Confusing error message when passing trait object to generic function

[wesleywiser]

This could be the same issue as #23286 but I'm not sure.

The following code gives this error (playground):

use std::io::Write;

fn trait_obj(w: &dyn Write) {
    generic(w);
}

fn generic<W: Write>(_w: &W) {
}

error[E0277]: the size for values of type `dyn std::io::Write` cannot be known at compilation time
 --> src/lib.rs:4:5
  |
4 |     generic(w);
  |     ^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `dyn std::io::Write`
  = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
note: required by `generic`
 --> src/lib.rs:7:1
  |
7 | fn generic<W: Write>(_w: &W) {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

There's a couple reasons this is very confusing:

---

4 |     generic(w);
  |     ^^^^^^^ doesn't have a size known at compile-time

points to the function name not the parameter w.

---

the size for values of type `dyn std::io::Write` cannot be known at compilation time

dyn Write only appears behind a reference which means it should be sized.

We should show a better error message when you pass a &dyn T value to a function expecting a generic &T value.

cc @munik

[estebank]

Current output:

error[E0277]: the size for values of type `dyn std::io::Write` cannot be known at compilation time
 --> src/lib.rs:4:13
  |
4 |     generic(w);
  |             ^ doesn't have a size known at compile-time
...
7 | fn generic<W: Write>(_w: &W) {
  |    ------- - required by this bound in `generic`
  |
  = help: the trait `std::marker::Sized` is not implemented for `dyn std::io::Write`
  = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>

A possible solution would be for the diagnostic to suggest ?Sized:

error[E0277]: the size for values of type `dyn std::io::Write` cannot be known at compilation time
 --> src/lib.rs:4:13
  |
4 |     generic(w);
  |             ^ doesn't have a size known at compile-time
...
7 | fn generic<W: Write>(_w: &W) {
  |    ------- -       - help: consider relaxing the implicit `Sized` constraint: `+ ?Sized`
  |            |
  |            required by this bound in `generic`
  |            
  |
  = help: the trait `std::marker::Sized` is not implemented for `dyn std::io::Write`
  = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>

CC #27964