ICE: impl VtableImpl(..) did not contain projection for Obligation(..)

[diwic]

420:2 error: internal compiler error: impl VtableImpl(impl_def_id=DefId { krate: 0, node: 12304 }:ConnectionItems<'a>.Iterator, substs=Substs[types=[[];[];[]], regions=[['_#0r];[];[]]], nested=[[];[];[]]) did not contain projection for Obligation(predicate=<ConnectionItems<'a> as TraitRef(ConnectionItems<'a>, core::iter::Iterator)>::Item,depth=0)

                                 ...

note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: http://doc.rust-lang.org/complement-bugreport.html
note: run with RUST_BACKTRACE=1 for a backtrace
thread 'rustc' panicked at 'Box', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libsyntax/diagnostic.rs:123

stack backtrace:
1: 0x7fa6520d0920 - sys::backtrace::write::h36422e877a8b3158yQs
2: 0x7fa6520f6650 - failure::on_fail::hde5005d61795902da3y
3: 0x7fa65205c4a0 - rt::unwind::begin_unwind_inner::ha0668a3a9d73b9a7RHy
4: 0x7fa64cf12af0 - rt::unwind::begin_unwind::h9147230717703493418
5: 0x7fa64cf12a80 - diagnostic::SpanHandler::span_bug::hee7de6e88971705bDFF
6: 0x7fa65040dc70 - middle::traits::project::project_type::ha3169b6d00cba236CYP
7: 0x7fa65040add0 - middle::traits::project::opt_normalize_projection_type::h52e80e1c888609adFQP
8: 0x7fa6503f8890 - middle::traits::project::normalize_projection_type::had1d9d27edaeb68fgPP
9: 0x7fa65040c270 - middle::traits::project::AssociatedTypeNormalizer<'a, 'b, 'tcx>.TypeFolder<'tcx>::fold_ty::h8977cf1010cf2ef8RNP
10: 0x7fa6516c9990 - middle::ty_fold::TypeFolder::fold_trait_ref::h15244371943192340039
11: 0x7fa6516ca5c0 - middle::ty_fold::VecPerParamSpace.TypeFoldable<'tcx>::fold_with::h3029018912722243389
12: 0x7fa6516e5b40 - check::FnCtxt<'a, 'tcx>::instantiate_bounds::haf69ce16fdf22227Nul
13: 0x7fa6516fe870 - check::wf::CheckTypeWellFormedVisitor<'ccx, 'tcx>::check_impl::closure.30018
14: 0x7fa6516face0 - check::wf::CheckTypeWellFormedVisitor<'ccx, 'tcx>::with_fcx::h0df2a1129da2a633Hfi
15: 0x7fa651701760 - check::wf::CheckTypeWellFormedVisitor<'ccx, 'tcx>.Visitor<'v>::visit_item::hc72c9294b1b3f21ftoi
16: 0x7fa6518efbd0 - check_crate::unboxed_closure.40283
17: 0x7fa6518ea820 - check_crate::h4ba306a17295b431iux
18: 0x7fa652611600 - driver::phase_3_run_analysis_passes::h56810662503f92d8Sva
19: 0x7fa6525ffaa0 - driver::compile_input::h8d0abf631800220evba
20: 0x7fa6526cb070 - thunk::F.Invoke<A, R>::invoke::h13263903969514368718
21: 0x7fa6526c9e20 - rt::unwind::try::try_fn::h2272437873322108176
22: 0x7fa65215a8b0 - rust_try_inner
23: 0x7fa65215a8a0 - rust_try
24: 0x7fa6526ca170 - thunk::F.Invoke<A, R>::invoke::h17236552692728063371
25: 0x7fa6520e2490 - sys::thread::thread_start::hd19c7749b7af23e8EGv
26: 0x7fa64c7360c0 - start_thread
27: 0x7fa651d06ec9 - __clone
28: 0x0 -

I'm trying to make dbus-rs track master, and apparently Iterators have changed in some way that makes compilation fail. Here's the function that fails:

impl<'a> Iterator for ConnectionItems<'a> {
    fn next(&mut self) -> Option<ConnectionItem> {
        loop {
            let i = self.c.i.pending_items.borrow_mut().pop_front();
            if i.is_some() { return i; }

            let r = unsafe { ffi::dbus_connection_read_write_dispatch(self.c.conn(), self.timeout_ms as libc::c_int) };
            if !self.c.i.pending_items.borrow().is_empty() { continue };

            if r == 0 { return None; }
            return Some(ConnectionItem::Nothing);
        }
    }
}

[huonw]

Closing as a dupe of #20347; thanks for filing!

In this case the fix is writing type Item = ConnectionItem; in the body of the impl.