convert the "only named lifetimes" error into a delay-span-bug

The existing rules should rule out this possibility.

Author: nikomatsakis

src/librustc_typeck/check/writeback.rs

@@ -320,9 +320,11 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
                             map.get(&Kind::from(r)).and_then(|k| k.as_region()) { r1 } else
                         {
                             let span = node_id.to_span(&self.fcx.tcx);
-                            span_err!(self.tcx().sess, span, E0564,
-                                      "only named lifetimes are allowed in `impl Trait`, \
-                                       but `{}` was found in the type `{}`", r, inside_ty);
+                            self.tcx().sess.delay_span_bug(
+                                span,
+                                &format!("only named lifetimes are allowed in `impl Trait`, \
+                                          but `{}` was found in the type `{}`",
+                                         r, inside_ty));
                             gcx.types.re_static
                         },
                 }

src/test/compile-fail/impl-trait/more_lifetimes.rs

@@ -23,9 +23,6 @@ fn whatever<T>(x: T) -> impl Any + 'static {
 
 fn move_lifetime_into_fn<'a, 'b>(x: &'a u32, y: &'b u32) -> impl Fn(&'a u32) {
     //~^ ERROR lifetime mismatch
-    //~^^ ERROR only named lifetimes are allowed in `impl Trait`, but `'b` was found
-    //~^^^ ERROR only named lifetimes are allowed in `impl Trait`, but `'b` was found
-    // TODO^ remove the above duplicate errors
     move |_| println!("{}", y)
 }