Unhelpful Error Messages for trait objects due to trait bounds

[theduke]

When a struct cannot be turned into a trait object due to a trait bound,
the compiler errors are very cryptic and unhelpful.

trait T: Clone {}

#[derive(Clone)]
struct S {}

fn main() {
  let x: &T = & S{};
}

This results in the following errors (both on stable and nightly):

rustc 1.13.0 (2c6933acc 2016-11-07)
error[E0038]: the trait `T` cannot be made into an object
  --> <anon>:12:15
   |
12 |   let x: &T = & S{};
   |               ^^^^^ the trait `T` cannot be made into an object
   |
   = note: the trait cannot require that `Self : Sized`
   = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<&T>` for `&S`

error[E0038]: the trait `T` cannot be made into an object
  --> <anon>:12:10
   |
12 |   let x: &T = & S{};
   |          ^^ the trait `T` cannot be made into an object
   |
   = note: the trait cannot require that `Self : Sized`

error: aborting due to 2 previous errors

The problem lies with the : Clone trait bound, since Clone is not object safe.

But the errors do not mention Clone at all, which leaves someone relatively new to Rust (like me) completely stranded and puzzled as to what might be the problem.

Better errors here would be a big win.

My ideal error would be something like:

the trait 'T' cannot be made into an object. 
...
note:  T requires the trait bound 'Clone', which cannot be made into an object`.

[llogiq]

Another data point (playground)

[estebank]

CC #38376

Current status: no change.

I believe this lead the dev towards the following:

trait Selectable : Sized {
    fn get_fd(&self) -> RawFd;
    fn on_read(self, _: &mut Server<Self>) -> Option<Self>;
    fn on_write(self, _: &mut Server<Self>) -> Option<Self>;
    fn on_except(self, _: &mut Server<Self>) -> Option<Self>;
}

struct Server<T: Selectable>{
    readfds:VecDeque<Box<T>>,
    writefds:VecDeque<Box<T>>,
    exceptfds:VecDeque<Box<T>>,
}

[estebank]

The current output now points at Clone:

error[E0038]: the trait `T` cannot be made into an object
 --> src/main.rs:7:15
  |
1 | trait T: Clone {}
  |       -  ----- ...because it requires `Self: Sized`
  |       |
  |       this trait cannot be made into an object...
...
7 |   let x: &T = & S{};
  |               ^^^ the trait `T` cannot be made into an object
  |
  = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<&dyn T>` for `&S`
  = note: required by cast to type `&dyn T`

error[E0038]: the trait `T` cannot be made into an object
 --> src/main.rs:7:10
  |
1 | trait T: Clone {}
  |       -  ----- ...because it requires `Self: Sized`
  |       |
  |       this trait cannot be made into an object...
...
7 |   let x: &T = & S{};
  |          ^^ the trait `T` cannot be made into an object