rust-lang / rust-lang/rust

ICE: "failed to resolve instance for type", apparently caused by generic parameter inference

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C-bug fixed-by-next-solver I-ICE S-bug-has-test T-compiler
Dominant language
Rust
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Description

Code

Playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c29a73a6b88b2ef3d327766daee94e1d

This example is a little large (213 lines), but it's also an error I can work around pretty easily so I'm not too worried about it.

To fix it, it seems like you can just change the impl at line 88,

impl<const A: usize, const B: usize, O> VarCmp<Var<B>> for Var<A>
where
    Const<A>: ToUInt,
    Const<B>: ToUInt,
    U<A>: Cmp<U<B>, Output=O>,
{
    type Output = O;
}

to use a more explicit path:

impl<const A: usize, const B: usize> VarCmp<Var<B>> for Var<A>
where
    Const<A>: ToUInt,
    Const<B>: ToUInt,
    U<A>: Cmp<U<B>>,
{
    type Output = <U<A> as Cmp<U<B>>>::Output;
}

(Fixed version: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=748b532ef879333714976bf96b904fcd)

For reference, this system is meant to order & group Var<N> types (e.g. Var<0> + Var<1> + Var<0> -> Sum<Sum<Var<0>, Var<0>>, Var<1>>).

Meta

rustc --version --verbose:

rustc 1.84.0 (9fc6b4312 2025-01-07)
binary: rustc
commit-hash: 9fc6b43126469e3858e2fe86cafb4f0fd5068869
commit-date: 2025-01-07
host: x86_64-pc-windows-msvc
release: 1.84.0
LLVM version: 19.1.5

This error also occurred in 1.83.0 stable, but updating to 1.84.0 didn't seem to change anything.

Error output
error: internal compiler error: compiler\rustc_middle\src\ty\instance.rs:581:21: failed to resolve instance for <Sum<Sum<Var<0>, Var<0>>, Sum<Sum<Var<1>, Sum<Var<1>, Var<1>>>, Sum<Var<2>, Var<2>>>> as Add<Sum<Var<0>, Sum<Sum<Var<1>, Var<1>>, Var<2>>>>>::add   
   --> src\lib.rs:212:23
    |
212 |     assert_eq!(y + z, z + y);
    |                       ^^^^^

thread 'rustc' panicked at compiler\rustc_middle\src\ty\instance.rs:581:21:
Box<dyn Any>
Backtrace 

