Compiler panic with associated constant

[etwyniel]

The compiler panics when I use a generic associated constant from within the impl.

I was able to reproduce it with this code:

struct S<T>(T);

impl<T> S<T> {
    const ID: fn(&S<T>) -> &S<T> = |s| s;
    pub fn id(&self) -> &Self {
        Self::ID(self)
    }
}

fn main() {
    let s = S(10u32);
    assert!(S::<u32>::ID(&s).0 == 10); // Works fine
    assert!(s.id().0 == 10); // Causes compiler to panic
}

Meta

rustc --version --verbose:

rustc 1.32.0 (9fda7c2 2019-01-16)
binary: rustc
commit-hash: 9fda7c2
commit-date: 2019-01-16
host: x86_64-unknown-linux-gnu
release: 1.32.0
LLVM version: 8.0

Backtrace:

thread 'main' panicked at 'assertion failed: !value.needs_subst()', src/librustc/traits/query/normalize_erasing_regions.rs:69:9
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   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:211
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:227
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:495
   6: std::panicking::begin_panic
   7: rustc::traits::query::normalize_erasing_regions::<impl rustc::ty::context::TyCtxt<'cx, 'tcx, 'tcx>>::normalize_erasing_late_bound_regions
   8: rustc::ty::instance::Instance::resolve_closure
   9: rustc_mir::interpret::cast::<impl rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::cast
  10: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::run
  11: rustc_mir::const_eval::eval_body_using_ecx
  12: rustc_mir::const_eval::const_eval_raw_provider
  13: rustc::ty::query::__query_compute::const_eval_raw
  14: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::const_eval_raw<'tcx>>::compute
  15: rustc::dep_graph::graph::DepGraph::with_task_impl
  16: <rustc::ty::query::plumbing::JobOwner<'a, 'tcx, Q>>::start
  17: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  18: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  19: rustc::ty::query::TyCtxtAt::const_eval_raw
  20: <rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::const_eval_raw
  21: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::const_value_to_op
  22: rustc_mir::const_eval::const_to_op
  23: rustc_mir::transform::const_prop::ConstPropagator::eval_constant
  24: <rustc_mir::transform::const_prop::ConstPropagator<'b, 'a, 'tcx> as rustc::mir::visit::Visitor<'tcx>>::visit_terminator_kind
  25: <rustc_mir::transform::const_prop::ConstProp as rustc_mir::transform::MirPass>::run_pass
  26: rustc_mir::transform::run_passes::{{closure}}
  27: rustc_mir::transform::run_passes
  28: rustc_mir::transform::optimized_mir
  29: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::optimized_mir<'tcx>>::compute
  30: rustc::dep_graph::graph::DepGraph::with_task_impl
  31: <rustc::ty::query::plumbing::JobOwner<'a, 'tcx, Q>>::start
  32: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  33: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_get_query
  34: rustc::ty::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::instance_mir
  35: rustc_mir::monomorphize::collector::collect_items_rec
  36: rustc_mir::monomorphize::collector::collect_items_rec
  37: rustc_mir::monomorphize::collector::collect_crate_mono_items::{{closure}}
  38: rustc::util::common::time
  39: rustc_mir::monomorphize::collector::collect_crate_mono_items
  40: rustc::util::common::time
  41: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  42: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::collect_and_partition_mono_items<'tcx>>::compute
  43: rustc::dep_graph::graph::DepGraph::with_task_impl
  44: <rustc::ty::query::plumbing::JobOwner<'a, 'tcx, Q>>::start
  45: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  46: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  47: rustc_codegen_ssa::base::codegen_crate
  48: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  49: rustc::util::common::time
  50: rustc_driver::driver::phase_4_codegen
  51: rustc_driver::driver::compile_input::{{closure}}
  52: rustc::ty::context::tls::enter_context
  53: <std::thread::local::LocalKey<T>>::with
  54: rustc::ty::context::TyCtxt::create_and_enter
  55: rustc_driver::driver::compile_input
  56: rustc_driver::run_compiler_with_pool
  57: <scoped_tls::ScopedKey<T>>::set
  58: rustc_driver::run_compiler
  59: rustc_driver::monitor::{{closure}}
  60: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:102
  61: rustc_driver::run
  62: rustc_driver::main
  63: std::rt::lang_start::{{closure}}
  64: std::panicking::try::do_call
             at src/libstd/rt.rs:59
             at src/libstd/panicking.rs:310
  65: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:102
  66: std::rt::lang_start_internal
             at src/libstd/panicking.rs:289
             at src/libstd/panic.rs:398
             at src/libstd/rt.rs:58
  67: main
  68: __libc_start_main
  69: <unknown>
query stack during panic:
#0 [const_eval_raw] const-evaluating `<S<T>>::ID`
 --> associated_generic.rs:6:9
  |
6 |         Self::ID(self)
  |         ^^^^^^^^^^^^^^
#1 [optimized_mir] processing `<S<T>>::id`
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack

