Update error message for E0166

Author: Roy Brunton

src/librustc_typeck/check/mod.rs

@@ -3392,8 +3392,10 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                     if let Some(ref e) = *expr_opt {
                         self.check_expr(&e);
                     }
-                    span_err!(tcx.sess, expr.span, E0166,
-                        "`return` in a function declared as diverging");
+                    struct_span_err!(tcx.sess, expr.span, E0166,
+                        "`return` in a function declared as diverging")
+                        .span_label(expr.span, &format!("diverging function cannot return"))
+                        .emit();
                 }
             }
             self.write_ty(id, self.next_diverging_ty_var());

src/test/compile-fail/E0166.rs

@@ -8,7 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-fn foo() -> ! { return; } //~ ERROR E0166
+fn foo() -> ! { return; }
+    //~^ ERROR E0166
+    //~| NOTE diverging function cannot return
 
 fn main() {
 }

src/test/compile-fail/bad-bang-ann-3.rs

@@ -11,7 +11,9 @@
 // Tests that a function with a ! annotation always actually fails
 
 fn bad_bang(i: usize) -> ! {
-    return 7; //~ ERROR `return` in a function declared as diverging [E0166]
+    return 7;
+    //~^ ERROR `return` in a function declared as diverging [E0166]
+    //~| NOTE diverging function cannot return
 }
 
 fn main() { bad_bang(5); }