error: the trait core::marker::Sized is not implemented for the type std::error::Error + Sized + 'static

[julienw]

The error is not easy to understand for this code:

use std::error::Error;
use std::fmt;

#[derive(Debug, Clone)]
struct SomeError {
  cause: String
}

impl fmt::Display for SomeError {
  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
    write!(f, "SomeError")
  }
}

impl Error for SomeError {
  fn description(&self) -> &str {
    "Some Error"
  }

  fn cause(&self) -> Option<&Error> {
    None
  }
}

#[derive(Debug, Clone)]
enum NewError {
  SomeCause(Error + Sized)
}

fn main() {
    let error = NewError::SomeCause(SomeError { cause: "some cause".to_string() });
    println!("{:?}", error);
}

(it's also in https://play.rust-lang.org/?gist=ec4209f420275037c3f652a7aeae01ff&version=stable&mode=debug)

If I understand properly, the issue is that I should have something like this instead:

enum NewError<T: Error> {
  SomeCause(T)
}

But the error is very misleading.

[julienw]

Full error message:

<anon>:25:17: 25:22 error: the trait `core::marker::Sized` is not implemented for the type `std::error::Error + Sized + 'static` [E0277]
<anon>:25 #[derive(Debug, Clone)]
                          ^~~~~
<anon>:25:17: 25:22 note: in this expansion of #[derive_Clone] (defined in <anon>)
<anon>:25:17: 25:22 help: see the detailed explanation for E0277
<anon>:25:17: 25:22 note: `std::error::Error + Sized + 'static` does not have a constant size known at compile-time
<anon>:25:17: 25:22 note: required because it appears within the type `NewError`
<anon>:25:17: 25:22 note: required by `core::clone::Clone`
error: aborting due to previous error

[julienw]

If I understand properly, the issue is that I should have something like this instead:

So actually the error is that I have a Trait object that's not behind a pointer (Box or ref). As a result it's not sized even though I use + Sized. Maybe the error message could explain better why it doesn't have a constant size known at compile-time. (damned, every word is important :) )

[arielb1]

The + Sized being allowed is an implementation bug,

[arielb1]

duplicate of #33243.

[dataf3l]

@julienw can you share the example with the solution?

[julienw]

I described it at the end of my initial description, but here is the full code for the solution