Exactly in the same way than this question, I would like that the return type of a function to be a trait, the return value being an instance of a type implementing that trait. A simple example:

fn myfunction() -> Box<Printable> { box TypeB{val: 2} as Box<Printable> } 

If I don't explicitly cast into a box of my generic trait, I get:

error: mismatched types: expected Box<Printable> but found Box<TypeB> (expected trait Printable but found struct TypeB)

So I wonder:

• If it is the normal way to proceed in Rust to return a trait type
• Why the Rust compiler cannot infer that automatically downcast

Any idea? I am using the current nightly version of the compiler.

Exactly in the same way than this question, I would like that the return type of a function to be a trait, the return value being an instance of a type implementing that trait. A simple example:

fn myfunction() -> Box<Printable> { box TypeB{val: 2} as Box<Printable> } 

If I don't explicitly cast into a box of my generic trait, I get:

error: mismatched types: expected Box<Printable> but found Box<TypeB> (expected trait Printable but found struct TypeB)

So I wonder:

• If it is the normal way to proceed in Rust to return a trait type
• Why the Rust compiler cannot infer that automatically downcast

Any idea? I am using the current nightly version of the compiler.