Const generic ICE: assertion failed: key.value.promoted.is_none()

[goodmanjonathan]

Promotion of const generic parameters causes the compiler to crash.

#![feature(const_generics)]

fn promote<const N: i32>() {
    // works:
    //
    // let n = N;
    // &n;
    
    &N;
}

fn main() {
    promote::<0>();
}

$ RUST_BACKTRACE=1 rustc +nightly promotion_const_generics_ice.rs
warning: the feature `const_generics` is incomplete and may cause the compiler to crash
 --> promotion_const_generics_ice.rs:1:12
  |
1 | #![feature(const_generics)]
  |            ^^^^^^^^^^^^^^

thread 'rustc' panicked at 'assertion failed: key.value.promoted.is_none()', src/librustc_mir/const_eval.rs:577:17
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.25/src/backtrace/libunwind.rs:97
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.25/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:47
   3: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:36
   4: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:197
   5: std::panicking::default_hook
             at src/libstd/panicking.rs:211
   6: rustc::util::common::panic_hook
   7: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   8: std::panicking::begin_panic
   9: rustc_mir::const_eval::const_eval_provider
  10: rustc::ty::query::__query_compute::const_eval
  11: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval>::compute
  12: rustc::dep_graph::graph::DepGraph::with_task_impl
  13: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  14: rustc_mir::monomorphize::collector::collect_items_rec
  15: rustc_mir::monomorphize::collector::collect_items_rec
  16: rustc_mir::monomorphize::collector::collect_crate_mono_items::{{closure}}
  17: rustc::util::common::time
  18: rustc_mir::monomorphize::collector::collect_crate_mono_items
  19: rustc::util::common::time
  20: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  21: rustc::ty::query::__query_compute::collect_and_partition_mono_items
  22: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::collect_and_partition_mono_items>::compute
  23: rustc::dep_graph::graph::DepGraph::with_task_impl
  24: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  25: rustc_codegen_ssa::base::codegen_crate
  26: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  27: rustc::util::common::time
  28: rustc_interface::passes::start_codegen
  29: rustc::ty::context::tls::enter_global
  30: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  31: rustc_interface::passes::create_global_ctxt::{{closure}}
  32: rustc_interface::passes::BoxedGlobalCtxt::enter
  33: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::ongoing_codegen
  34: rustc_interface::interface::run_compiler_in_existing_thread_pool
  35: std::thread::local::LocalKey<T>::with
  36: scoped_tls::ScopedKey<T>::set
  37: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
query stack during panic:
#0 [const_eval] const-evaluating + checking `promote`
#1 [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 1.37.0-nightly (7840a0b75 2019-05-31) running on x86_64-unknown-linux-gnu

[varkor]

This is happening here:
https://github.com/rust-lang/rust/blob/73b67c255a113fd0f94e00eb51be2f1eb11ce39a/src/librustc_mir/const_eval.rs#L572-L576
cc @oli-obk

[czipperz]

So what is the expected behavior here?

[oli-obk]

@varkor why shouldn't we? Like ok, we shouldn't right now, but why shouldn't ever? We are already promoting &std::mem::size_of::<T>() which seems just as problematic. This is just a limitation of the miri engine, that I wager will be simple to solve. Especially when looking at where this is happening (in the collector). Everything is monomorphic at that point I believe?

[varkor]

Ah, that is true. It should be okay, you're right.

[oli-obk]

Ok, awesome, I was worried for a bit that we screwed up promotion. Now let's figure out what's going wrong here.

As far as I can tell, the only way to get to a TooGeneric is by substituting and ending up with an unsubstituted (needs_subst) value. Our substitution machinery is a little whacky as clippy has already encountered, but if size_of calls can get promoted, I don't know why const generics would go down a different path. We'll need to run the repro from this issue through a rustc with RUSTC_LOG=rustc_mir::interpret,rustc::mir::interpret RUST_CTFE_BACKTRACE=immediate to get some detailed info on those errors.

[jplatte]

This also happens when passing a const generic variable (directly) to println! and friends.

[oli-obk]

🤣 uh, so, I found this

rust/src/librustc_mir/interpret/operand.rs

Line 526 in 811b996

ConstValue::Param(_) => return err!(TooGeneric), // FIXME(oli-obk): try to monomorphize

Basically the fix is to call self.monomorphize on constant.literal in

rust/src/librustc_mir/interpret/operand.rs

Line 497 in 811b996

Constant(ref constant) => self.eval_const_to_op(constant.literal, layout)?,

and turn the return in

rust/src/librustc_mir/interpret/operand.rs

Line 526 in 811b996

ConstValue::Param(_) => return err!(TooGeneric), // FIXME(oli-obk): try to monomorphize

into a bug! call

This change is blocked on the eval_context.rs changes from #62012

[varkor]

Making this change:

-            Constant(ref constant) => self.eval_const_to_op(constant.literal, layout)?,
+            Constant(ref constant) => {
+                self.eval_const_to_op(self.monomorphize(constant.literal)?, layout)?
+            }

results in:

error: internal compiler error: src/librustc/ty/subst.rs:557: type parameter `Self/#0` (Self/0) out of range when substituting (root type=Some(<Self as float::Float>::Int)) substs=[]

while compiling rustc.

