Should Box<dyn ToSql> be an instance of ToSql?

[DevJac]

I had a function like fn my_value_to_sql(v: &MyValue) -> Box<dyn ToSql> which would parse a MyValue into one of several different types, but all the types implemented ToSql. I had to returned this Boxed since otherwise the size of the return value wouldn't have been known.

It would have been nice if Box<dyn ToSql> was an instance of ToSql.

(I'm fairly new to Rust, so it may be that my Box approach is misguided, but it's the best I could come up with.)

[thomcc]

Returning a &dyn ToSql definitely should work (and will be more efficient than using a Box, although will impose more lifetime restrictions). I'd expect Box to work too, but since i know Box<dyn Error> doesn't implement std::error::Error, I could believe that I'm wrong.

[DevJac]

You sure? ToSql is a trait; &dyn ToSql can be any of an infinite number of types. How can its size be known? Or am I mistaken that the size of a returned value must be known?

EDIT: Nevermind, of course a reference has a known size. I'm new at this. 😉
Still, I returned a Box<dyn ToSql> because I wanted to pass ownership back. I ended up turning those Boxes into &dyn ToSqls, but it would be nice if Box<dyn ToSql> was itself a ToSql instance.