ICE error in Rust compiler: ty::Const::eval called on a non-valtree-compatible type

[iamanonymouscs]

Code

struct Checked<const F: fn(usize) -> bool>;
fn not_one(val: usize) -> bool { val != 1 }
fn not_two(val: usize) -> bool { val != 2 }
fn main() {
    let _ = Checked::<{const _: Checked<not_one> = Checked::<not_one>;
    let _: Checked<not_one> = Checked::<not_two>;
    let _: Checked<not_one> = Checked}>;
    
}

Meta

rustc --version --verbose:

rustc 1.74.0 (79e9716c9 2023-11-13)
binary: rustc
commit-hash: 79e9716c980570bfd1f666e3b16ac583f0168962
commit-date: 2023-11-13
host: x86_64-unknown-linux-gnu
release: 1.74.0
LLVM version: 17.0.4

The same problem is reproduced by rustc 1.76.0-nightly (37b2813 2023-11-24).

Error output

error: expected one of `!`, `.`, `::`, `;`, `?`, `else`, `{`, or an operator, found `}`
 --> code.rs:7:38
  |
7 |     let _: Checked<not_one> = Checked}>;
  |                                      ^ expected one of 8 possible tokens

error: using function pointers as const generic parameters is forbidden
 --> code.rs:1:25
  |
1 | struct Checked<const F: fn(usize) -> bool>;
  |                         ^^^^^^^^^^^^^^^^^
  |
  = note: the only supported types are integers, `bool` and `char`
thread 'rustc' panicked at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/compiler/rustc_middle/src/ty/consts.rs:295:22:
`ty::Const::eval` called on a non-valtree-compatible type

