When a trait isn't implemented, but another similar impl is found, point at it:

```
error[E0277]: the trait bound `u32: Trait` is not satisfied
  --> $DIR/trait_objects_fail.rs:26:9
   |
LL |     foo(&10_u32);
   |         ^^^^^^^ the trait `Trait` is not implemented for `u32`
   |
help: the trait `Trait<12>` is not implemented for `u32`
      but trait `Trait<2>` is implemented for it
  --> $DIR/trait_objects_fail.rs:7:1
   |
LL | impl Trait<2> for u32 {}
   | ^^^^^^^^^^^^^^^^^^^^^
   = note: required for the cast from `&u32` to `&dyn Trait`
```

Pointing at the `impl` definition that *could* apply given a different self type is particularly useful when it has a blanket self type, as it might not be obvious and is not trivially greppable:

```
error[E0277]: the trait bound `RawImpl<_>: Raw<_>` is not satisfied
  --> $DIR/issue-62742.rs:4:5
   |
LL |     WrongImpl::foo(0i32);
   |     ^^^^^^^^^ unsatisfied trait bound
   |
help: the trait `Raw<_>` is not implemented for `RawImpl<_>`
      but trait `Raw<[_]>` is implemented for it
  --> $DIR/issue-62742.rs:29:1
   |
LL | impl<T> Raw<[T]> for RawImpl<T> {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: required by a bound in `SafeImpl`
  --> $DIR/issue-62742.rs:33:35
   |
LL | pub struct SafeImpl<T: ?Sized, A: Raw<T>>(PhantomData<(A, T)>);
   |                                   ^^^^^^ required by this bound in `SafeImpl`
```

Author: estebank

compiler/rustc_errors/src/emitter.rs

@@ -1526,16 +1526,17 @@ impl HumanEmitter {
                 label_width += 2;
             }
             let mut line = 0;
+            let mut pad = false;
             for (text, style) in msgs.iter() {
                 let text =
                     self.translator.translate_message(text, args).map_err(Report::new).unwrap();
                 // Account for newlines to align output to its label.
-                for text in normalize_whitespace(&text).lines() {
+                for text in normalize_whitespace(&text).split('\n') {
                     buffer.append(
                         line,
                         &format!(
                             "{}{}",
-                            if line == 0 { String::new() } else { " ".repeat(label_width) },
+                            if pad { " ".repeat(label_width) } else { String::new() },
                             text
                         ),
                         match style {
@@ -1544,7 +1545,9 @@ impl HumanEmitter {
                         },
                     );
                     line += 1;
+                    pad = true;
                 }
+                pad = false;
                 // We add lines above, but if the last line has no explicit newline (which would
                 // yield an empty line), then we revert one line up to continue with the next
                 // styled text chunk on the same line as the last one from the prior one. Otherwise

compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

@@ -2112,7 +2112,7 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                         msg.extend(types.1.0);
                         msg.push(StringPart::normal("`"));
                     }
-                    err.highlighted_help(msg);
+                    err.highlighted_span_help(self.tcx.def_span(single.impl_def_id), msg);
 
                     if let [TypeError::Sorts(exp_found)] = &terrs[..] {
                         let exp_found = self.resolve_vars_if_possible(*exp_found);

tests/ui/const-generics/associated-type-bound-fail.stderr

@@ -4,8 +4,12 @@ error[E0277]: the trait bound `u16: Bar<N>` is not satisfied
 LL |     type Assoc = u16;
    |                  ^^^ the trait `Bar<N>` is not implemented for `u16`
    |
-   = help: the trait `Bar<N>` is not implemented for `u16`
-           but trait `Bar<3>` is implemented for it
+help: the trait `Bar<N>` is not implemented for `u16`
+      but trait `Bar<3>` is implemented for it
+  --> $DIR/associated-type-bound-fail.rs:7:1
+   |
+LL | impl Bar<3> for u16 {}
+   | ^^^^^^^^^^^^^^^^^^^
 note: required by a bound in `Foo::Assoc`
   --> $DIR/associated-type-bound-fail.rs:4:17
    |

