Fix issues and add test

src/test/ui/lint/force-warn/force-allowed-group.rs

@@ -1,7 +1,7 @@
 // compile-flags: --force-warns bare_trait_objects
 // check-pass
 
-#![allow(rust_2018_compatibility)]
+#![allow(rust_2018_idioms)]
 
 pub trait SomeTrait {}
 

src/test/ui/lint/force-warn/force-warn-group.rs

@@ -0,0 +1,13 @@
+// ignore-test
+// compile-flags: --force-warns rust_2018_idioms
+// check-pass
+
+#![allow(rust_2018_idioms)]
+
+pub trait SomeTrait {}
+
+pub fn function(_x: Box<SomeTrait>) {}
+//~^ WARN trait objects without an explicit `dyn` are deprecated
+//~| WARN this was previously accepted by the compiler
+
+fn main() {}