add unreported error variant

Author: mark-i-m

src/librustc_mir/borrow_check/diagnostics/region_errors.rs

@@ -107,6 +107,17 @@ crate enum RegionErrorKind<'tcx> {
         /// The region that should be shorter, but we can't prove it
         shorter_fr: RegionVid,
     },
+
+    /// The same as `RegionError`, but this is an unreported error. We currently only report the
+    /// first error encountered and leave the rest unreported so as not to overwhelm the user.
+    UnreportedError {
+        /// The origin of the region
+        fr_origin: NLLRegionVariableOrigin,
+        /// The region that should outlive `shorter_fr`
+        longer_fr: RegionVid,
+        /// The region that should be shorter, but we can't prove it
+        shorter_fr: RegionVid,
+    }
 }
 
 /// Various pieces of state used when reporting borrow checker errors.

src/librustc_mir/borrow_check/mod.rs

@@ -1664,7 +1664,24 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                     db.buffer(&mut self.errors_buffer);
 
                     // Emit outlives suggestions
-                    //outlives_suggestion.add_suggestion(body, self, infcx, errors_buffer, region_naming);
+                    outlives_suggestion.add_suggestion(
+                        &self.body,
+                        &self.nonlexical_regioncx,
+                        self.infcx,
+                        &mut self.errors_buffer,
+                        &mut region_naming
+                    );
+                }
+
+                RegionErrorKind::UnreportedError {
+                    longer_fr, shorter_fr,
+                    fr_origin: _,
+                } => {
+                    // FIXME: currently we do nothing with these, but perhaps we can do better?
+                    debug!(
+                        "Unreported region error: can't prove that {:?}: {:?}",
+                        longer_fr, shorter_fr
+                    );
                 }
             }
         }

src/librustc_mir/borrow_check/region_infer/mod.rs

@@ -1467,15 +1467,13 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                 propagated_outlives_requirements,
                 errors_buffer,
             ) {
-                // continuing to iterate just reports more errors than necessary
-                //
-                // FIXME It would also allow us to report more Outlives Suggestions, though, so
-                // it's not clear that that's a bad thing. Somebody should try commenting out this
-                // line and see it is actually a regression.
-                //
-                // TODO(mark-i-m): perhaps we could add them to the `RegionErrors` with a flag that
-                // says "don't report me"?
-                return;
+                // We only report the first region error. Subsequent errors are hidden so as not to
+                // overwhelm the user, but we do record them so as to potentially print better
+                // diagnostics elsewhere...
+                errors_buffer.push(RegionErrorKind::UnreportedError {
+                    longer_fr, shorter_fr,
+                    fr_origin: NLLRegionVariableOrigin::FreeRegion,
+                });
             }
         }
     }