Add comment explaining reasoning

Author: estebank

src/librustc_typeck/check/_match.rs

@@ -300,6 +300,8 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 let pat_ty = tcx.mk_ty(ty::Tuple(element_tys));
                 if let Some(mut err) = self.demand_eqtype_diag(pat.span, expected, pat_ty) {
                     err.emit();
+                    // Walk subpatterns with an expected type of `err` in this case to silence
+                    // further errors being emitted when using the bindings. #50333
                     let element_tys_iter = (0..max_len).map(|_| tcx.types.err);
                     for (_, elem) in elements.iter().enumerate_and_adjust(max_len, ddpos) {
                         self.check_pat_walk(elem, &tcx.types.err, def_bm, true);