Auto merge of #40565 - estebank:binops-help, r=arielb1

Explicit help message for binop type mismatch

When trying to do `1 + Some(2)`, or some other binary operation on two
types different types without an appropriate trait implementation, provide
an explicit help message:

```rust
help: `{integer} + std::option::Option<{integer}>` has no implementation
```

Re: #39579, #38564, #37626, #39942, #34698.

8 files changed, 164 insertions, 17 deletions

diff --git a/src/libcore/cmp.rs b/src/libcore/cmp.rs
index 74ded948b18..d4544dadaeb 100644
--- a/src/libcore/cmp.rs
+++ b/src/libcore/cmp.rs
@@ -102,6 +102,7 @@ use self::Ordering::*;
 /// ```
 #[lang = "eq"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_on_unimplemented = "can't compare `{Self}` with `{Rhs}`"]
 pub trait PartialEq<Rhs: ?Sized = Self> {
     /// This method tests for `self` and `other` values to be equal, and is used
     /// by `==`.
@@ -550,6 +551,7 @@ impl PartialOrd for Ordering {
 /// ```
 #[lang = "ord"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_on_unimplemented = "can't compare `{Self}` with `{Rhs}`"]
 pub trait PartialOrd<Rhs: ?Sized = Self>: PartialEq<Rhs> {
     /// This method returns an ordering between `self` and `other` values if one exists.
     ///
diff --git a/src/libcore/ops.rs b/src/libcore/ops.rs
index d203b68c0df..175b3a5a69a 100644
--- a/src/libcore/ops.rs
+++ b/src/libcore/ops.rs
@@ -242,6 +242,7 @@ pub trait Drop {
 /// [std::time::SystemTime]: ../../std/time/struct.SystemTime.html
 #[lang = "add"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} + {RHS}`"]
 pub trait Add<RHS=Self> {
     /// The resulting type after applying the `+` operator
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -315,6 +316,7 @@ add_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 /// [std::time::SystemTime]: ../../std/time/struct.SystemTime.html
 #[lang = "sub"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} - {RHS}`"]
 pub trait Sub<RHS=Self> {
     /// The resulting type after applying the `-` operator
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -437,6 +439,7 @@ sub_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 /// ```
 #[lang = "mul"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} * {RHS}`"]
 pub trait Mul<RHS=Self> {
     /// The resulting type after applying the `*` operator
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -565,6 +568,7 @@ mul_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 /// ```
 #[lang = "div"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} / {RHS}`"]
 pub trait Div<RHS=Self> {
     /// The resulting type after applying the `/` operator
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -644,6 +648,7 @@ div_impl_float! { f32 f64 }
 /// ```
 #[lang = "rem"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} % {RHS}`"]
 pub trait Rem<RHS=Self> {
     /// The resulting type after applying the `%` operator
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -883,6 +888,7 @@ not_impl! { bool usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 /// ```
 #[lang = "bitand"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} & {RHS}`"]
 pub trait BitAnd<RHS=Self> {
     /// The resulting type after applying the `&` operator
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -966,6 +972,7 @@ bitand_impl! { bool usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 /// ```
 #[lang = "bitor"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} | {RHS}`"]
 pub trait BitOr<RHS=Self> {
     /// The resulting type after applying the `|` operator
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -1052,6 +1059,7 @@ bitor_impl! { bool usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 /// ```
 #[lang = "bitxor"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} ^ {RHS}`"]
 pub trait BitXor<RHS=Self> {
     /// The resulting type after applying the `^` operator
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -1134,6 +1142,7 @@ bitxor_impl! { bool usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 /// ```
 #[lang = "shl"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} << {RHS}`"]
 pub trait Shl<RHS> {
     /// The resulting type after applying the `<<` operator
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -1237,6 +1246,7 @@ shl_impl_all! { u8 u16 u32 u64 u128 usize i8 i16 i32 i64 isize i128 }
 /// ```
 #[lang = "shr"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} >> {RHS}`"]
