test/compile-fail/on-unimplemented/multiple-impls.rs
+++ b/src/test/compile-fail/on-unimplemented/multiple-impls.rs
@@ -42,17 +42,14 @@ impl Index<Bar<usize>> for [i32] {
 fn main() {
     Index::index(&[] as &[i32], 2u32);
     //~^ ERROR E0277
-    //~| NOTE the trait `Index<u32>` is not implemented for `[i32]`
     //~| NOTE trait message
     //~| NOTE required by
     Index::index(&[] as &[i32], Foo(2u32));
     //~^ ERROR E0277
-    //~| NOTE the trait `Index<Foo<u32>>` is not implemented for `[i32]`
     //~| NOTE on impl for Foo
     //~| NOTE required by
     Index::index(&[] as &[i32], Bar(2u32));
     //~^ ERROR E0277
-    //~| NOTE the trait `Index<Bar<u32>>` is not implemented for `[i32]`
     //~| NOTE on impl for Bar
     //~| NOTE required by
 }
diff --git a/src/test/compile-fail/on-unimplemented/on-impl.rs b/src/test/compile-fail/on-unimplemented/on-impl.rs
index a7c599330a0..79021cd03cc 100644
--- a/src/test/compile-fail/on-unimplemented/on-impl.rs
+++ b/src/test/compile-fail/on-unimplemented/on-impl.rs
@@ -31,7 +31,6 @@ impl Index<usize> for [i32] {
 fn main() {
     Index::<u32>::index(&[1, 2, 3] as &[i32], 2u32);
     //~^ ERROR E0277
-    //~| NOTE the trait `Index<u32>` is not implemented for `[i32]`
     //~| NOTE a usize is required
     //~| NOTE required by
 }
diff --git a/src/test/compile-fail/on-unimplemented/on-trait.rs b/src/test/compile-fail/on-unimplemented/on-trait.rs
index 0f4b0919b65..a8daef356a5 100644
--- a/src/test/compile-fail/on-unimplemented/on-trait.rs
+++ b/src/test/compile-fail/on-unimplemented/on-trait.rs
@@ -35,9 +35,8 @@ pub fn main() {
     //~^ ERROR
     //~^^ NOTE a collection of type `std::option::Option<std::vec::Vec<u8>>` cannot be built from an iterator over elements of type `&u8`
     //~^^^ NOTE required by `collect`
-    //~| NOTE the trait `MyFromIterator<&u8>` is not implemented for `std::option::Option<std::vec::Vec<u8>>`
+
     let x: String = foobar(); //~ ERROR
     //~^ NOTE test error `std::string::String` with `u8` `_` `u32`
     //~^^ NOTE required by `foobar`
-    //~| NOTE the trait `Foo<u8, _, u32>` is not implemented for `std::string::String`
 }
diff --git a/src/test/compile-fail/on-unimplemented/slice-index.rs b/src/test/compile-fail/on-unimplemented/slice-index.rs
index 1a9ed2dd6e4..5d30c2e982e 100644
--- a/src/test/compile-fail/on-unimplemented/slice-index.rs
+++ b/src/test/compile-fail/on-unimplemented/slice-index.rs
@@ -20,10 +20,8 @@ fn main() {
     let x = &[1, 2, 3] as &[i32];
     x[1i32]; //~ ERROR E0277
              //~| NOTE slice indices are of type `usize` or ranges of `usize`
-             //~| NOTE trait `std::slice::SliceIndex<[i32]>` is not implemented for `i32`
              //~| NOTE required because of the requirements on the impl of `std::ops::Index<i32>`
     x[..1i32]; //~ ERROR E0277
                //~| NOTE slice indices are of type `usize` or ranges of `usize`
-               //~| NOTE trait `std::slice::SliceIndex<[i32]>` is not implemented for `std::ops::RangeTo<i32>`
                //~| NOTE requirements on the impl of `std::ops::Index<std::ops::RangeTo<i32>>`
 }
diff --git a/src/test/compile-fail/partialeq_help.rs b/src/test/compile-fail/partialeq_help.rs
new file mode 100644
index 00000000000..52c24473bd3
--- /dev/null
+++ b/src/test/compile-fail/partialeq_help.rs
@@ -0,0 +1,20 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn foo<T: PartialEq>(a: &T, b: T) {
+    a == b; //~ ERROR E0277
+            //~| NOTE 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
+}
+
+fn main() {
+    foo(&1, 1);
+}
diff --git a/src/test/compile-fail/trait-suggest-where-clause.rs b/src/test/compile-fail/trait-suggest-where-clause.rs
index 7530d8890b9..2c38d8d2e28 100644
--- a/src/test/compile-fail/trait-suggest-where-clause.rs
+++ b/src/test/compile-fail/trait-suggest-where-clause.rs
@@ -16,51 +16,53 @@ fn check<T: Iterator, U: ?Sized>() {
     // suggest a where-clause, if needed
     mem::size_of::<U>();
     //~^ ERROR `U: std::marker::Sized` is not satisfied
-    //~| NOTE the trait `std::marker::Sized` is not implemented for `U`
     //~| HELP consider adding a `where U: std::marker::Sized` bound
     //~| NOTE required by `std::mem::size_of`
+    //~| NOTE `U` does not have a constant size known at compile-time
+    //~| HELP the trait `std::marker::Sized` is not implemented for `U`
 