stack backtrace:
   0:     0x7ffa23abba41 - std::backtrace_rs::backtrace::dbghelp64::trace
                               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869\library/std\src\..\..\backtrace\src\backtrace\dbghelp64.rs:91
   1:     0x7ffa23abba41 - std::backtrace_rs::backtrace::trace_unsynchronized
                               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869\library/std\src\..\..\backtrace\src\backtrace\mod.rs:66
   2:     0x7ffa23abba41 - std::sys::backtrace::_print_fmt
                               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869\library/std\src\sys\backtrace.rs:66
   3:     0x7ffa23abba41 - std::sys::backtrace::impl$0::print::impl$0::fmt
                               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869\library/std\src\sys\backtrace.rs:39
   4:     0x7ffa23aedd1a - core::fmt::rt::Argument::fmt
                               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869\library/core\src\fmt\rt.rs:177
   5:     0x7ffa23aedd1a - core::fmt::write
                               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869\library/core\src\fmt\mod.rs:1189
   6:     0x7ffa23ab1de7 - std::io::Write::write_fmt<std::sys::pal::windows::stdio::Stderr>
                               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869\library/std\src\io\mod.rs:1884
   7:     0x7ffa23abb885 - std::sys::backtrace::BacktraceLock::print
                               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869\library/std\src\sys\backtrace.rs:42
   8:     0x7ffa23abe7a3 - std::panicking::default_hook::closure$1
                               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869\library/std\src\panicking.rs:268
   9:     0x7ffa23abe582 - std::panicking::default_hook
                               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869\library/std\src\panicking.rs:295
  10:     0x7ffa2509ab1e - strncpy
  11:     0x7ffa23abeee2 - alloc::boxed::impl$30::call
                               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869\library/alloc\src\boxed.rs:1986
  12:     0x7ffa23abeee2 - std::panicking::rust_panic_with_hook
                               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869\library/std\src\panicking.rs:809
  13:     0x7ffa2688b993 - <rustc_hir_pretty[ab0fe6e31331acdd]::State>::print_variant
  14:     0x7ffa2687d999 - <rustc_hir_pretty[ab0fe6e31331acdd]::State>::print_variant
  15:     0x7ffa2687d958 - <rustc_hir_pretty[ab0fe6e31331acdd]::State>::print_variant
  16:     0x7ffa268993c5 - <rustc_errors[3bd8408db383e9c9]::diagnostic::BugAbort as rustc_errors[3bd8408db383e9c9]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  17:     0x7ffa266ee57f - ar_archive_writer[5ad4fe434effa9d8]::object_reader::get_member_alignment
  18:     0x7ffa267a0f36 - rustc_middle[87d6ac0499a2eaf7]::util::bug::bug_fmt
  19:     0x7ffa267818bd - <rustc_middle[87d6ac0499a2eaf7]::ty::consts::Const>::to_valtree
  20:     0x7ffa267816d6 - <rustc_middle[87d6ac0499a2eaf7]::ty::consts::Const>::to_valtree
  21:     0x7ffa267a0e69 - rustc_middle[87d6ac0499a2eaf7]::util::bug::bug_fmt
  22:     0x7ffa24ee608c - <rustc_middle[87d6ac0499a2eaf7]::ty::instance::Instance>::expect_resolve
  23:     0x7ffa23e6c637 - <rustc_monomorphize[85a23766e536164]::collector::MirUsedCollector as rustc_middle[87d6ac0499a2eaf7]::mir::visit::Visitor>::visit_terminator
  24:     0x7ffa23e770a9 - rustc_monomorphize[85a23766e536164]::collector::items_of_instance
  25:     0x7ffa24840ee7 - rustc_query_impl[8be1416e77d742ed]::plumbing::query_key_hash_verify_all
  26:     0x7ffa247784db - rustc_ty_utils[4902e6e6aecd2893]::ty::self_ty_of_trait_impl_enabling_order_dep_trait_object_hack
  27:     0x7ffa24853bed - rustc_query_impl[8be1416e77d742ed]::plumbing::query_key_hash_verify_all
  28:     0x7ffa23e997f4 - rustc_monomorphize[85a23766e536164]::polymorphize::unused_generic_params
  29:     0x7ffa23e96624 - rustc_monomorphize[85a23766e536164]::polymorphize::unused_generic_params
  30:     0x7ffa23e96f82 - rustc_monomorphize[85a23766e536164]::polymorphize::unused_generic_params
  31:     0x7ffa23e96f82 - rustc_monomorphize[85a23766e536164]::polymorphize::unused_generic_params
  32:     0x7ffa23e8d616 - rustc_monomorphize[85a23766e536164]::partitioning::collect_and_partition_mono_items
  33:     0x7ffa236b8007 - rustc_ty_utils[4902e6e6aecd2893]::ty::adt_sized_constraint
  34:     0x7ffa2369e9fd - rustc_ty_utils[4902e6e6aecd2893]::ty::adt_sized_constraint
  35:     0x7ffa235c568f - rustc_ty_utils[4902e6e6aecd2893]::ty::adt_sized_constraint
  36:     0x7ffa236dd2ee - rustc_query_impl[8be1416e77d742ed]::query_system
  37:     0x7ffa20bb29bb - <rustc_codegen_llvm[85ae7dac9399dd6b]::LlvmCodegenBackend as rustc_codegen_ssa[ce0820b527006769]::traits::backend::CodegenBackend>::codegen_crate
  38:     0x7ffa20b13073 - <rustc_interface[c014954558e9d384]::queries::Linker>::codegen_and_build_linker
  39:     0x7ffa20ac74bd - _rust_alloc_error_handler
  40:     0x7ffa20ac2fcf - _rust_alloc_error_handler
  41:     0x7ffa20accfc5 - _rust_alloc_error_handler
  42:     0x7ffa23ad0bad - alloc::boxed::impl$28::call_once
                               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869\library/alloc\src\boxed.rs:1972
  43:     0x7ffa23ad0bad - alloc::boxed::impl$28::call_once
                               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869\library/alloc\src\boxed.rs:1972
  44:     0x7ffa23ad0bad - std::sys::pal::windows::thread::impl$0::new::thread_start
                               at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869\library/std\src\sys\pal\windows\thread.rs:55
  45:     0x7ffa9f687374 - BaseThreadInitThunk
  46:     0x7ffaa03bcc91 - RtlUserThreadStart

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 1.84.0 (9fc6b4312 2025-01-07) running on x86_64-pc-windows-msvc

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental=[REDACTED]

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

query stack during panic:
#0 [items_of_instance] collecting items used by `test`
#1 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack

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  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce the ICE from the stable Rust Playground example and inspect the failing call at src/lib.rs:212, especially the impl around line 88. Trace the failed instance resolution reported at compiler/rustc_middle/src/ty/instance.rs:581 and compare it with the explicit associated-type workaround. Done means the original example no longer causes an internal compiler error.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
38/100

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