On E0271 for a closure behind a binding, point at binding in call too

```
error[E0271]: expected `{[email protected]:18:13}` to be a closure that returns `Result<(), _>`, but it returns `!`
    --> tests/ui/closures/return-type-doesnt-match-bound.rs:18:20
     |
18   |     let c = |e| -> ! { //~ ERROR to be a closure that returns
     |             -------^
     |                    |
     |                    expected `Result<(), _>`, found `!`
...
22   |     f().or_else(c);
     |         ------- -
     |         |
     |         required by a bound introduced by this call
     |
     = note: expected enum `Result<(), _>`
                found type `!`
note: required by a bound in `Result::<T, E>::or_else`
    --> /home/gh-estebank/rust/library/core/src/result.rs:1406:39
     |
1406 |     pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
     |                                       ^^^^^^^^^^^^ required by this bound in `Result::<T, E>::or_else`
```

Author: estebank

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

@@ -1407,6 +1407,10 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 //    |                 ^^^^^ expected `Unit3`, found `Unit4`
                 //    |
                 diag.span_label(span, "");
+                if !span.overlaps(obligation.cause.span) {
+                    // Point at the binding corresponding to the closure where it is used.
+                    diag.span_label(obligation.cause.span, "");
+                }
             }
 
             let secondary_span = (|| {

tests/ui/closures/return-type-doesnt-match-bound.rs

@@ -0,0 +1,25 @@
+use std::error::Error;
+use std::process::exit;
+
+fn foo<F>(f: F) -> ()
+where
+    F: FnOnce() -> Result<(), Box<dyn Error>>,
+{
+    f().or_else(|e| -> ! { //~ ERROR to be a closure that returns
+        eprintln!("{:?}", e);
+        exit(1)
+    });
+}
+
+fn bar<F>(f: F) -> ()
+where
+    F: FnOnce() -> Result<(), Box<dyn Error>>,
+{
+    let c = |e| -> ! { //~ ERROR to be a closure that returns
+        eprintln!("{:?}", e);
+        exit(1)
+    };
+    f().or_else(c);
+}
+
+fn main() {}

tests/ui/closures/return-type-doesnt-match-bound.stderr

@@ -0,0 +1,35 @@
+error[E0271]: expected `{[email protected]:8:17}` to be a closure that returns `Result<(), _>`, but it returns `!`
+  --> $DIR/return-type-doesnt-match-bound.rs:8:24
+   |
+LL |     f().or_else(|e| -> ! {
+   |         ------- -------^
+   |         |              |
+   |         |              expected `Result<(), _>`, found `!`
+   |         required by a bound introduced by this call
+   |
+   = note: expected enum `Result<(), _>`
+              found type `!`
+note: required by a bound in `Result::<T, E>::or_else`
+  --> $SRC_DIR/core/src/result.rs:LL:COL
+
+error[E0271]: expected `{[email protected]:18:13}` to be a closure that returns `Result<(), _>`, but it returns `!`
+  --> $DIR/return-type-doesnt-match-bound.rs:18:20
+   |
+LL |     let c = |e| -> ! {
+   |             -------^
+   |                    |
+   |                    expected `Result<(), _>`, found `!`
+...
+LL |     f().or_else(c);
+   |         ------- -
+   |         |
+   |         required by a bound introduced by this call
+   |
+   = note: expected enum `Result<(), _>`
+              found type `!`
+note: required by a bound in `Result::<T, E>::or_else`
+  --> $SRC_DIR/core/src/result.rs:LL:COL
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0271`.