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:578:9
stack backtrace:
   0: std::sys_common::backtrace::print
   1: std::panicking::default_hook::{{closure}}
   2: std::panicking::default_hook
   3: rustc::util::common::panic_hook
   4: std::panicking::rust_panic_with_hook
   5: std::panicking::begin_panic
   6: rustc_errors::Handler::span_bug
   7: rustc::util::bug::opt_span_bug_fmt::{{closure}}
   8: rustc::ty::context::tls::with_opt::{{closure}}
   9: rustc::ty::context::tls::with_context_opt
  10: rustc::ty::context::tls::with_opt
  11: rustc::util::bug::opt_span_bug_fmt
  12: rustc::util::bug::span_bug_fmt
  13: <rustc::ty::subst::SubstFolder as rustc::ty::fold::TypeFolder>::fold_ty
  14: <smallvec::SmallVec<A> as core::iter::traits::collect::FromIterator<<A as smallvec::Array>::Item>>::from_iter
  15: rustc::ty::fold::TypeFoldable::fold_with
  16: rustc::ty::structural_impls::<impl rustc::ty::fold::TypeFoldable for &rustc::ty::TyS>::super_fold_with
  17: <rustc::ty::subst::SubstFolder as rustc::ty::fold::TypeFolder>::fold_ty
  18: <rustc::ty::subst::SubstFolder as rustc::ty::fold::TypeFolder>::fold_const
  19: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpretCx<M>>::eval_operand
  20: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpretCx<M>>::run
  21: rustc_mir::const_eval::eval_body_using_ecx
  22: rustc_mir::const_eval::const_eval_raw_provider
  23: rustc::ty::query::__query_compute::const_eval_raw
  24: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval_raw>::compute
  25: rustc::dep_graph::graph::DepGraph::with_task_impl
  26: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  27: rustc_mir::const_eval::const_eval_provider
  28: rustc::ty::query::__query_compute::const_eval
  29: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval>::compute
  30: rustc::dep_graph::graph::DepGraph::with_task_impl
  31: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  32: rustc_mir::const_eval::const_eval_provider
  33: rustc::ty::query::__query_compute::const_eval
  34: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::const_eval>::compute
  35: rustc::dep_graph::graph::DepGraph::with_task_impl
  36: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  37: rustc_mir::monomorphize::collector::collect_const
  38: rustc::mir::visit::Visitor::super_rvalue
  39: <rustc_mir::monomorphize::collector::MirNeighborCollector as rustc::mir::visit::Visitor>::visit_rvalue
  40: rustc::mir::visit::Visitor::visit_body
  41: rustc_mir::monomorphize::collector::collect_items_rec
  42: rustc_mir::monomorphize::collector::collect_items_rec
  43: rustc_mir::monomorphize::collector::collect_crate_mono_items::{{closure}}
  44: rustc::util::common::time
  45: rustc_mir::monomorphize::collector::collect_crate_mono_items
  46: rustc::util::common::time
  47: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  48: rustc::ty::query::__query_compute::collect_and_partition_mono_items
  49: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::collect_and_partition_mono_items>::compute
  50: rustc::dep_graph::graph::DepGraph::with_task_impl
  51: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  52: rustc_codegen_ssa::base::codegen_crate
  53: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  54: rustc::util::common::time
  55: rustc_interface::passes::start_codegen
  56: rustc::ty::context::tls::enter_global
  57: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  58: rustc_interface::passes::create_global_ctxt::{{closure}}
  59: rustc_interface::passes::BoxedGlobalCtxt::enter
  60: rustc_interface::queries::Query<T>::compute
  61: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::ongoing_codegen
  62: rustc_interface::interface::run_compiler_in_existing_thread_pool
  63: std::thread::local::LocalKey<T>::with
  64: scoped_tls::ScopedKey<T>::set
  65: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
query stack during panic:
#0 [const_eval_raw] const-evaluating `<f32 as float::Float>::EXPONENT_MASK`
#1 [const_eval] const-evaluating + checking `<f32 as float::Float>::EXPONENT_MASK`
#2 [const_eval] const-evaluating + checking `<f32 as float::Float>::EXPONENT_MASK`
#3 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: aborting due to previous error

(I haven't looked into the error yet.)

[oli-obk]

heh, one can always trust in libapfloat to expose const eval bugs

can you run with RUSTC_LOG=rustc_mir::interpret and dump that log, too? It looks to me like the instance field of the Frame has the wrong Substs

[JohnTitor]

@oli-obk FWIW I got this log.