Type inference regression in 1.0.38

[dtolnay]

The following compiles in 1.0.37 but is broken by #1213 in 1.0.38 because of rust-lang/rust#41617.

#[macro_use]
extern crate serde_derive;

extern crate serde;
use serde::de::DeserializeOwned;

trait Trait {
    type Associated;
}

#[derive(Deserialize)]
struct Struct<T: Trait> where T::Associated: DeserializeOwned {
    a: T::Associated,
}

[dtolnay]

The inference limitation here is unfortunate but I think we are better off sticking with the approach in #1213. In general data structures should not use DeserializeOwned as a trait bound anyway as discussed in the C-STRUCT-BOUNDS guideline.