ICE when trying to match on non-PartialEq slice.

[matthewjasper]

The following code will result in an ICE in codegen MIR construction:

struct B(i32);

const A: &[B] = &[];

fn main() {
    match &[][..] {
        A => (),
        _ => (),
    }
}

playground

Error and Backtrace

error: internal compiler error: src/librustc/traits/codegen/mod.rs:156: Encountered errors `[FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<B as std::cmp::PartialEq>)),depth=2),Unimplemented)]` resolving bounds after type-checking

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:570:9
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:

 0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:197
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:211
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   6: std::panicking::begin_panic
   7: rustc_errors::Handler::span_bug
   8: rustc::util::bug::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::util::bug::opt_span_bug_fmt
  13: rustc::util::bug::span_bug_fmt
  14: rustc::traits::codegen::<impl rustc::infer::InferCtxt>::drain_fulfillment_cx_or_panic
  15: rustc::ty::context::GlobalCtxt::enter_local
  16: rustc::traits::codegen::codegen_fulfill_obligation
  17: rustc::ty::query::__query_compute::codegen_fulfill_obligation
  18: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::codegen_fulfill_obligation>::compute
  19: rustc::dep_graph::graph::DepGraph::with_task_impl
  20: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  21: rustc::ty::instance::Instance::resolve
  22: rustc_mir::lints::check
  23: rustc::ty::context::GlobalCtxt::enter_local
  24: rustc_mir::build::mir_build
  25: rustc_mir::transform::mir_built
  26: rustc::ty::query::__query_compute::mir_built
  27: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::mir_built>::compute
  28: rustc::dep_graph::graph::DepGraph::with_task_impl
  29: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  30: rustc_mir::transform::check_unsafety::unsafety_check_result
  31: rustc::ty::query::__query_compute::unsafety_check_result
  32: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::unsafety_check_result>::compute
  33: rustc::dep_graph::graph::DepGraph::with_task_impl
  34: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  35: rustc_mir::transform::mir_const
  36: rustc::ty::query::__query_compute::mir_const
  37: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::mir_const>::compute
  38: rustc::dep_graph::graph::DepGraph::with_task_impl
  39: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  40: rustc_mir::transform::mir_validated
  41: rustc::ty::query::__query_compute::mir_validated
  42: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::mir_validated>::compute
  43: rustc::dep_graph::graph::DepGraph::with_task_impl
  44: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  45: rustc_mir::borrow_check::mir_borrowck
  46: rustc::ty::query::__query_compute::mir_borrowck
  47: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::mir_borrowck>::compute
  48: rustc::dep_graph::graph::DepGraph::with_task_impl
  49: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  50: rustc::ty::<impl rustc::ty::context::TyCtxt>::par_body_owners
  51: rustc::util::common::time
  52: rustc_interface::passes::analysis
  53: rustc::ty::query::__query_compute::analysis
  54: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::analysis>::compute
  55: rustc::dep_graph::graph::DepGraph::with_task_impl
  56: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  57: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  58: rustc_interface::passes::create_global_ctxt::{{closure}}
  59: rustc_interface::interface::run_compiler_in_existing_thread_pool
  60: std::thread::local::LocalKey<T>::with
  61: scoped_tls::ScopedKey<T>::set
  62: syntax::with_globals
query stack during panic:
#0 [codegen_fulfill_obligation] checking if `std::cmp::PartialEq` fulfills its obligations
#1 [mir_built] processing `main`
#2 [unsafety_check_result] processing `main`
#3 [mir_const] processing `main`
#4 [mir_validated] processing `main`
#5 [mir_borrowck] processing `main`
#6 [analysis] running analysis passes on this crate
end of query stack

[estebank]

CC #53708

[pnkfelix]

As implied by @estebank above, this seems very likely to be a duplicate (or at least strongly related to) #53708 ...

Update: Not duplicate. Further investigation of #53708 shows that it is no longer an ICE, but "merely" an exhaustiveness checker bug. This bug, #61188, remains an ICE today.

[pnkfelix]

Ah ha!

If you add #[derive(PartialEq, Eq)] to the struct B(i32) in the example above, then the code compiles successfully.

Which may be something a user might infer from the message ...

Encountered errors [FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<B as std::cmp::PartialEq>)),depth=2),Unimplemented)]

... but probably not 🤣

[pnkfelix]

triage: P-high; removing nomination. I want to know why we are not catching the failure to fulfill B: PartialEq earlier in the compilation. Is this perhaps a const-eval issue, based on the stack trace? (or a mir-borrowck one? I suppose the query-system is just making that appear on the stack, though it is not necessarily the root cause?)

[eddyb]

I suspect rustc_typeck is not enforcing it, and MIR borrowck, as I found recently, doesn't produce user errors from trait matching, but rather expects it to succeed, and only uses it to get lifetime obligations.

EDIT: I should've read more carefully, looks like Instance::resolve is being used by rustc_mir::build without rustc_typeck having checked that Eq is implemented for that type.

[pnkfelix]

assigning to self with intent to try to mentor.