Opaque type locations in error message for clarity.

Author: gilescope

src/librustc/infer/error_reporting/mod.rs

@@ -1136,12 +1136,19 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         if let Some((expected, found)) = expected_found {
             match (terr, is_simple_error, expected == found) {
                 (&TypeError::Sorts(ref values), false, true) => {
+                    let sort_string = | a_type: Ty<'tcx> |
+                        if let ty::Opaque(def_id, _) = a_type.sty {
+                            format!(" (opaque type at {})", self.tcx.sess.source_map()
+                                .mk_substr_filename(self.tcx.def_span(def_id)))
+                        } else {
+                            format!(" ({})", a_type.sort_string(self.tcx))
+                        };
                     diag.note_expected_found_extra(
                         &"type",
                         expected,
                         found,
-                        &format!(" ({})", values.expected.sort_string(self.tcx)),
-                        &format!(" ({})", values.found.sort_string(self.tcx)),
+                        &sort_string(values.expected),
+                        &sort_string(values.found),
                     );
                 }
                 (_, false, _) => {

src/test/ui/suggestions/opaque-type-error.stderr

@@ -10,8 +10,8 @@ LL | |         thing_two()
 LL | |     }.await
    | |_____- if and else have incompatible types
    |
-   = note: expected type `impl std::future::Future` (opaque type)
-              found type `impl std::future::Future` (opaque type)
+   = note: expected type `impl std::future::Future` (opaque type at <$DIR/opaque-type-error.rs:8:19>)
+              found type `impl std::future::Future` (opaque type at <$DIR/opaque-type-error.rs:12:19>)
    = note: distinct uses of `impl Trait` result in different opaque types
    = help: if both `Future`s have the same `Output` type, consider `.await`ing on both of them