Trait mismatch for function argument

Question

I\'ve got one piece of Rust code that compiles and one that\'s very similar that does not.

The one that works:

pub fn do_something(_: Box

        

Answer 1

You can't coerce a Box> into a Box>>, for reasons discussed in this question, but you can coerce the inner Box:

pub fn do_something(_: Box>>) {}

fn main() {
    let iter = Box::new(Box::new(vec![1.0].into_iter()) as Box>);
    //                                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    do_something(iter);
}

Playground.

This works because a cast is a coercion site. By writing as Box>, you're hinting to the compiler that it should attempt to make the expression to the left fit that type instead of inferring Box>, because once it's wrapped up in the "outer" Box, you can't change it anymore.

Alternatively (but less clearly), you could make Box::new(...) a coercion site by explicitly parameterizing Box:

    let iter = Box::>>::new(Box::new(vec![1.0].into_iter()));

Which effectively does the same thing.