tests/ui/const-generics/defaults/rp_impl_trait_fail.stderr

@@ -18,8 +18,12 @@ LL |
 LL |     1_u32
    |     ----- return type was inferred to be `u32` here
    |
-   = help: the trait `Traitor<N, N>` is not implemented for `u32`
-           but trait `Traitor<N, 2>` is implemented for it
+help: the trait `Traitor<N, N>` is not implemented for `u32`
+      but trait `Traitor<N, 2>` is implemented for it
+  --> $DIR/rp_impl_trait_fail.rs:13:1
+   |
+LL | impl<const N: u8> Traitor<N, 2> for u32 {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: the trait bound `u64: Traitor` is not satisfied
   --> $DIR/rp_impl_trait_fail.rs:21:13
@@ -30,8 +34,12 @@ LL |
 LL |     1_u64
    |     ----- return type was inferred to be `u64` here
    |
-   = help: the trait `Traitor<1, 1>` is not implemented for `u64`
-           but trait `Traitor<1, 2>` is implemented for it
+help: the trait `Traitor<1, 1>` is not implemented for `u64`
+      but trait `Traitor<1, 2>` is implemented for it
+  --> $DIR/rp_impl_trait_fail.rs:14:1
+   |
+LL | impl Traitor<1, 2> for u64 {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0284]: type annotations needed
   --> $DIR/rp_impl_trait_fail.rs:28:5

tests/ui/const-generics/defaults/trait_objects_fail.stderr

@@ -4,8 +4,12 @@ error[E0277]: the trait bound `u32: Trait` is not satisfied
 LL |     foo(&10_u32);
    |         ^^^^^^^ the trait `Trait` is not implemented for `u32`
    |
-   = help: the trait `Trait<12>` is not implemented for `u32`
-           but trait `Trait<2>` is implemented for it
+help: the trait `Trait<12>` is not implemented for `u32`
+      but trait `Trait<2>` is implemented for it
+  --> $DIR/trait_objects_fail.rs:7:1
+   |
+LL | impl Trait<2> for u32 {}
+   | ^^^^^^^^^^^^^^^^^^^^^
    = note: required for the cast from `&u32` to `&dyn Trait`
 
 error[E0277]: the trait bound `bool: Traitor<_>` is not satisfied
@@ -14,8 +18,12 @@ error[E0277]: the trait bound `bool: Traitor<_>` is not satisfied
 LL |     bar(&true);
    |         ^^^^^ the trait `Traitor<_>` is not implemented for `bool`
    |
-   = help: the trait `Traitor<_, _>` is not implemented for `bool`
-           but trait `Traitor<2, 3>` is implemented for it
+help: the trait `Traitor<_, _>` is not implemented for `bool`
+      but trait `Traitor<2, 3>` is implemented for it
+  --> $DIR/trait_objects_fail.rs:19:1
+   |
+LL | impl Traitor<2, 3> for bool {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: required for the cast from `&bool` to `&dyn Traitor<_>`
 
 error: aborting due to 2 previous errors

tests/ui/const-generics/defaults/wfness.stderr

@@ -10,8 +10,12 @@ error[E0277]: the trait bound `(): Trait<2>` is not satisfied
 LL |     (): Trait<N>;
    |         ^^^^^^^^ the trait `Trait<2>` is not implemented for `()`
    |
-   = help: the trait `Trait<2>` is not implemented for `()`
-           but trait `Trait<3>` is implemented for it
+help: the trait `Trait<2>` is not implemented for `()`
+      but trait `Trait<3>` is implemented for it
+  --> $DIR/wfness.rs:5:1
+   |
+LL | impl Trait<3> for () {}
+   | ^^^^^^^^^^^^^^^^^^^^
 note: required by a bound in `WhereClause`
   --> $DIR/wfness.rs:8:9
    |
@@ -27,8 +31,12 @@ error[E0277]: the trait bound `(): Trait<1>` is not satisfied
 LL | fn foo() -> DependentDefaultWfness {
    |             ^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait<1>` is not implemented for `()`
    |
-   = help: the trait `Trait<1>` is not implemented for `()`
-           but trait `Trait<3>` is implemented for it
+help: the trait `Trait<1>` is not implemented for `()`
+      but trait `Trait<3>` is implemented for it
+  --> $DIR/wfness.rs:5:1
+   |
+LL | impl Trait<3> for () {}
+   | ^^^^^^^^^^^^^^^^^^^^
 note: required by a bound in `WhereClause`
   --> $DIR/wfness.rs:8:9
    |

tests/ui/const-generics/occurs-check/unused-substs-1.stderr

@@ -4,8 +4,12 @@ error[E0277]: the trait bound `A<_>: Bar<_>` is not satisfied
 LL |     let _ = A;
    |             ^ the trait `Bar<_>` is not implemented for `A<_>`
    |
-   = help: the trait `Bar<_>` is not implemented for `A<_>`
-           but it is implemented for `A<{ 6 + 1 }>`
+help: the trait `Bar<_>` is not implemented for `A<_>`
+      but it is implemented for `A<{ 6 + 1 }>`
+  --> $DIR/unused-substs-1.rs:5:1
+   |
+LL | impl<const N: usize> Bar<N> for A<{ 6 + 1 }> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 note: required by a bound in `A`
   --> $DIR/unused-substs-1.rs:9:11
    |

