Rust Error: The size for values of type `(dyn std::error::Error + 'static)` cannot be known at compilation time

Question

first of all I want to mention that there many similar question on StackOverflow and around the web but I just can't figure out how to solve this error for my case.

So I have a struct, which represents my own error type:

#[derive(Debug)]
pub struct Error {
    msg: String,
}

Then I continued implementing Display and std::error::Error for my error type:

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

impl std::error::Error for Error {
    fn description(&self) -> &str {
        &self.msg
    }
}

Now I tried to implement std::convert::From so I can use my error type seamlessly with the ? operator:

impl From<dyn std::error::Error> for Error {
    fn from(err: dyn std::error::Error) -> Self {
        Error {
            msg: err.to_string(),
        }
    }
}

But then the rust-compiler gives me this error:

error[E0277]: the size for values of type `(dyn std::error::Error + 'static)` cannot be known
at compilation time
  --> wasm_api/geohub_wasm_filehandler_api/src/lib.rs:33:6
   |
33 | impl From<dyn std::error::Error> for Error {
   |      ^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
   |

I know that by default, generic functions will work only on types that have a known size at compile time. But I cant figure out how to solve this problem properly.

Thanks for your help!

Link to code on Rust-Playground:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=568900e8c7847c1f79781fa9bb6d499d

Answer 1

As @SirDarius says above, you can't do this for an Error because Error is not a type, it's a trait. (If you're coming from OOP, think of a Trait like an interface. You can't convert an interface into another type of object because the interface doesn't have any underlying state -- there's no "there" there.)

The right way to handle this is to implement From for each concrete type you need to support. This video really helped me understand how this all fits together.