ICE: Unable to fulfill trait

Rust version: rustc 1.30.0-nightly (bb0896af1 2018-09-29)

How to reproduce:

git clone https://github.com/rayon-rs/rayon.git
cd rayon/rayon-futures
git checkout df8644
RUSTFLAGS="--cfg rayon_unstable" cargo doc --open -p rayon-futures

Error:

 Documenting rayon-futures v0.1.0 (/home/stjepan/tmp/rayon/rayon-futures)
thread '<unnamed>' panicked at 'Unable to fulfill trait DefId(2/0:829 ~ core[3330]::marker[0]::Send[0]) for 'ScopeFutureContents<'scope, F, S>': [FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<S as rayon_core::internal::task::ScopeHandle<'_>>)),depth=5),Ambiguity), FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<S as rayon_core::internal::task::ScopeHandle<'_>>)),depth=5),Ambiguity)]', librustc/traits/auto_trait.rs:218:17
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.30.0-nightly (bb0896af1 2018-09-29) running on x86_64-unknown-linux-gnu

error: Could not document `rayon-futures`.

[Aaron1011]

Minimized reproduction:

pub trait ScopeHandle<'scope> {}

pub struct ScopeFutureContents<'scope, S>
    where S: ScopeHandle<'scope>,
{
    dummy: &'scope S,
    this: Box<ScopeFuture<'scope, S>>,
}


struct ScopeFuture<'scope, S>
    where S: ScopeHandle<'scope>,
{
    contents: ScopeFutureContents<'scope, S>,
}

unsafe impl<'scope, S> Send for ScopeFuture<'scope, S>
    where S: ScopeHandle<'scope>,
{}
unsafe impl<'scope, S> Sync for ScopeFuture<'scope, S>
    where S: ScopeHandle<'scope>,
{}

[Aaron1011]

I'd like to work on this.