ICE: Impossible case reached

[dpc]

error: internal compiler error: librustc/traits/structural_impls.rs:178: impossible case reached                                                                                   
                                                                                                                                                                                   
thread 'main' panicked at 'Box<Any>', librustc_errors/lib.rs:554:9                                                                                                                 
stack backtrace:                                                                                                                                                                   
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace                                                                                                                    
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49                                                                                                                      
   1: std::sys_common::backtrace::print  
             at libstd/sys_common/backtrace.rs:71                                                                                                                         [87/8490]
             at libstd/sys_common/backtrace.rs:59                                                                                                                                  
   2: std::panicking::default_hook::{{closure}}                                                                                                                                    
             at libstd/panicking.rs:211                                                                                                                                            
   3: std::panicking::default_hook                                                                                                                                                 
             at libstd/panicking.rs:227                                                                                                                                            
   4: rustc::util::common::panic_hook                                                                                                                                              
   5: std::panicking::rust_panic_with_hook                                                                                                                                         
             at libstd/panicking.rs:479                                                                                                                                            
   6: std::panicking::begin_panic                                                                                                                                                  
   7: rustc_errors::Handler::bug                                                                                                                                                   
   8: rustc::session::opt_span_bug_fmt::{{closure}}                                                                                                                                
   9: rustc::ty::context::tls::with_opt::{{closure}}                                                                                                                               
  10: rustc::ty::context::tls::with_context_opt                                                                                                                                    
  11: rustc::ty::context::tls::with_opt                                                                                                                                            
  12: rustc::session::opt_span_bug_fmt                                                                                                                                             
  13: rustc::session::bug_fmt                                                                                                                                                      
  14: rustc::traits::structural_impls::<impl rustc::ty::context::Lift<'tcx> for rustc::traits::SelectionError<'a>>::lift_to_tcx                                                    
  15: rustc::ty::context::TyCtxt::lift_to_global                                                                                                                                   
  16: rustc::traits::select::SelectionContext::candidate_from_obligation                                                                                                           
  17: rustc::traits::select::SelectionContext::evaluate_stack                                                                                                                      
  18: rustc::ty::context::tls::with_context  
  19: rustc::dep_graph::graph::DepGraph::with_anon_task                                                                                                                   [65/8490]
  20: rustc::traits::select::SelectionContext::evaluate_predicate_recursively                                                                                                      
  21: rustc::infer::InferCtxt::probe                                                                                                                                               
  22: <&'a mut I as core::iter::iterator::Iterator>::next                                                                                                                          
  23: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter                                                                                                              
  24: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache                                                                                                  
  25: rustc::ty::context::tls::with_context                                                                                                                                        
  26: rustc::dep_graph::graph::DepGraph::with_anon_task                                                                                                                            
  27: rustc::traits::select::SelectionContext::candidate_from_obligation                                                                                                           
  28: rustc::traits::select::SelectionContext::evaluate_stack                                                                                                                      
  29: rustc::ty::context::tls::with_context                                                                                                                                        
  30: rustc::dep_graph::graph::DepGraph::with_anon_task                                                                                                                            
  31: rustc::traits::select::SelectionContext::evaluate_predicate_recursively                                                                                                      
  32: rustc::infer::InferCtxt::probe                                                                                                                                               
  33: <&'a mut I as core::iter::iterator::Iterator>::next                                                                                                                          
  34: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter                                                                                                              
  35: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache                                                                                                  
  36: rustc::ty::context::tls::with_context                                                                                                                                        
  37: rustc::dep_graph::graph::DepGraph::with_anon_task                                                                                                                            
  38: rustc::traits::select::SelectionContext::candidate_from_obligation                                                                                                           
  39: rustc::traits::select::SelectionContext::evaluate_stack                                                                                                                      
  40: rustc::ty::context::tls::with_context                                                                                                                                        
  41: rustc::dep_graph::graph::DepGraph::with_anon_task
  42: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  43: rustc::infer::InferCtxt::probe
  44: <&'a mut I as core::iter::iterator::Iterator>::next
  45: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  46: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache
  47: rustc::ty::context::tls::with_context
  48: rustc::dep_graph::graph::DepGraph::with_anon_task
  49: rustc::traits::select::SelectionContext::candidate_from_obligation
  50: rustc::traits::select::SelectionContext::evaluate_stack
  51: rustc::ty::context::tls::with_context
  52: rustc::dep_graph::graph::DepGraph::with_anon_task
  53: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  54: rustc::infer::InferCtxt::probe
  55: <&'a mut I as core::iter::iterator::Iterator>::next
  56: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  57: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache    
  58: rustc::ty::context::tls::with_context                                   
  59: rustc::dep_graph::graph::DepGraph::with_anon_task
  60: rustc::traits::select::SelectionContext::candidate_from_obligation
  61: rustc::traits::select::SelectionContext::evaluate_stack
  62: rustc::ty::context::tls::with_context                                                                                                                               [22/8490]
  63: rustc::dep_graph::graph::DepGraph::with_anon_task
  64: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  65: rustc::infer::InferCtxt::probe
  66: <&'a mut I as core::iter::iterator::Iterator>::next
  67: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  68: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache
  69: rustc::ty::context::tls::with_context
  70: rustc::dep_graph::graph::DepGraph::with_anon_task
  71: rustc::traits::select::SelectionContext::candidate_from_obligation
  72: rustc::traits::select::SelectionContext::evaluate_stack
  73: rustc::ty::context::tls::with_context
  74: rustc::dep_graph::graph::DepGraph::with_anon_task
  75: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  76: rustc::infer::InferCtxt::probe
  77: <&'a mut I as core::iter::iterator::Iterator>::next
  78: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  79: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache    
  80: rustc::ty::context::tls::with_context                                   
  81: rustc::dep_graph::graph::DepGraph::with_anon_task
  82: rustc::traits::select::SelectionContext::candidate_from_obligation
  83: rustc::traits::select::SelectionContext::evaluate_stack
  84: rustc::ty::context::tls::with_context
  85: rustc::dep_graph::graph::DepGraph::with_anon_task
  86: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  87: rustc::infer::InferCtxt::probe
  88: <&'a mut I as core::iter::iterator::Iterator>::next                                                        
  89: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  90: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache    
  91: rustc::ty::context::tls::with_context
  92: rustc::dep_graph::graph::DepGraph::with_anon_task             
  93: rustc::traits::select::SelectionContext::candidate_from_obligation
  94: rustc::traits::select::SelectionContext::evaluate_stack
  95: rustc::ty::context::tls::with_context
  96: rustc::dep_graph::graph::DepGraph::with_anon_task
  97: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  98: rustc::infer::InferCtxt::probe                
  99: <&'a mut I as core::iter::iterator::Iterator>::next                                                                                
query stack during panic:                              
#0 [evaluate_obligation] evaluating trait selection obligation `&'_ _: std::io::Read`
#1 [typeck_tables_of] pr  62: rustc::ty::context::tls::with_context                                                                                                                               [22/8490]
  63: rustc::dep_graph::graph::DepGraph::with_anon_task
  64: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  65: rustc::infer::InferCtxt::probe
  66: <&'a mut I as core::iter::iterator::Iterator>::next
  67: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  68: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache
  69: rustc::ty::context::tls::with_context
  70: rustc::dep_graph::graph::DepGraph::with_anon_task
  71: rustc::traits::select::SelectionContext::candidate_from_obligation
  72: rustc::traits::select::SelectionContext::evaluate_stack
  73: rustc::ty::context::tls::with_context
  74: rustc::dep_graph::graph::DepGraph::with_anon_task
  75: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  76: rustc::infer::InferCtxt::probe
  77: <&'a mut I as core::iter::iterator::Iterator>::next
  78: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  79: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache    
  80: rustc::ty::context::tls::with_context                                   
  81: rustc::dep_graph::graph::DepGraph::with_anon_task
  82: rustc::traits::select::SelectionContext::candidate_from_obligation
  83: rustc::traits::select::SelectionContext::evaluate_stack
  84: rustc::ty::context::tls::with_context
  85: rustc::dep_graph::graph::DepGraph::with_anon_task
  86: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  87: rustc::infer::InferCtxt::probe
  88: <&'a mut I as core::iter::iterator::Iterator>::next                                                        
  89: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T, I>>::from_iter
  90: rustc::traits::select::SelectionContext::candidate_from_obligation_no_cache    
  91: rustc::ty::context::tls::with_context
  92: rustc::dep_graph::graph::DepGraph::with_anon_task             
  93: rustc::traits::select::SelectionContext::candidate_from_obligation
  94: rustc::traits::select::SelectionContext::evaluate_stack
  95: rustc::ty::context::tls::with_context
  96: rustc::dep_graph::graph::DepGraph::with_anon_task
  97: rustc::traits::select::SelectionContext::evaluate_predicate_recursively
  98: rustc::infer::InferCtxt::probe                
  99: <&'a mut I as core::iter::iterator::Iterator>::next                                                                                
query stack during panic:                              
#0 [evaluate_obligation] evaluating trait selection obligation `&'_ _: std::io::Read`
#1 [typeck_tables_of] processing `child::mongod::Mongod::lookup_block_by_hash`
#2 [typeck_item_bodies] type-checking all item bodies
end of query stack                         
error: aborting due to previous error ocessing `child::mongod::Mongod::lookup_block_by_hash`
#2 [typeck_item_bodies] type-checking all item bodies
end of query stack                         
error: aborting due to previous error 

