cargo doc crashes on impl Trait usage

[lilith]

cargo doc --all --release --no-deps

error[E0282]: type annotations needed
   --> imageflow_core/src/codecs/mod.rs:106:68
    |
106 |     fn create(c: &Context, io:  IoProxy, io_id: i32) -> Result<Box<impl Decoder>> {
    |                                                                    ^^^^^^^^^^^^ cannot infer type for `_`

rustc -V
rustc 1.22.0-nightly (0701b37 2017-09-18)

[kennytm]

Similar cause as #43869.

Repro case for rustc:

#![feature(conservative_impl_trait)]
fn foo() -> Box<impl Clone> { //~ ERROR E0283
    loop {}
}
fn main() {
}