 pub trait Shr<RHS> {
     /// The resulting type after applying the `>>` operator
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -1321,6 +1331,7 @@ shr_impl_all! { u8 u16 u32 u64 u128 usize i8 i16 i32 i64 i128 isize }
 /// ```
 #[lang = "add_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} += {Rhs}`"]
 pub trait AddAssign<Rhs=Self> {
     /// The method for the `+=` operator
     #[stable(feature = "op_assign_traits", since = "1.8.0")]
@@ -1377,6 +1388,7 @@ add_assign_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 /// ```
 #[lang = "sub_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} -= {Rhs}`"]
 pub trait SubAssign<Rhs=Self> {
     /// The method for the `-=` operator
     #[stable(feature = "op_assign_traits", since = "1.8.0")]
@@ -1422,6 +1434,7 @@ sub_assign_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 /// ```
 #[lang = "mul_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} *= {Rhs}`"]
 pub trait MulAssign<Rhs=Self> {
     /// The method for the `*=` operator
     #[stable(feature = "op_assign_traits", since = "1.8.0")]
@@ -1467,6 +1480,7 @@ mul_assign_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 /// ```
 #[lang = "div_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} /= {Rhs}`"]
 pub trait DivAssign<Rhs=Self> {
     /// The method for the `/=` operator
     #[stable(feature = "op_assign_traits", since = "1.8.0")]
@@ -1511,6 +1525,7 @@ div_assign_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 /// ```
 #[lang = "rem_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} %= {Rhs}`"]
 pub trait RemAssign<Rhs=Self> {
     /// The method for the `%=` operator
     #[stable(feature = "op_assign_traits", since = "1.8.0")]
@@ -1597,6 +1612,7 @@ rem_assign_impl! { usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 f32 f64 }
 /// ```
 #[lang = "bitand_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} &= {Rhs}`"]
 pub trait BitAndAssign<Rhs=Self> {
     /// The method for the `&=` operator
     #[stable(feature = "op_assign_traits", since = "1.8.0")]
@@ -1641,6 +1657,7 @@ bitand_assign_impl! { bool usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 /// ```
 #[lang = "bitor_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} |= {Rhs}`"]
 pub trait BitOrAssign<Rhs=Self> {
     /// The method for the `|=` operator
     #[stable(feature = "op_assign_traits", since = "1.8.0")]
@@ -1685,6 +1702,7 @@ bitor_assign_impl! { bool usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 /// ```
 #[lang = "bitxor_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} ^= {Rhs}`"]
 pub trait BitXorAssign<Rhs=Self> {
     /// The method for the `^=` operator
     #[stable(feature = "op_assign_traits", since = "1.8.0")]
@@ -1729,6 +1747,7 @@ bitxor_assign_impl! { bool usize u8 u16 u32 u64 u128 isize i8 i16 i32 i64 i128 }
 /// ```
 #[lang = "shl_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} <<= {Rhs}`"]
 pub trait ShlAssign<Rhs> {
     /// The method for the `<<=` operator
     #[stable(feature = "op_assign_traits", since = "1.8.0")]
@@ -1794,6 +1813,7 @@ shl_assign_impl_all! { u8 u16 u32 u64 u128 usize i8 i16 i32 i64 i128 isize }
 /// ```
 #[lang = "shr_assign"]
 #[stable(feature = "op_assign_traits", since = "1.8.0")]
+#[rustc_on_unimplemented = "no implementation for `{Self} >>= {Rhs}`"]
 pub trait ShrAssign<Rhs=Self> {
     /// The method for the `>>=` operator
     #[stable(feature = "op_assign_traits", since = "1.8.0")]
diff --git a/src/librustc/traits/error_reporting.rs b/src/librustc/traits/error_reporting.rs
index 152dd6ac300..931c77badad 100644
--- a/src/librustc/traits/error_reporting.rs
+++ b/src/librustc/traits/error_reporting.rs
@@ -524,15 +524,8 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                             "the trait bound `{}` is not satisfied{}",
                             trait_ref.to_predicate(),
                             post_message);
-                        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) {
                             // If a where-clause may be useful, remind the
@@ -544,17 +537,21 @@ 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) {
+                        } 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 {
-                            // If we can't show anything useful, try to find
-                            // similar impls.
+                            // 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
                     }
 
