improve comment

src/librustc/middle/traits/error_reporting.rs

@@ -57,8 +57,10 @@ pub fn report_projection_error<'a, 'tcx>(infcx: &InferCtxt<'a, 'tcx>,
     let predicate =
         infcx.resolve_type_vars_if_possible(&obligation.predicate);
     // The ty_err created by normalize_to_error can end up being unified
-    // into all obligations - we still have to report an error in that
-    // case.
+    // into all obligations: for example, if our obligation is something
+    // like `$X = <() as Foo<$X>>::Out` and () does not implement Foo<_>,
+    // then $X will be unified with ty_err, but the error still needs to be
+    // reported.
     if !infcx.tcx.sess.has_errors() || !predicate.references_error() {
         span_err!(infcx.tcx.sess, obligation.cause.span, E0271,
                 "type mismatch resolving `{}`: {}",