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# Error code E0782

Trait objects must include the `dyn` keyword.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    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.

    #![allow(unused)]
    fn main() {
    trait Foo {}
    fn test(arg: Box<dyn Foo>) {} // ok!
    }

This used to be allowed before edition 2021, but is now an error.