diff --git a/src/librustc_mir/borrow_check/mod.rs b/src/librustc_mir/borrow_check/mod.rs
index da9bdc8b4b015..f9db62e0a3a42 100644
--- a/src/librustc_mir/borrow_check/mod.rs
+++ b/src/librustc_mir/borrow_check/mod.rs
@@ -221,9 +221,17 @@ fn do_mir_borrowck<'a, 'tcx>(
     // write unit-tests, as well as helping with debugging.
     nll::dump_mir_results(infcx, MirSource::item(def_id), &body, &regioncx, &opt_closure_req);
 
-    // We also have a `#[rustc_nll]` annotation that causes us to dump
+    // We also have a `#[rustc_regions]` annotation that causes us to dump
     // information.
-    nll::dump_annotation(infcx, &body, def_id, &regioncx, &opt_closure_req, &mut errors_buffer);
+    nll::dump_annotation(
+        infcx,
+        &body,
+        def_id,
+        &regioncx,
+        &opt_closure_req,
+        &opaque_type_values,
+        &mut errors_buffer,
+    );
 
     // The various `flow_*` structures can be large. We drop `flow_inits` here
     // so it doesn't overlap with the others below. This reduces peak memory
diff --git a/src/librustc_mir/borrow_check/nll.rs b/src/librustc_mir/borrow_check/nll.rs
index 4c1ee9582c1f2..101d1856c38f2 100644
--- a/src/librustc_mir/borrow_check/nll.rs
+++ b/src/librustc_mir/borrow_check/nll.rs
@@ -355,6 +355,7 @@ pub(super) fn dump_annotation<'a, 'tcx>(
     mir_def_id: DefId,
     regioncx: &RegionInferenceContext<'tcx>,
     closure_region_requirements: &Option<ClosureRegionRequirements<'_>>,
+    opaque_type_values: &FxHashMap<DefId, ty::ResolvedOpaqueTy<'tcx>>,
     errors_buffer: &mut Vec<Diagnostic>,
 ) {
     let tcx = infcx.tcx;
@@ -370,7 +371,7 @@ pub(super) fn dump_annotation<'a, 'tcx>(
     // viewing the intraprocedural state, the -Zdump-mir output is
     // better.
 
-    if let Some(closure_region_requirements) = closure_region_requirements {
+    let mut err = if let Some(closure_region_requirements) = closure_region_requirements {
         let mut err = tcx.sess.diagnostic().span_note_diag(body.span, "external requirements");
 
         regioncx.annotate(tcx, &mut err);
@@ -388,13 +389,19 @@ pub(super) fn dump_annotation<'a, 'tcx>(
         })
         .unwrap();
 
-        err.buffer(errors_buffer);
+        err
     } else {
         let mut err = tcx.sess.diagnostic().span_note_diag(body.span, "no external requirements");
         regioncx.annotate(tcx, &mut err);
 
-        err.buffer(errors_buffer);
+        err
+    };
+
+    if !opaque_type_values.is_empty() {
+        err.note(&format!("Inferred opaque type values:\n{:#?}", opaque_type_values));
     }
+
+    err.buffer(errors_buffer);
 }
 
 fn for_each_region_constraint(
diff --git a/src/librustc_mir/borrow_check/type_check/mod.rs b/src/librustc_mir/borrow_check/type_check/mod.rs
index b2e8cf9df7c61..6c073b3d2c8dc 100644
--- a/src/librustc_mir/borrow_check/type_check/mod.rs
+++ b/src/librustc_mir/borrow_check/type_check/mod.rs
@@ -1226,6 +1226,15 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                          revealed_ty={:?}",
                         output_ty, opaque_type_map, revealed_ty
                     );
+                    // Make sure that the inferred types are well-formed. I'm
+                    // not entirely sure this is needed (the HIR type check
+                    // didn't do this) but it seems sensible to prevent opaque
+                    // types hiding ill-formed types.
+                    obligations.obligations.push(traits::Obligation::new(
+                        ObligationCause::dummy(),
+                        param_env,
+                        ty::Predicate::WellFormed(revealed_ty),
+                    ));
                     obligations.add(
                         infcx
                             .at(&ObligationCause::dummy(), param_env)