futures-rs tests ICE on beta

[sfackler]

error: internal compiler error: /checkout/src/librustc_trans/context.rs:739: failed to get layout for `&mut futures::future::join::MaybeDone<futures::MapErr<futures::stream::Forward<futures::stream::Iter<std::iter::Map<std::vec::IntoIter<i32>, fn(i32) -> std::result::Result<i32, futures::unsync::mpsc::SendError<i32>> {std::result::Result<i32, futures::unsync::mpsc::SendError<i32>>::Ok}>>, futures::unsync::mpsc::UnboundedSender<i32>>, [closure@tests/unsync.rs:84:22: 84:57]>>`: the type `futures::future::join::MaybeDone<futures::MapErr<futures::stream::Forward<futures::stream::Iter<std::iter::Map<std::vec::IntoIter<i32>, fn(i32) -> std::result::Result<i32, futures::unsync::mpsc::SendError<i32>> {std::result::Result<i32, futures::unsync::mpsc::SendError<i32>>::Ok}>>, futures::unsync::mpsc::UnboundedSender<i32>>, [closure@tests/unsync.rs:84:22: 84:57]>>` has an unknown layout                                                                                                           
                                                                                                                                                                                                                                                       
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 1.20.0-beta.1 (e93aa3aa8 2017-07-18) running on x86_64-unknown-linux-gnu                                                                                                                                                                   
                                                                                                                                                                                                                                                       
note: run with `RUST_BACKTRACE=1` for a backtrace                                                                                                                                                                                                      
                                                                                                                                                                                                                                                       
thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:489:8                                                                                                                                                                      
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.                                                                                                                                                                
stack backtrace:                                                                                                                                                                                                                                       
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace                                                                                                                                                                                         
             at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49                                                                                                                                                                            
   1: std::sys_common::backtrace::_print                                                                                                                                                                                                               
             at /checkout/src/libstd/sys_common/backtrace.rs:71                                                                                                                                                                                        
   2: std::panicking::default_hook::{{closure}}                                                                                                                                                                                                        
             at /checkout/src/libstd/sys_common/backtrace.rs:60                                                                                                                                                                                        
             at /checkout/src/libstd/panicking.rs:380                                                                                                                                                                                                  
   3: std::panicking::default_hook                                                                                                                                                                                                                     
             at /checkout/src/libstd/panicking.rs:390                                                                                                                                                                                                  
   4: std::panicking::rust_panic_with_hook                                                                                                                                                                                                             
             at /checkout/src/libstd/panicking.rs:611                                                                                                                                                                                                  
   5: std::panicking::begin_panic_new                                                                                                                                                                                                                  
   6: rustc_errors::Handler::bug                                                                                                                                                                                                                       
   7: rustc::session::opt_span_bug_fmt::{{closure}}                                                                                                                                                                                                    
   8: rustc::session::opt_span_bug_fmt                                                                                                                                                                                                                 
   9: rustc::session::bug_fmt                                                                                                                                                                                                                          
  10: <&'a rustc_trans::context::SharedCrateContext<'a, 'tcx> as rustc::ty::layout::LayoutTyper<'tcx>>::layout_of                                                                                                                                      
  11: rustc_trans::abi::FnType::unadjusted::{{closure}}                                                                                                                                                                                                
  12: rustc_trans::abi::FnType::unadjusted                                                                                                                                                                                                             
  13: rustc_trans::declare::declare_fn                                                                                                                                                                                                                 
  14: rustc_trans::trans_item::TransItem::predefine                                                                                                                                                                                                    
  15: rustc_trans::base::trans_crate::module_translation                                                                                                                                                                                               
  16: rustc_trans::base::trans_crate                                                                                                                                                                                                                   
  17: rustc_driver::driver::phase_4_translate_to_llvm                                                                                                                                                                                                  
  18: rustc_driver::driver::compile_input::{{closure}}                                                                                                                                                                                                 
  19: rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}                                                                                                                                                                                   
  20: rustc_driver::driver::phase_3_run_analysis_passes                                                                                                                                                                                                
  21: rustc_driver::driver::compile_input                                                                                                                                                                                                              
  22: rustc_driver::run_compiler

[arielb1]

Looks like a dupe of #43132 - fix in queue for beta.

[sfackler]

Duplicate of #43132