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 1.29.0-nightly (31f1bc7b4 2018-07-15) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C incremental --crate-type bin

note: some of the compiler flags provided by cargo are hidden

I'm sorry for no more info. I'll try to provide when I can.

[dpc]

Same issue on stable

[dpc]

I couldn't really minimize, and I can't publish the code verbatim, but more or less it had something to do with

if let Some(ref block_record) = cursor
            .map(|document| {
                let document = document.unwrap();
                bson::from_bson(bson::Bson::Document(document)).unwrap()
            })
            .next() {
    let block_record: BlocksRecord = block_record;
    Ok(block_record)
} else {
/...
}

BlocksRecord ia a simple struct that holds bunch of fields of the record fetched from mongodb.

cursor was https://docs.rs/mongo_driver/0.12.1/mongo_driver/cursor/struct.Cursor.html (note it has a <'a> . Adding that ref that triggered the problem.

[ljedrz]

Looks like a trait evaluation overflow error. Is this actually a bug or is this evaluation just impossible? In case of the latter, wouldn't it just be better to report that with a regular error?

[ljedrz]

The trace looks quite similar to #52356.

[Centril]

@dpc Were you able to produce a minimal example?

[dpc]

@Centril I have failed. And that's almost 2 years old now. I don't remember which project or code that was. I think we can close.