std::thread::Result has problems with ?ing to anyhow::Error

[golddranks]

This example code:

use anyhow::{anyhow, Result as AResult};

fn main() -> AResult<()> {
    std::thread::spawn( move || { Ok(anyhow!("test error")) }).join()??;
    Ok(())
}

Errors with:

error[E0277]: the size for values of type `dyn std::any::Any + std::marker::Send` cannot be known at compilation time
 --> src/main.rs:5:70
  |
5 |     std::thread::spawn( move || { Ok(anyhow!("test error")) }).join()??;
  |                                                                      ^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `dyn std::any::Any + std::marker::Send`
  = 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 because of the requirements on the impl of `std::error::Error` for `std::boxed::Box<dyn std::any::Any + std::marker::Send>`
  = note: required because of the requirements on the impl of `std::convert::From<std::boxed::Box<dyn std::any::Any + std::marker::Send>>` for `anyhow::Error`
  = note: required by `std::convert::From::from`

error[E0277]: the trait bound `dyn std::any::Any + std::marker::Send: std::error::Error` is not satisfied
 --> src/main.rs:5:70
  |
5 |     std::thread::spawn( move || { Ok(anyhow!("test error")) }).join()??;
  |                                                                      ^ the trait `std::error::Error` is not implemented for `dyn std::any::Any + std::marker::Send`
  |
  = note: required because of the requirements on the impl of `std::error::Error` for `std::boxed::Box<dyn std::any::Any + std::marker::Send>`
  = note: required because of the requirements on the impl of `std::convert::From<std::boxed::Box<dyn std::any::Any + std::marker::Send>>` for `anyhow::Error`
  = note: required by `std::convert::From::from`

error[E0277]: `dyn std::any::Any + std::marker::Send` cannot be shared between threads safely
 --> src/main.rs:5:70
  |
5 |     std::thread::spawn( move || { Ok(anyhow!("test error")) }).join()??;
  |                                                                      ^ `dyn std::any::Any + std::marker::Send` cannot be shared between threads safely
  |
  = help: the trait `std::marker::Sync` is not implemented for `dyn std::any::Any + std::marker::Send`
  = note: required because of the requirements on the impl of `std::marker::Sync` for `std::ptr::Unique<dyn std::any::Any + std::marker::Send>`
  = note: required because it appears within the type `std::boxed::Box<dyn std::any::Any + std::marker::Send>`
  = note: required because of the requirements on the impl of `std::convert::From<std::boxed::Box<dyn std::any::Any + std::marker::Send>>` for `anyhow::Error`
  = note: required by `std::convert::From::from`

error: aborting due to 3 previous errors

Judging from the error messages, the reason is that the error value of std::thread::Result doesn't implement std::error::Error (!). Is there any suggested workarounds? I'd almost expect .map_err(|e| anyhow!(e)) to work for wrapping the error value to anyhow::Error, but it doesn't.

[dtolnay]

thread::spawn doesn't return an error, the E in the Result is a box of whatever value your thread panicked with, which probably doesn't implement Error and maybe doesn't implement Display or Debug. You are supposed to propagate it by unwrapping.

[golddranks]

I see. In that case better documentation would be warranted in the standard library. At the moment it says just: "Indicates the manner in which a thread exited." No mention about what value is returned. I'll close this.