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 1.32.0 (9fda7c223 2019-01-16) running on x86_64-unknown-linux-gnu

[Centril]

This is broken on stable and beta but works on nightly. Reduced:

struct _S<T>(T);

impl<T> _S<T> {
    const _ID: fn(_S<T>) = |_| {};

    pub fn _id(self) {
        _S::<T>::_ID(self)
    }
}

fn _foo() {
    _S(())._id();
}

Might be worth beta-backporting whatever we did to fix it on nightly?

[hellow554]

Bug was fixed between 01f8e25...bf669d1 (nightly-2019-01-24 and 01-25)

[pnkfelix]

triage: assigning to self to attempt to identify backport.

[pnkfelix]

01f8e25...bf669d1

The bors merges in this range were:

% git log 01f8e25...bf669d1 --author bors --format=oneline

• bf669d1 Auto merge of Update Cargo.lock to use the latest compiler_builtins #57435 - Dylan-DPC:fi/compiler-builtins-lock, r=alexcrichton
• 0b1669d Auto merge of [NLL] Clean up handling of type annotations #57714 - matthewjasper:wellformed-unreachable, r=pnkfelix
• 7187db6 Auto merge of bootstrap: Don't rely on any default settings regarding incr. comp. in Cargo #57891 - michaelwoerister:no-default-incr-bootstrap, r=pietroalbini
• 5d74d75 Auto merge of submodules: update clippy from 280069dd to f1753522 #57881 - matthiaskrgr:clippy_submodule_upd, r=oli-obk
• a41ade7 Auto merge of Rollup of 5 pull requests #57888 - Centril:rollup, r=Centril
  • Add grammar in docs for {f32,f64}::from_str, mention known bug. #56217 (Add grammar in docs for {f32,f64}::from_str, mention known bug.)
  • When using value after move, point at span of local #57294 (When using value after move, point at span of local)
  • Update/remove some old readmes #57652 (Update/remove some old readmes)
  • Print visible name for types as well as modules. #57802 (Print visible name for types as well as modules.)
  • Don't ICE when logging unusual types #57865 (Don't ICE when logging unusual types)
• 278067d Auto merge of Rollup of 9 pull requests #57879 - Centril:rollup, r=Centril
  • Fix Instant/Duration math precision & associativity on Windows #57380 (Fix Instant/Duration math precision & associativity on Windows)
  • Get rid of the fake stack frame for reading from constants #57606 (Get rid of the fake stack frame for reading from constants)
  • Several changes to libunwind for SGX target #57803 (Several changes to libunwind for SGX target)
  • rustdoc: fix ICE from loading proc-macro stubs #57846 (rustdoc: fix ICE from loading proc-macro stubs)
  • Add os::fortanix_sgx::ffi module #57860 (Add os::fortanix_sgx::ffi module)
  • Don't export table by default in wasm #57861 (Don't export table by default in wasm)
  • Add suggestion for incorrect field syntax. #57863 (Add suggestion for incorrect field syntax.)
  • Fix std::future::from_generator documentation #57867 (Fix std::future::from_generator documentation)
  • Stabilize no_panic_pow #57873 (Stabilize no_panic_pow)

[pnkfelix]

Initial guesses as to which PR(s) in list above seem most relevant: #57606 ...

(That's assuming that the const-eval bit in the stack trace is the most relevant clue as to the cause of the ICE.) all the others don't seem to touch const-eval.

[pnkfelix]

But, wait, what am I doing here...

at this point enough time has passed that the current beta is able to handle the example without ICE'ing.

This bug does not seem worthy of attempting a backport all the way to stable.

(In other words, the time to attempt a backport would have been prior to when the beta-next was cut from the nightly, which was around a week ago.)