@@ -997,3 +994,4 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                           suggested_limit));
     }
 }
+
diff --git a/src/test/compile-fail/impl-trait/equality.rs b/src/test/ui/impl-trait/equality.rs
index 36df4f0eb4d..96db53ad2e4 100644
--- a/src/test/compile-fail/impl-trait/equality.rs
+++ b/src/test/ui/impl-trait/equality.rs
@@ -32,7 +32,7 @@ fn sum_to(n: u32) -> impl Foo {
         0
     } else {
         n + sum_to(n - 1)
-        //~^ ERROR the trait bound `u32: std::ops::Add<impl Foo>` is not satisfied
+        //~^ ERROR no implementation for `u32 + impl Foo`
     }
 }
 
diff --git a/src/test/ui/impl-trait/equality.stderr b/src/test/ui/impl-trait/equality.stderr
new file mode 100644
index 00000000000..bd024d6766e
--- /dev/null
+++ b/src/test/ui/impl-trait/equality.stderr
@@ -0,0 +1,55 @@
+error[E0308]: mismatched types
+  --> $DIR/equality.rs:25:5
+   |
+25 |     0_u32
+   |     ^^^^^ expected i32, found u32
+   |
+   = note: expected type `i32`
+              found type `u32`
+
+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`
+   |
+   = note: no implementation for `u32 + impl Foo`
+
+error[E0308]: mismatched types
+  --> $DIR/equality.rs:53:18
+   |
+53 |     let _: u32 = hide(0_u32);
+   |                  ^^^^^^^^^^^ expected u32, found anonymized type
+   |
+   = note: expected type `u32`
+              found type `impl Foo`
+
+error[E0308]: mismatched types
+  --> $DIR/equality.rs:59:18
+   |
+59 |     let _: i32 = Leak::leak(hide(0_i32));
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^ expected i32, found associated type
+   |
+   = note: expected type `i32`
+              found type `<impl Foo as Leak>::T`
+
+error[E0308]: mismatched types
+  --> $DIR/equality.rs:66:10
+   |
+66 |     x = (x.1,
+   |          ^^^ expected u32, found i32
+   |
+   = note: expected type `impl Foo` (u32)
+              found type `impl Foo` (i32)
+
+error[E0308]: mismatched types
+  --> $DIR/equality.rs:69:10
+   |
+69 |          x.0);
+   |          ^^^ expected i32, found u32
+   |
+   = note: expected type `impl Foo` (i32)
+              found type `impl Foo` (u32)
+
+error: aborting due to 6 previous errors
+
diff --git a/src/test/ui/mismatched_types/binops.rs b/src/test/ui/mismatched_types/binops.rs
new file mode 100644
index 00000000000..98449e59664
--- /dev/null
+++ b/src/test/ui/mismatched_types/binops.rs
@@ -0,0 +1,18 @@
+// 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 main() {
+    1 + Some(1);
+    2 as usize - Some(1);
+    3 * ();
+    4 / "";
+    5 < String::new();
+    6 == Ok(1);
+}
diff --git a/src/test/ui/mismatched_types/binops.stderr b/src/test/ui/mismatched_types/binops.stderr
new file mode 100644
index 00000000000..a0f7ff65870
--- /dev/null
+++ b/src/test/ui/mismatched_types/binops.stderr
@@ -0,0 +1,58 @@
+error[E0277]: the trait bound `{integer}: std::ops::Add<std::option::Option<{integer}>>` is not satisfied
+  --> $DIR/binops.rs:12:5
+   |
+12 |     1 + Some(1);
+   |     ^^^^^^^^^^^ the trait `std::ops::Add<std::option::Option<{integer}>>` is not implemented for `{integer}`
+   |
+   = note: no implementation for `{integer} + std::option::Option<{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`
+   |
+   = note: no implementation for `usize - std::option::Option<{integer}>`
+
+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}`
+   |
+   = note: no implementation 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}`
+   |
+   = note: no implementation for `{integer} / &str`
+
+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}`
+   |
+   = note: can't compare `{integer}` with `std::string::String`
+
+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}`
+   |
+   = note: can't compare `{integer}` with `std::string::String`
+
+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}`
+   |
+   = note: can't compare `{integer}` with `std::result::Result<{integer}, _>`
+
+error: aborting due to 7 previous errors
+
diff --git a/src/test/ui/span/multiline-span-simple.stderr b/src/test/ui/span/multiline-span-simple.stderr
index 85c11c05b9f..161b6ca48b2 100644
--- a/src/test/ui/span/multiline-span-simple.stderr
+++ b/src/test/ui/span/multiline-span-simple.stderr
@@ -9,11 +9,7 @@ error[E0277]: the trait bound `u32: std::ops::Add<()>` is not satisfied
 27 | |             y),
    | |______________^ ...ending here: the trait `std::ops::Add<()>` is not implemented for `u32`
    |
-   = help: the following implementations were found:
-             <u32 as std::ops::Add>
-             <&'a u32 as std::ops::Add<u32>>
-             <u32 as std::ops::Add<&'a u32>>
-             <&'b u32 as std::ops::Add<&'a u32>>
+   = note: no implementation for `u32 + ()`
 
 error: aborting due to previous error