| 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. |
