Generated code triggers private_in_public future compatibility warnings.

[eddyb]

If a private field of a public type contains a private type, the generated bounds result in warnings such as (example from Servo):

<quote expansion>:1:12: 1:22 warning: private type in public interface, #[warn(private_in_public)] on by default
<quote expansion>:1  where Vec<GlyphEntry>: _serde::ser::Serialize,
                               ^~~~~~~~~~
<quote expansion>:1:12: 1:22 warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
<quote expansion>:1:12: 1:22 note: for more information, see the explanation for E0446 (`--explain E0446`)

See rust-lang/rfcs#1671 (comment) for more on the possible future of private types in bounds (i.e. no action may be necessary in the future if that RFC gets through).