Backtrace

thread 'rustc' panicked at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/compiler/rustc_middle/src/ty/consts.rs:295:22:
`ty::Const::eval` called on a non-valtree-compatible type
stack backtrace:
   0:     0x7f2d78b7e33c - std::backtrace_rs::backtrace::libunwind::trace::h67a838aed1f4d6ec
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7f2d78b7e33c - std::backtrace_rs::backtrace::trace_unsynchronized::h1d1786bb1962baf8
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f2d78b7e33c - std::sys_common::backtrace::_print_fmt::h5a0b1f807a002d23
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/std/src/sys_common/backtrace.rs:67:5
   3:     0x7f2d78b7e33c - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hf84ab6ad0b91784c
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f2d78be3f9c - core::fmt::rt::Argument::fmt::h28f463bd1fdabed5
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/core/src/fmt/rt.rs:138:9
   5:     0x7f2d78be3f9c - core::fmt::write::ha37c23b175e921b3
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/core/src/fmt/mod.rs:1114:21
   6:     0x7f2d78b710be - std::io::Write::write_fmt::haa1b000741bcbbe1
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/std/src/io/mod.rs:1763:15
   7:     0x7f2d78b7e124 - std::sys_common::backtrace::_print::h1ff1030b04dfb157
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7f2d78b7e124 - std::sys_common::backtrace::print::hb982056c6f29541c
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7f2d78b81003 - std::panicking::default_hook::{{closure}}::h11f92f82c62fbd68
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/std/src/panicking.rs:272:22
  10:     0x7f2d78b80d24 - std::panicking::default_hook::hb8810fe276772c66
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/std/src/panicking.rs:292:9
  11:     0x7f2d7bd196ff - std[75110f2ce3cbb2c9]::panicking::update_hook::<alloc[e150e82988d12314]::boxed::Box<rustc_driver_impl[47b5c41fd5152b42]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x7f2d78b81831 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h87b887549356728a
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/alloc/src/boxed.rs:2021:9
  13:     0x7f2d78b81831 - std::panicking::rust_panic_with_hook::hd2f0efd2fec86cb0
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/std/src/panicking.rs:735:13
  14:     0x7f2d78b815b1 - std::panicking::begin_panic_handler::{{closure}}::h3651b7fc4f61d784
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/std/src/panicking.rs:609:13
  15:     0x7f2d78b7e866 - std::sys_common::backtrace::__rust_end_short_backtrace::hbc468e4b98c7ae04
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/std/src/sys_common/backtrace.rs:170:18
  16:     0x7f2d78b81302 - rust_begin_unwind
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/std/src/panicking.rs:597:5
  17:     0x7f2d78be0625 - core::panicking::panic_fmt::h979245e2fdb2fabd
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/core/src/panicking.rs:72:14
  18:     0x7f2d78be0373 - core::panicking::panic_display::h9b355c58fd35af38
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/core/src/panicking.rs:168:5
  19:     0x7f2d78be0373 - core::panicking::panic_str::h187a5146d72e7d2f
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/core/src/panicking.rs:152:5
  20:     0x7f2d78be0373 - core::option::expect_failed::h7cdfa49208a82a89
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/core/src/option.rs:1988:5
  21:     0x7f2d7a577b57 - <rustc_middle[cfbee9f1a0b252f]::ty::consts::Const>::normalize
  22:     0x7f2d7a574b54 - <rustc_trait_selection[cde77d364afa16a7]::traits::project::AssocTypeNormalizer as rustc_type_ir[22f713ae44c5fda6]::fold::TypeFolder<rustc_middle[cfbee9f1a0b252f]::ty::context::TyCtxt>>::fold_const
  23:     0x7f2d7a55b03f - <&rustc_middle[cfbee9f1a0b252f]::ty::list::List<rustc_middle[cfbee9f1a0b252f]::ty::generic_args::GenericArg> as rustc_type_ir[22f713ae44c5fda6]::fold::TypeFoldable<rustc_middle[cfbee9f1a0b252f]::ty::context::TyCtxt>>::try_fold_with::<rustc_trait_selection[cde77d364afa16a7]::traits::project::AssocTypeNormalizer>
  24:     0x7f2d7a55b454 - <rustc_trait_selection[cde77d364afa16a7]::traits::project::AssocTypeNormalizer as rustc_type_ir[22f713ae44c5fda6]::fold::TypeFolder<rustc_middle[cfbee9f1a0b252f]::ty::context::TyCtxt>>::fold_ty
  25:     0x7f2d7a92c68a - <rustc_trait_selection[cde77d364afa16a7]::traits::engine::ObligationCtxt>::normalize::<rustc_middle[cfbee9f1a0b252f]::ty::Ty>
  26:     0x7f2d7a93f5ae - rustc_hir_analysis[de820db48f542fb9]::check::wfcheck::check_item_type
  27:     0x7f2d7a9220ea - rustc_hir_analysis[de820db48f542fb9]::check::wfcheck::check_well_formed
  28:     0x7f2d79e2248c - rustc_query_impl[252fba113696aa73]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[252fba113696aa73]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[cfbee9f1a0b252f]::query::erase::Erased<[u8; 0usize]>>
  29:     0x7f2d7a7cbd7b - rustc_query_system[835e2ed124ae29cf]::query::plumbing::try_execute_query::<rustc_query_impl[252fba113696aa73]::DynamicConfig<rustc_query_system[835e2ed124ae29cf]::query::caches::VecCache<rustc_hir[174f47e87bf43e7f]::hir_id::OwnerId, rustc_middle[cfbee9f1a0b252f]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[252fba113696aa73]::plumbing::QueryCtxt, false>
  30:     0x7f2d7a7cba84 - rustc_query_impl[252fba113696aa73]::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
  31:     0x7f2d7ad1f8a8 - rustc_data_structures[7bd4ad184b36c2b5]::sync::parallel::disabled::par_for_each_in::<&[rustc_hir[174f47e87bf43e7f]::hir::ImplItemId], <rustc_middle[cfbee9f1a0b252f]::hir::ModuleItems>::par_impl_items<rustc_hir_analysis[de820db48f542fb9]::check::wfcheck::check_mod_type_wf::{closure#1}>::{closure#0}>
  32:     0x7f2d7ad1f597 - rustc_hir_analysis[de820db48f542fb9]::check::wfcheck::check_mod_type_wf
  33:     0x7f2d7ab98c2c - rustc_query_impl[252fba113696aa73]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[252fba113696aa73]::query_impl::check_mod_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[cfbee9f1a0b252f]::query::erase::Erased<[u8; 0usize]>>
  34:     0x7f2d7ab98c0e - <rustc_query_impl[252fba113696aa73]::query_impl::check_mod_type_wf::dynamic_query::{closure#2} as core[5a612027cd243909]::ops::function::FnOnce<(rustc_middle[cfbee9f1a0b252f]::ty::context::TyCtxt, rustc_span[56b754c3614a230e]::def_id::LocalModDefId)>>::call_once
  35:     0x7f2d7ad04ff8 - rustc_query_system[835e2ed124ae29cf]::query::plumbing::try_execute_query::<rustc_query_impl[252fba113696aa73]::DynamicConfig<rustc_query_system[835e2ed124ae29cf]::query::caches::DefaultCache<rustc_span[56b754c3614a230e]::def_id::LocalModDefId, rustc_middle[cfbee9f1a0b252f]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[252fba113696aa73]::plumbing::QueryCtxt, false>
  36:     0x7f2d7b665304 - rustc_query_impl[252fba113696aa73]::query_impl::check_mod_type_wf::get_query_non_incr::__rust_end_short_backtrace
  37:     0x7f2d7b22cd54 - rustc_hir_analysis[de820db48f542fb9]::check_crate
  38:     0x7f2d7b227972 - rustc_interface[dac25a6ec31f00eb]::passes::analysis
  39:     0x7f2d7b1f93ea - rustc_query_impl[252fba113696aa73]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[252fba113696aa73]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[cfbee9f1a0b252f]::query::erase::Erased<[u8; 1usize]>>
  40:     0x7f2d7b1f93d9 - <rustc_query_impl[252fba113696aa73]::query_impl::analysis::dynamic_query::{closure#2} as core[5a612027cd243909]::ops::function::FnOnce<(rustc_middle[cfbee9f1a0b252f]::ty::context::TyCtxt, ())>>::call_once
  41:     0x7f2d7b3ee984 - rustc_query_system[835e2ed124ae29cf]::query::plumbing::try_execute_query::<rustc_query_impl[252fba113696aa73]::DynamicConfig<rustc_query_system[835e2ed124ae29cf]::query::caches::SingleCache<rustc_middle[cfbee9f1a0b252f]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[252fba113696aa73]::plumbing::QueryCtxt, false>
  42:     0x7f2d7b3ee6f9 - rustc_query_impl[252fba113696aa73]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  43:     0x7f2d7aea9632 - <rustc_middle[cfbee9f1a0b252f]::ty::context::GlobalCtxt>::enter::<rustc_driver_impl[47b5c41fd5152b42]::run_compiler::{closure#1}::{closure#2}::{closure#6}, core[5a612027cd243909]::result::Result<(), rustc_span[56b754c3614a230e]::ErrorGuaranteed>>
  44:     0x7f2d7aea80b1 - rustc_span[56b754c3614a230e]::set_source_map::<core[5a612027cd243909]::result::Result<(), rustc_span[56b754c3614a230e]::ErrorGuaranteed>, rustc_interface[dac25a6ec31f00eb]::interface::run_compiler<core[5a612027cd243909]::result::Result<(), rustc_span[56b754c3614a230e]::ErrorGuaranteed>, rustc_driver_impl[47b5c41fd5152b42]::run_compiler::{closure#1}>::{closure#0}::{closure#0}>
  45:     0x7f2d7aea2c90 - std[75110f2ce3cbb2c9]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[dac25a6ec31f00eb]::util::run_in_thread_with_globals<rustc_interface[dac25a6ec31f00eb]::interface::run_compiler<core[5a612027cd243909]::result::Result<(), rustc_span[56b754c3614a230e]::ErrorGuaranteed>, rustc_driver_impl[47b5c41fd5152b42]::run_compiler::{closure#1}>::{closure#0}, core[5a612027cd243909]::result::Result<(), rustc_span[56b754c3614a230e]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[5a612027cd243909]::result::Result<(), rustc_span[56b754c3614a230e]::ErrorGuaranteed>>
  46:     0x7f2d7b56a9ee - <<std[75110f2ce3cbb2c9]::thread::Builder>::spawn_unchecked_<rustc_interface[dac25a6ec31f00eb]::util::run_in_thread_with_globals<rustc_interface[dac25a6ec31f00eb]::interface::run_compiler<core[5a612027cd243909]::result::Result<(), rustc_span[56b754c3614a230e]::ErrorGuaranteed>, rustc_driver_impl[47b5c41fd5152b42]::run_compiler::{closure#1}>::{closure#0}, core[5a612027cd243909]::result::Result<(), rustc_span[56b754c3614a230e]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[5a612027cd243909]::result::Result<(), rustc_span[56b754c3614a230e]::ErrorGuaranteed>>::{closure#1} as core[5a612027cd243909]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  47:     0x7f2d78b8c395 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hfa37c25e0ad051b0
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/alloc/src/boxed.rs:2007:9
  48:     0x7f2d78b8c395 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h9486bed8ab2e65ad
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/alloc/src/boxed.rs:2007:9
  49:     0x7f2d78b8c395 - std::sys::unix::thread::Thread::new::thread_start::hd28b46dbf5673d17
                               at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/std/src/sys/unix/thread.rs:108:17
  50:     0x7f2d78a50609 - start_thread
  51:     0x7f2d78973133 - clone
  52:                0x0 - <unknown>

