Add closure labels

Author: estebank

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

@@ -1406,10 +1406,10 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                 // LL |                 Unit4
                 //    |                 ^^^^^ expected `Unit3`, found `Unit4`
                 //    |
-                diag.span_label(span, "");
+                diag.span_label(span, "this closure");
                 if !span.overlaps(obligation.cause.span) {
                     // Point at the binding corresponding to the closure where it is used.
-                    diag.span_label(obligation.cause.span, "");
+                    diag.span_label(obligation.cause.span, "closure used here");
                 }
             }
 

tests/ui/async-await/dont-ice-for-type-mismatch-in-closure-in-async.stderr

@@ -25,8 +25,9 @@ error[E0271]: expected `{closure@dont-ice-for-type-mismatch-in-closure-in-async.
    |
 LL |     call(|| -> Option<()> {
    |     ---- ------^^^^^^^^^^
-   |     |          |
-   |     |          expected `bool`, found `Option<()>`
+   |     |    |     |
+   |     |    |     expected `bool`, found `Option<()>`
+   |     |    this closure
    |     required by a bound introduced by this call
    |
    = note: expected type `bool`

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

@@ -3,8 +3,9 @@ error[E0271]: expected `{closure@return-type-doesnt-match-bound.rs:8:17}` to be
    |
 LL |     f().or_else(|e| -> ! {
    |         ------- -------^
-   |         |              |
-   |         |              expected `Result<(), _>`, found `!`
+   |         |       |      |
+   |         |       |      expected `Result<(), _>`, found `!`
+   |         |       this closure
    |         required by a bound introduced by this call
    |
    = note: expected enum `Result<(), _>`
@@ -17,11 +18,12 @@ error[E0271]: expected `{closure@return-type-doesnt-match-bound.rs:18:13}` to be
    |
 LL |     let c = |e| -> ! {
    |             -------^
-   |                    |
-   |                    expected `Result<(), _>`, found `!`
+   |             |      |
+   |             |      expected `Result<(), _>`, found `!`
+   |             this closure
 ...
 LL |     f().or_else(c);
-   |         ------- -
+   |         ------- - closure used here
    |         |
    |         required by a bound introduced by this call
    |

tests/ui/higher-ranked/trait-bounds/issue-62203-hrtb-ice.stderr

@@ -36,7 +36,7 @@ LL |     let v = Unit2.m(
    |                   - required by a bound introduced by this call
 LL |         L {
 LL |             f: |x| {
-   |                ---
+   |                --- this closure
 LL |                 drop(x);
 LL |                 Unit4
    |                 ^^^^^ expected `Unit3`, found `Unit4`

tests/ui/never_type/fallback-closure-wrap.fallback.stderr

@@ -2,7 +2,7 @@ error[E0271]: expected `{closure@fallback-closure-wrap.rs:18:40}` to be a closur
   --> $DIR/fallback-closure-wrap.rs:19:9
    |
 LL |     let error = Closure::wrap(Box::new(move || {
-   |                                        -------
+   |                                        ------- this closure
 LL |         panic!("Can't connect to server.");
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found `!`
    |