Send/Sync additional trait requirements cannot be automatically relaxed

[dhardy]

The following code fails:

#[derive(Debug)]
struct Error {
    cause: Option<Box<std::error::Error + Send + Sync>>,
}

impl Error {
    fn cause(&self) -> Option<&::std::error::Error> {
       self.cause.as_ref().map(|e| e.as_ref())
    }
}

fn main() {}

Error:

error[E0308]: mismatched types
 --> sendsyncerror.rs:8:9
  |
8 |         self.cause.as_ref().map(|e| e.as_ref())
  |         ^^^^^^^^^^^^^^^^^^^^^^^ expected trait `std::error::Error`, found trait `std::error::Error + std::marker::Sync + std::marker::Send`
  |
  = note: expected type `std::option::Option<&std::error::Error>`
             found type `std::option::Option<&std::error::Error + std::marker::Sync + std::marker::Send>`

error: aborting due to previous error

Surely automatic type coercion is possible here?

[kennytm]

See #18737 for a previous discussion that removed this kind of coercion/implicit-casting.

[dhardy]

Ok, fair enough. After sorting out the lifetime stuff the cast works. I guess a general solution is not likely to happen soon then.

[arielb1]

This (subtyping is restricted to lifetimes) is an intentional design decision, and changing it would require an RFC.