Avoid unnecessary opaque type errors in borrowck

Author: matthewjasper

src/librustc_mir/borrow_check/mod.rs

@@ -136,6 +136,9 @@ fn do_mir_borrowck<'a, 'tcx>(
 
     // Gather the upvars of a closure, if any.
     let tables = tcx.typeck_tables_of(def_id);
+    if tables.tainted_by_errors {
+        infcx.set_tainted_by_errors();
+    }
     let upvars: Vec<_> = tables
         .upvar_list
         .get(&def_id)

src/librustc_mir/borrow_check/nll.rs

@@ -282,6 +282,11 @@ pub(in crate::borrow_check) fn compute_regions<'cx, 'tcx>(
     let (closure_region_requirements, nll_errors) =
         regioncx.solve(infcx, &body, def_id, polonius_output.clone());
 
+    if !nll_errors.is_empty() {
+        // Suppress unhelpful extra errors in `infer_opaque_types`.
+        infcx.set_tainted_by_errors();
+    }
+
     let remapped_opaque_tys = regioncx.infer_opaque_types(&infcx, opaque_type_values, body.span);
 
     NllOutput {