error: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.74.0 (79e9716c9 2023-11-13) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [check_well_formed] checking that `main::{constant#0}::_` is well-formed
#1 [check_mod_type_wf] checking that types are well-formed in top-level module
#2 [analysis] running analysis passes on this crate
end of query stack

[saethlin]

Oo this looks like #118098 but it's much smaller and doesn't need any features.

[saethlin]

searched nightlies: from nightly-2023-01-01 to nightly-2023-11-20
regressed nightly: nightly-2023-09-14
searched commit range: d9c8274...8142a31
regressed commit: 76e59c7

bisected with cargo-bisect-rustc v0.6.7

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start=2023-01-01 --end=2023-11-20 --regress=ice 

cc @RalfJung

[gurry]

@rustbot claim

[RalfJung]

So that would be #115803 then. The panic originates here.

[RalfJung]

using function pointers as const generic parameters is forbidden

After that error, something is still trying to create a valtree from a function pointer, it seems. I'd say that's a bug, my PR just turns it into an ICE -- but silencing the ICE does not fix the bug. Certain errors need to stop rustc from doing any further work with the faulty code.

Cc @rust-lang/project-const-generics

[oli-obk]

Can we turn it into a Constkind::Error instead of ICEing?

[RalfJung]

I mean we could I guess, though it seems suspicious to me that we even call this function after the type checker already determined that this is not valid as a const generic.

[lcnr]

We check the type of const parameters separately from when we compute them. Not doing so results in query cycles iirc 🤔 so it's probably best to delay_span_bug somewhere here instead of ICEing