Unable to derive deserialize on generic type, where generic parameter is bounded by DeserializeOwned

[rphmeier]

The problem can be summed up in this code snippet:

extern crate serde;

#[macro_use]
extern crate serde_derive;

use serde::de::DeserializeOwned;

// without `DeserializeOwned`, this compiles. with it, it fails.
trait Foo: DeserializeOwned {}

#[derive(Deserialize)]
struct Bar<F: Foo> {
    inner: F,
}

fn main() {}

http://play.integer32.com/?gist=a0e9f556bdb6f5cbd8c6aedacf1a2e25&version=stable&mode=debug

[dtolnay]

I believe the error is a compiler bug: rust-lang/rust#41617. The workaround is to write Bar without a trait bound.

#[derive(Deserialize)]
struct Bar<F> {
    inner: F,
}

Also see c-struct-bounds, we typically want to avoid trait bounds on data structures anyway.

[rphmeier]

yes, generally I try to avoid them, but in my situation there isn't any easy workaround.

trait Foo {
    type A;
    type B;
}

struct Bar<F: Foo> {
    f: F,
    a: F::A,
    b: F::B,
}

The alternative is to use Bar<F, F::A, F::B> where F: Foo absolutely everywhere, which is a lot of unnecessary clutter

[dtolnay]

This may also work as a workaround:

#[derive(Deserialize)]
struct Bar<F: Foo> {
    #[serde(deserialize_with = "F::deserialize")]
    f: F,
    a: F::A,
    b: F::B,
}

[dtolnay]

Another workaround:

#[derive(Deserialize)]
struct Bar<F: Foo> {
    #[serde(bound = "")]
    f: F,
    a: F::A,
    b: F::B,
}

[dtolnay]

Closing because I don't think this is something we can fix in Serde. It will need to be fixed in the compiler: rust-lang/rust#41617