src/test/ui-fulldeps/issue-24972.rs

// run-pass

#![allow(dead_code)]
#![feature(rustc_private)]

extern crate rustc_serialize;

use rustc_serialize::{json, Decodable, Encodable};
use std::fmt::Display;

pub trait Entity: Decodable<json::Decoder> + for<'a> Encodable<json::Encoder<'a>> + Sized {
    type Key: Clone
        + Decodable<json::Decoder>
        + for<'a> Encodable<json::Encoder<'a>>
        + ToString
        + Display
        + Eq
        + Ord
        + Sized;

    fn id(&self) -> Self::Key;

    fn find_by_id(id: Self::Key) -> Option<Self>;
}

pub struct DbRef<E: Entity> {
    pub id: E::Key,
}

impl<E> DbRef<E>
where
    E: Entity,
{
    fn get(self) -> Option<E> {
        E::find_by_id(self.id)
    }
}

fn main() {}