If I'm following correctly, you had a situation like the following, right?

https://play.rust-lang.org/?version=stable&mode=debug&editio...

    error[E0277]: the size for values of type `dyn std::fmt::Display` cannot be known at compilation time
       --> src/main.rs:5:12
        |
    5   |     let y: Vec<dyn Display> = x.into_iter().collect();
        |            ^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
        |
        = help: the trait `Sized` is not implemented for `dyn std::fmt::Display`
    
    error[E0277]: a value of type `Vec<dyn std::fmt::Display>` cannot be built from an iterator over elements of type `{integer}`
     --> src/main.rs:5:45
      |
    5 |     let y: Vec<dyn Display> = x.into_iter().collect();
      |                                             ^^^^^^^ value of type `Vec<dyn std::fmt::Display>` cannot be built from `std::iter::Iterator<Item={integer}>`
      |
      = help: the trait `FromIterator<{integer}>` is not implemented for `Vec<dyn std::fmt::Display>`
    
    error[E0277]: the size for values of type `dyn std::fmt::Display` cannot be known at compilation time
       --> src/main.rs:5:31
        |
    5   |     let y: Vec<dyn Display> = x.into_iter().collect();
        |                               ^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
        |
        = help: the trait `Sized` is not implemented for `dyn std::fmt::Display`

I can see a bunch of places where we could improve the output that we haven't gotten to yet:

- The third error shouldn't have been emitted in the first place, the first two are more than enough

- The first error has a note, but that note with a little bit of work could be turned into a structured suggestion for boxing or borrowing

- For the second suggestion we could detect this case in particular where the result would be !Sized and also suggest Boxing.

Edit: filed https://github.com/rust-lang/rust/issues/84346

It is also somehow unfortunate that `impl Trait` in locals isn't yet in stable, but once it is it would let you write `let z: Vec<impl Display> = x.into_iter().collect();`, but as you can see here, that doesn't currently work even on nightly: https://play.rust-lang.org/?version=nightly&mode=debug&editi...