Box "doesn't have a size known at compile-time" wrong diagnostic

[spastorino]

If I do ...

fn main() {
    let x = "hello";
    let y = Box::new(*x);
}

I get the following error ...

 --> src/main.rs:3:13
  |
3 |     let y = Box::new(*x);
  |             ^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `str`
  = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = note: required by `std::boxed::Box::<T>::new`

I think the right error should be ...

 --> src/main.rs:3:13
  |
3 |     let y = Box::new(*x);
  |                      ^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `str`
  = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = note: required by `std::boxed::Box::<T>::new`

Is not that Box is not Sized, the problem is that the parameter of the Box's new function is not Sized. So the last note in the error is important and also kind of lost in a lot of noise.

Possibly related to #61860.

[estebank]

There are multiple cases of this, where we point at the method ident instead of the correct argument, as we should in this case. It will require some reworking on how we deal with this. The solution will fix a swath of similar issues. Sadly I think it'll be a medium size undertaking to fix.