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
```

Author: bors

Diff for: src/librustc/traits/error_reporting.rs

@@ -559,6 +559,23 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                             trait_ref.to_predicate(),
                             post_message);
 
+                        let unimplemented_note = self.on_unimplemented_note(trait_ref, obligation);
+                        if let Some(ref s) = unimplemented_note {
+                            // If it has a custom "#[rustc_on_unimplemented]"
+                            // error message, let's display it as the label!
+                            err.span_label(span, s.as_str());
+                            err.help(&format!("{}the trait `{}` is not implemented for `{}`",
+                                              pre_message,
+                                              trait_ref,
+                                              trait_ref.self_ty()));
+                        } else {
+                            err.span_label(span,
+                                           &*format!("{}the trait `{}` is not implemented for `{}`",
+                                                     pre_message,
+                                                     trait_ref,
+                                                     trait_ref.self_ty()));
+                        }
+
                         // Try to report a help message
                         if !trait_ref.has_infer_types() &&
                             self.predicate_can_apply(trait_ref) {
@@ -571,21 +588,12 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                             // which is somewhat confusing.
                             err.help(&format!("consider adding a `where {}` bound",
                                                 trait_ref.to_predicate()));
-                        } else if let Some(s) = self.on_unimplemented_note(trait_ref, obligation) {
-                            // If it has a custom "#[rustc_on_unimplemented]"
-                            // error message, let's display it!
-                            err.note(&s);
-                        } else {
+                        } else if unimplemented_note.is_none() {
                             // Can't show anything else useful, try to find similar impls.
                             let impl_candidates = self.find_similar_impl_candidates(trait_ref);
                             self.report_similar_impl_candidates(impl_candidates, &mut err);
                         }
 
-                        err.span_label(span,
-                                       format!("{}the trait `{}` is not implemented for `{}`",
-                                                pre_message,
-                                                trait_ref,
-                                                trait_ref.self_ty()));
                         err
                     }
 

Diff for: 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 for: 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 for: 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 for: src/test/compile-fail/impl-trait/auto-trait-leak.rs

@@ -26,15 +26,13 @@ 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,)>`
     //~| NOTE required by `send`
 
     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 for: 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 for: 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 for: 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 for: 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 for: 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 for: 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() {

Diff for: 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

Diff for: 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
 

Diff for: 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

Diff for: 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

Diff for: 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