> It would be cute if the Rust compiler said "I think you want a Boxed collection of strings here to use the algorithm you're attempting, change how this is represented in all the related structs too, here's how:" but that's unrealistic

I think we could provide such messages in some cases. :)

For example, if you have an impl Trait return type and have multiple returned values of different types, we suggest using Box<dyn Trait> when valid. It's just a lot of work to get those in place. You're effectively implementing a language that does those things automatically, plus all the work to print out the actual diagnostic.