     mem::size_of::<Misc<U>>();
     //~^ ERROR `U: std::marker::Sized` is not satisfied
-    //~| NOTE the trait `std::marker::Sized` is not implemented for `U`
     //~| HELP consider adding a `where U: std::marker::Sized` bound
     //~| NOTE required because it appears within the type `Misc<U>`
     //~| NOTE required by `std::mem::size_of`
+    //~| NOTE `U` does not have a constant size known at compile-time
+    //~| HELP within `Misc<U>`, the trait `std::marker::Sized` is not implemented for `U`
 
     // ... even if T occurs as a type parameter
 
     <u64 as From<T>>::from;
     //~^ ERROR `u64: std::convert::From<T>` is not satisfied
-    //~| NOTE the trait `std::convert::From<T>` is not implemented for `u64`
     //~| HELP consider adding a `where u64: std::convert::From<T>` bound
     //~| NOTE required by `std::convert::From::from`
+    //~| NOTE the trait `std::convert::From<T>` is not implemented for `u64`
 
     <u64 as From<<T as Iterator>::Item>>::from;
     //~^ ERROR `u64: std::convert::From<<T as std::iter::Iterator>::Item>` is not satisfied
-    //~| NOTE the trait `std::convert::From<<T as std::iter::Iterator>::Item>` is not implemented
     //~| HELP consider adding a `where u64:
     //~| NOTE required by `std::convert::From::from`
+    //~| NOTE the trait `std::convert::From<<T as std::iter::Iterator>::Item>` is not implemented
 
     // ... but not if there are inference variables
 
     <Misc<_> as From<T>>::from;
     //~^ ERROR `Misc<_>: std::convert::From<T>` is not satisfied
-    //~| NOTE the trait `std::convert::From<T>` is not implemented for `Misc<_>`
     //~| NOTE required by `std::convert::From::from`
+    //~| NOTE the trait `std::convert::From<T>` is not implemented for `Misc<_>`
 
     // ... and also not if the error is not related to the type
 
     mem::size_of::<[T]>();
     //~^ ERROR `[T]: std::marker::Sized` is not satisfied
-    //~| NOTE the trait `std::marker::Sized` is not implemented for `[T]`
     //~| NOTE `[T]` does not have a constant size
     //~| NOTE required by `std::mem::size_of`
+    //~| HELP the trait `std::marker::Sized` is not implemented for `[T]`
 
     mem::size_of::<[&U]>();
     //~^ ERROR `[&U]: std::marker::Sized` is not satisfied
-    //~| NOTE the trait `std::marker::Sized` is not implemented for `[&U]`
     //~| NOTE `[&U]` does not have a constant size
     //~| NOTE required by `std::mem::size_of`
+    //~| HELP the trait `std::marker::Sized` is not implemented for `[&U]`
 }
 
 fn main() {