drop_tracking_mir: diagnose recursive generator.

Author: cjgillot

compiler/rustc_hir_analysis/src/check/check.rs

@@ -1511,6 +1511,14 @@ fn opaque_type_cycle_error(
                     {
                         label_match(interior_ty.ty, interior_ty.span);
                     }
+                    if tcx.sess.opts.unstable_opts.drop_tracking_mir
+                        && let DefKind::Generator = tcx.def_kind(closure_def_id)
+                    {
+                        let generator_layout = tcx.mir_generator_witnesses(closure_def_id);
+                        for interior_ty in &generator_layout.field_tys {
+                            label_match(interior_ty.ty, interior_ty.source_info.span);
+                        }
+                    }
                 }
             }
         }

tests/ui/impl-trait/recursive-impl-trait-type-indirect.drop_tracking_mir.stderr

@@ -114,6 +114,9 @@ error[E0720]: cannot resolve opaque type
    |
 LL | fn generator_hold() -> impl Sized {
    |                        ^^^^^^^^^^ recursive opaque type
+...
+LL |         let x = generator_hold();
+   |             - generator captures itself here
 
 error[E0720]: cannot resolve opaque type
   --> $DIR/recursive-impl-trait-type-indirect.rs:90:26