tests/ui/diagnostic_namespace/do_not_recommend/as_expression.current.stderr

@@ -4,8 +4,12 @@ error[E0277]: the trait bound `&str: AsExpression<Integer>` is not satisfied
 LL |     SelectInt.check("bar");
    |               ^^^^^ the trait `AsExpression<Integer>` is not implemented for `&str`
    |
-   = help: the trait `AsExpression<Integer>` is not implemented for `&str`
-           but trait `AsExpression<Text>` is implemented for it
+help: the trait `AsExpression<Integer>` is not implemented for `&str`
+      but trait `AsExpression<Text>` is implemented for it
+  --> $DIR/as_expression.rs:40:1
+   |
+LL | impl AsExpression<Text> for &'_ str {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: for that trait implementation, expected `Text`, found `Integer`
 
 error: aborting due to 1 previous error

tests/ui/diagnostic_namespace/do_not_recommend/as_expression.next.stderr

@@ -6,8 +6,12 @@ LL |     SelectInt.check("bar");
    |               |
    |               required by a bound introduced by this call
    |
-   = help: the trait `AsExpression<Integer>` is not implemented for `&str`
-           but trait `AsExpression<Text>` is implemented for it
+help: the trait `AsExpression<Integer>` is not implemented for `&str`
+      but trait `AsExpression<Text>` is implemented for it
+  --> $DIR/as_expression.rs:40:1
+   |
+LL | impl AsExpression<Text> for &'_ str {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: for that trait implementation, expected `Text`, found `Integer`
 note: required by a bound in `Foo::check`
   --> $DIR/as_expression.rs:47:12
@@ -24,8 +28,12 @@ error[E0277]: the trait bound `&str: AsExpression<Integer>` is not satisfied
 LL |     SelectInt.check("bar");
    |     ^^^^^^^^^^^^^^^^^^^^^^ the trait `AsExpression<Integer>` is not implemented for `&str`
    |
-   = help: the trait `AsExpression<Integer>` is not implemented for `&str`
-           but trait `AsExpression<Text>` is implemented for it
+help: the trait `AsExpression<Integer>` is not implemented for `&str`
+      but trait `AsExpression<Text>` is implemented for it
+  --> $DIR/as_expression.rs:40:1
+   |
+LL | impl AsExpression<Text> for &'_ str {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = help: for that trait implementation, expected `Text`, found `Integer`
 
 error: aborting due to 2 previous errors

tests/ui/did_you_mean/casting-fn-item-to-fn-pointer.stderr

@@ -4,8 +4,9 @@ error[E0277]: a value of type `Vec<(&str, fn())>` cannot be built from an iterat
 LL |     let _: Vec<(&str, fn())> = [("foo", foo)].into_iter().collect();
    |                                                           ^^^^^^^ value of type `Vec<(&str, fn())>` cannot be built from `std::iter::Iterator<Item=(&str, fn() {foo})>`
    |
-   = help: the trait `FromIterator<(&_, fn() {foo})>` is not implemented for `Vec<(&str, fn())>`
-           but trait `FromIterator<(&_, fn())>` is implemented for it
+help: the trait `FromIterator<(&_, fn() {foo})>` is not implemented for `Vec<(&str, fn())>`
+      but trait `FromIterator<(&_, fn())>` is implemented for it
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
    = help: for that trait implementation, expected `fn()`, found `fn() {foo}`
    = note: fn items are distinct from fn pointers
    = help: consider casting the fn item to a fn pointer: `foo as fn()`
@@ -25,8 +26,9 @@ error[E0277]: a value of type `Vec<fn()>` cannot be built from an iterator over
 LL |     let _: Vec<fn()> = [foo].into_iter().collect();
    |                                          ^^^^^^^ value of type `Vec<fn()>` cannot be built from `std::iter::Iterator<Item=fn() {foo}>`
    |
-   = help: the trait `FromIterator<fn() {foo}>` is not implemented for `Vec<fn()>`
-           but trait `FromIterator<fn()>` is implemented for it
+help: the trait `FromIterator<fn() {foo}>` is not implemented for `Vec<fn()>`
+      but trait `FromIterator<fn()>` is implemented for it
+  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
    = help: for that trait implementation, expected `fn()`, found `fn() {foo}`
    = note: fn items are distinct from fn pointers
    = help: consider casting the fn item to a fn pointer: `foo as fn()`