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+Trait objects must include the `dyn` keyword.
+
+Erroneous code example:
+
+```edition2021,compile_fail,E0782
+trait Foo {}
+fn test(arg: Box<Foo>) {} // error!
+```
+
+Trait objects are a way to call methods on types that are not known until
+runtime but conform to some trait.
+
+Trait objects should be formed with `Box<dyn Foo>`, but in the code above
+`dyn` is left off.
+
+This makes it harder to see that `arg` is a trait object and not a
+simply a heap allocated type called `Foo`.
+
+To fix this issue, add `dyn` before the trait name.
+
+```edition2021
+trait Foo {}
+fn test(arg: Box<dyn Foo>) {} // ok!
+```
+
+This used to be allowed before edition 2021, but is now an error.