Trait objects must include the dyn keyword.
Erroneous code example:
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.
trait Foo {}
fn test(arg: Box<dyn Foo>) {} // ok!This used to be allowed before edition 2021, but is now an error.